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llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp

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//===- LoopAccessAnalysis.cpp - Loop Access Analysis Implementation --------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// The implementation for the loop memory dependence that was originally
// developed for the loop vectorizer.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "loop-accesses"
static cl::opt<unsigned, true>
VectorizationFactor("force-vector-width", cl::Hidden,
cl::desc("Sets the SIMD width. Zero is autoselect."),
cl::location(VectorizerParams::VectorizationFactor));
unsigned VectorizerParams::VectorizationFactor;
static cl::opt<unsigned, true>
VectorizationInterleave("force-vector-interleave", cl::Hidden,
cl::desc("Sets the vectorization interleave count. "
"Zero is autoselect."),
cl::location(
VectorizerParams::VectorizationInterleave));
unsigned VectorizerParams::VectorizationInterleave;
static cl::opt<unsigned, true> RuntimeMemoryCheckThreshold(
"runtime-memory-check-threshold", cl::Hidden,
cl::desc("When performing memory disambiguation checks at runtime do not "
"generate more than this number of comparisons (default = 8)."),
cl::location(VectorizerParams::RuntimeMemoryCheckThreshold), cl::init(8));
unsigned VectorizerParams::RuntimeMemoryCheckThreshold;
/// The maximum iterations used to merge memory checks
static cl::opt<unsigned> MemoryCheckMergeThreshold(
"memory-check-merge-threshold", cl::Hidden,
cl::desc("Maximum number of comparisons done when trying to merge "
"runtime memory checks. (default = 100)"),
cl::init(100));
/// Maximum SIMD width.
const unsigned VectorizerParams::MaxVectorWidth = 64;
/// We collect dependences up to this threshold.
static cl::opt<unsigned>
MaxDependences("max-dependences", cl::Hidden,
cl::desc("Maximum number of dependences collected by "
"loop-access analysis (default = 100)"),
cl::init(100));
/// This enables versioning on the strides of symbolically striding memory
/// accesses in code like the following.
/// for (i = 0; i < N; ++i)
/// A[i * Stride1] += B[i * Stride2] ...
///
/// Will be roughly translated to
/// if (Stride1 == 1 && Stride2 == 1) {
/// for (i = 0; i < N; i+=4)
/// A[i:i+3] += ...
/// } else
/// ...
static cl::opt<bool> EnableMemAccessVersioning(
"enable-mem-access-versioning", cl::init(true), cl::Hidden,
cl::desc("Enable symbolic stride memory access versioning"));
/// Enable store-to-load forwarding conflict detection. This option can
/// be disabled for correctness testing.
static cl::opt<bool> EnableForwardingConflictDetection(
"store-to-load-forwarding-conflict-detection", cl::Hidden,
cl::desc("Enable conflict detection in loop-access analysis"),
cl::init(true));
bool VectorizerParams::isInterleaveForced() {
return ::VectorizationInterleave.getNumOccurrences() > 0;
}
Value *llvm::stripIntegerCast(Value *V) {
if (auto *CI = dyn_cast<CastInst>(V))
if (CI->getOperand(0)->getType()->isIntegerTy())
return CI->getOperand(0);
return V;
}
const SCEV *llvm::replaceSymbolicStrideSCEV(PredicatedScalarEvolution &PSE,
const ValueToValueMap &PtrToStride,
Value *Ptr, Value *OrigPtr) {
const SCEV *OrigSCEV = PSE.getSCEV(Ptr);
// If there is an entry in the map return the SCEV of the pointer with the
// symbolic stride replaced by one.
ValueToValueMap::const_iterator SI =
PtrToStride.find(OrigPtr ? OrigPtr : Ptr);
if (SI != PtrToStride.end()) {
Value *StrideVal = SI->second;
// Strip casts.
StrideVal = stripIntegerCast(StrideVal);
ScalarEvolution *SE = PSE.getSE();
const auto *U = cast<SCEVUnknown>(SE->getSCEV(StrideVal));
const auto *CT =
static_cast<const SCEVConstant *>(SE->getOne(StrideVal->getType()));
PSE.addPredicate(*SE->getEqualPredicate(U, CT));
auto *Expr = PSE.getSCEV(Ptr);
LLVM_DEBUG(dbgs() << "LAA: Replacing SCEV: " << *OrigSCEV
<< " by: " << *Expr << "\n");
return Expr;
}
// Otherwise, just return the SCEV of the original pointer.
return OrigSCEV;
}
/// Calculate Start and End points of memory access.
/// Let's assume A is the first access and B is a memory access on N-th loop
/// iteration. Then B is calculated as:
/// B = A + Step*N .
/// Step value may be positive or negative.
/// N is a calculated back-edge taken count:
/// N = (TripCount > 0) ? RoundDown(TripCount -1 , VF) : 0
/// Start and End points are calculated in the following way:
/// Start = UMIN(A, B) ; End = UMAX(A, B) + SizeOfElt,
/// where SizeOfElt is the size of single memory access in bytes.
///
/// There is no conflict when the intervals are disjoint:
/// NoConflict = (P2.Start >= P1.End) || (P1.Start >= P2.End)
void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, bool WritePtr,
unsigned DepSetId, unsigned ASId,
const ValueToValueMap &Strides,
PredicatedScalarEvolution &PSE) {
// Get the stride replaced scev.
const SCEV *Sc = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
ScalarEvolution *SE = PSE.getSE();
const SCEV *ScStart;
const SCEV *ScEnd;
if (SE->isLoopInvariant(Sc, Lp))
ScStart = ScEnd = Sc;
else {
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(Sc);
assert(AR && "Invalid addrec expression");
const SCEV *Ex = PSE.getBackedgeTakenCount();
ScStart = AR->getStart();
ScEnd = AR->evaluateAtIteration(Ex, *SE);
const SCEV *Step = AR->getStepRecurrence(*SE);
// For expressions with negative step, the upper bound is ScStart and the
// lower bound is ScEnd.
if (const auto *CStep = dyn_cast<SCEVConstant>(Step)) {
if (CStep->getValue()->isNegative())
std::swap(ScStart, ScEnd);
} else {
// Fallback case: the step is not constant, but we can still
// get the upper and lower bounds of the interval by using min/max
// expressions.
ScStart = SE->getUMinExpr(ScStart, ScEnd);
ScEnd = SE->getUMaxExpr(AR->getStart(), ScEnd);
}
// Add the size of the pointed element to ScEnd.
unsigned EltSize =
Ptr->getType()->getPointerElementType()->getScalarSizeInBits() / 8;
const SCEV *EltSizeSCEV = SE->getConstant(ScEnd->getType(), EltSize);
ScEnd = SE->getAddExpr(ScEnd, EltSizeSCEV);
}
Pointers.emplace_back(Ptr, ScStart, ScEnd, WritePtr, DepSetId, ASId, Sc);
}
SmallVector<RuntimePointerChecking::PointerCheck, 4>
RuntimePointerChecking::generateChecks() const {
SmallVector<PointerCheck, 4> Checks;
for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
for (unsigned J = I + 1; J < CheckingGroups.size(); ++J) {
const RuntimePointerChecking::CheckingPtrGroup &CGI = CheckingGroups[I];
const RuntimePointerChecking::CheckingPtrGroup &CGJ = CheckingGroups[J];
if (needsChecking(CGI, CGJ))
Checks.push_back(std::make_pair(&CGI, &CGJ));
}
}
return Checks;
}
void RuntimePointerChecking::generateChecks(
MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
assert(Checks.empty() && "Checks is not empty");
groupChecks(DepCands, UseDependencies);
Checks = generateChecks();
}
bool RuntimePointerChecking::needsChecking(const CheckingPtrGroup &M,
const CheckingPtrGroup &N) const {
for (unsigned I = 0, EI = M.Members.size(); EI != I; ++I)
for (unsigned J = 0, EJ = N.Members.size(); EJ != J; ++J)
if (needsChecking(M.Members[I], N.Members[J]))
return true;
return false;
}
/// Compare \p I and \p J and return the minimum.
/// Return nullptr in case we couldn't find an answer.
static const SCEV *getMinFromExprs(const SCEV *I, const SCEV *J,
ScalarEvolution *SE) {
const SCEV *Diff = SE->getMinusSCEV(J, I);
const SCEVConstant *C = dyn_cast<const SCEVConstant>(Diff);
if (!C)
return nullptr;
if (C->getValue()->isNegative())
return J;
return I;
}
bool RuntimePointerChecking::CheckingPtrGroup::addPointer(unsigned Index) {
const SCEV *Start = RtCheck.Pointers[Index].Start;
const SCEV *End = RtCheck.Pointers[Index].End;
// Compare the starts and ends with the known minimum and maximum
// of this set. We need to know how we compare against the min/max
// of the set in order to be able to emit memchecks.
const SCEV *Min0 = getMinFromExprs(Start, Low, RtCheck.SE);
if (!Min0)
return false;
const SCEV *Min1 = getMinFromExprs(End, High, RtCheck.SE);
if (!Min1)
return false;
// Update the low bound expression if we've found a new min value.
if (Min0 == Start)
Low = Start;
// Update the high bound expression if we've found a new max value.
if (Min1 != End)
High = End;
Members.push_back(Index);
return true;
}
void RuntimePointerChecking::groupChecks(
MemoryDepChecker::DepCandidates &DepCands, bool UseDependencies) {
// We build the groups from dependency candidates equivalence classes
// because:
// - We know that pointers in the same equivalence class share
// the same underlying object and therefore there is a chance
// that we can compare pointers
// - We wouldn't be able to merge two pointers for which we need
// to emit a memcheck. The classes in DepCands are already
// conveniently built such that no two pointers in the same
// class need checking against each other.
// We use the following (greedy) algorithm to construct the groups
// For every pointer in the equivalence class:
// For each existing group:
// - if the difference between this pointer and the min/max bounds
// of the group is a constant, then make the pointer part of the
// group and update the min/max bounds of that group as required.
CheckingGroups.clear();
// If we need to check two pointers to the same underlying object
// with a non-constant difference, we shouldn't perform any pointer
// grouping with those pointers. This is because we can easily get
// into cases where the resulting check would return false, even when
// the accesses are safe.
//
// The following example shows this:
// for (i = 0; i < 1000; ++i)
// a[5000 + i * m] = a[i] + a[i + 9000]
//
// Here grouping gives a check of (5000, 5000 + 1000 * m) against
// (0, 10000) which is always false. However, if m is 1, there is no
// dependence. Not grouping the checks for a[i] and a[i + 9000] allows
// us to perform an accurate check in this case.
//
// The above case requires that we have an UnknownDependence between
// accesses to the same underlying object. This cannot happen unless
// FoundNonConstantDistanceDependence is set, and therefore UseDependencies
// is also false. In this case we will use the fallback path and create
// separate checking groups for all pointers.
// If we don't have the dependency partitions, construct a new
// checking pointer group for each pointer. This is also required
// for correctness, because in this case we can have checking between
// pointers to the same underlying object.
if (!UseDependencies) {
for (unsigned I = 0; I < Pointers.size(); ++I)
CheckingGroups.push_back(CheckingPtrGroup(I, *this));
return;
}
unsigned TotalComparisons = 0;
DenseMap<Value *, unsigned> PositionMap;
for (unsigned Index = 0; Index < Pointers.size(); ++Index)
PositionMap[Pointers[Index].PointerValue] = Index;
// We need to keep track of what pointers we've already seen so we
// don't process them twice.
SmallSet<unsigned, 2> Seen;
// Go through all equivalence classes, get the "pointer check groups"
// and add them to the overall solution. We use the order in which accesses
// appear in 'Pointers' to enforce determinism.
for (unsigned I = 0; I < Pointers.size(); ++I) {
// We've seen this pointer before, and therefore already processed
// its equivalence class.
if (Seen.count(I))
continue;
MemoryDepChecker::MemAccessInfo Access(Pointers[I].PointerValue,
Pointers[I].IsWritePtr);
SmallVector<CheckingPtrGroup, 2> Groups;
auto LeaderI = DepCands.findValue(DepCands.getLeaderValue(Access));
// Because DepCands is constructed by visiting accesses in the order in
// which they appear in alias sets (which is deterministic) and the
// iteration order within an equivalence class member is only dependent on
// the order in which unions and insertions are performed on the
// equivalence class, the iteration order is deterministic.
for (auto MI = DepCands.member_begin(LeaderI), ME = DepCands.member_end();
MI != ME; ++MI) {
unsigned Pointer = PositionMap[MI->getPointer()];
bool Merged = false;
// Mark this pointer as seen.
Seen.insert(Pointer);
// Go through all the existing sets and see if we can find one
// which can include this pointer.
for (CheckingPtrGroup &Group : Groups) {
// Don't perform more than a certain amount of comparisons.
// This should limit the cost of grouping the pointers to something
// reasonable. If we do end up hitting this threshold, the algorithm
// will create separate groups for all remaining pointers.
if (TotalComparisons > MemoryCheckMergeThreshold)
break;
TotalComparisons++;
if (Group.addPointer(Pointer)) {
Merged = true;
break;
}
}
if (!Merged)
// We couldn't add this pointer to any existing set or the threshold
// for the number of comparisons has been reached. Create a new group
// to hold the current pointer.
Groups.push_back(CheckingPtrGroup(Pointer, *this));
}
// We've computed the grouped checks for this partition.
// Save the results and continue with the next one.
llvm::copy(Groups, std::back_inserter(CheckingGroups));
}
}
bool RuntimePointerChecking::arePointersInSamePartition(
const SmallVectorImpl<int> &PtrToPartition, unsigned PtrIdx1,
unsigned PtrIdx2) {
return (PtrToPartition[PtrIdx1] != -1 &&
PtrToPartition[PtrIdx1] == PtrToPartition[PtrIdx2]);
}
bool RuntimePointerChecking::needsChecking(unsigned I, unsigned J) const {
const PointerInfo &PointerI = Pointers[I];
const PointerInfo &PointerJ = Pointers[J];
// No need to check if two readonly pointers intersect.
if (!PointerI.IsWritePtr && !PointerJ.IsWritePtr)
return false;
// Only need to check pointers between two different dependency sets.
if (PointerI.DependencySetId == PointerJ.DependencySetId)
return false;
// Only need to check pointers in the same alias set.
if (PointerI.AliasSetId != PointerJ.AliasSetId)
return false;
return true;
}
void RuntimePointerChecking::printChecks(
raw_ostream &OS, const SmallVectorImpl<PointerCheck> &Checks,
unsigned Depth) const {
unsigned N = 0;
for (const auto &Check : Checks) {
const auto &First = Check.first->Members, &Second = Check.second->Members;
OS.indent(Depth) << "Check " << N++ << ":\n";
OS.indent(Depth + 2) << "Comparing group (" << Check.first << "):\n";
for (unsigned K = 0; K < First.size(); ++K)
OS.indent(Depth + 2) << *Pointers[First[K]].PointerValue << "\n";
OS.indent(Depth + 2) << "Against group (" << Check.second << "):\n";
for (unsigned K = 0; K < Second.size(); ++K)
OS.indent(Depth + 2) << *Pointers[Second[K]].PointerValue << "\n";
}
}
void RuntimePointerChecking::print(raw_ostream &OS, unsigned Depth) const {
OS.indent(Depth) << "Run-time memory checks:\n";
printChecks(OS, Checks, Depth);
OS.indent(Depth) << "Grouped accesses:\n";
for (unsigned I = 0; I < CheckingGroups.size(); ++I) {
const auto &CG = CheckingGroups[I];
OS.indent(Depth + 2) << "Group " << &CG << ":\n";
OS.indent(Depth + 4) << "(Low: " << *CG.Low << " High: " << *CG.High
<< ")\n";
for (unsigned J = 0; J < CG.Members.size(); ++J) {
OS.indent(Depth + 6) << "Member: " << *Pointers[CG.Members[J]].Expr
<< "\n";
}
}
}
namespace {
/// Analyses memory accesses in a loop.
///
/// Checks whether run time pointer checks are needed and builds sets for data
/// dependence checking.
class AccessAnalysis {
public:
/// Read or write access location.
typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
typedef SmallVector<MemAccessInfo, 8> MemAccessInfoList;
AccessAnalysis(const DataLayout &Dl, Loop *TheLoop, AliasAnalysis *AA,
LoopInfo *LI, MemoryDepChecker::DepCandidates &DA,
PredicatedScalarEvolution &PSE)
: DL(Dl), TheLoop(TheLoop), AST(*AA), LI(LI), DepCands(DA),
IsRTCheckAnalysisNeeded(false), PSE(PSE) {}
/// Register a load and whether it is only read from.
void addLoad(MemoryLocation &Loc, bool IsReadOnly) {
Value *Ptr = const_cast<Value*>(Loc.Ptr);
AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
Accesses.insert(MemAccessInfo(Ptr, false));
if (IsReadOnly)
ReadOnlyPtr.insert(Ptr);
}
/// Register a store.
void addStore(MemoryLocation &Loc) {
Value *Ptr = const_cast<Value*>(Loc.Ptr);
AST.add(Ptr, LocationSize::unknown(), Loc.AATags);
Accesses.insert(MemAccessInfo(Ptr, true));
}
/// Check if we can emit a run-time no-alias check for \p Access.
///
/// Returns true if we can emit a run-time no alias check for \p Access.
/// If we can check this access, this also adds it to a dependence set and
/// adds a run-time to check for it to \p RtCheck. If \p Assume is true,
/// we will attempt to use additional run-time checks in order to get
/// the bounds of the pointer.
bool createCheckForAccess(RuntimePointerChecking &RtCheck,
MemAccessInfo Access,
const ValueToValueMap &Strides,
DenseMap<Value *, unsigned> &DepSetId,
Loop *TheLoop, unsigned &RunningDepId,
unsigned ASId, bool ShouldCheckStride,
bool Assume);
/// Check whether we can check the pointers at runtime for
/// non-intersection.
///
/// Returns true if we need no check or if we do and we can generate them
/// (i.e. the pointers have computable bounds).
bool canCheckPtrAtRT(RuntimePointerChecking &RtCheck, ScalarEvolution *SE,
Loop *TheLoop, const ValueToValueMap &Strides,
bool ShouldCheckWrap = false);
/// Goes over all memory accesses, checks whether a RT check is needed
/// and builds sets of dependent accesses.
void buildDependenceSets() {
processMemAccesses();
}
/// Initial processing of memory accesses determined that we need to
/// perform dependency checking.
///
/// Note that this can later be cleared if we retry memcheck analysis without
/// dependency checking (i.e. FoundNonConstantDistanceDependence).
bool isDependencyCheckNeeded() { return !CheckDeps.empty(); }
/// We decided that no dependence analysis would be used. Reset the state.
void resetDepChecks(MemoryDepChecker &DepChecker) {
CheckDeps.clear();
DepChecker.clearDependences();
}
MemAccessInfoList &getDependenciesToCheck() { return CheckDeps; }
private:
typedef SetVector<MemAccessInfo> PtrAccessSet;
/// Go over all memory access and check whether runtime pointer checks
/// are needed and build sets of dependency check candidates.
void processMemAccesses();
/// Set of all accesses.
PtrAccessSet Accesses;
const DataLayout &DL;
/// The loop being checked.
const Loop *TheLoop;
/// List of accesses that need a further dependence check.
MemAccessInfoList CheckDeps;
/// Set of pointers that are read only.
SmallPtrSet<Value*, 16> ReadOnlyPtr;
/// An alias set tracker to partition the access set by underlying object and
//intrinsic property (such as TBAA metadata).
AliasSetTracker AST;
LoopInfo *LI;
/// Sets of potentially dependent accesses - members of one set share an
/// underlying pointer. The set "CheckDeps" identfies which sets really need a
/// dependence check.
MemoryDepChecker::DepCandidates &DepCands;
/// Initial processing of memory accesses determined that we may need
/// to add memchecks. Perform the analysis to determine the necessary checks.
///
/// Note that, this is different from isDependencyCheckNeeded. When we retry
/// memcheck analysis without dependency checking
/// (i.e. FoundNonConstantDistanceDependence), isDependencyCheckNeeded is
/// cleared while this remains set if we have potentially dependent accesses.
bool IsRTCheckAnalysisNeeded;
/// The SCEV predicate containing all the SCEV-related assumptions.
PredicatedScalarEvolution &PSE;
};
} // end anonymous namespace
/// Check whether a pointer can participate in a runtime bounds check.
/// If \p Assume, try harder to prove that we can compute the bounds of \p Ptr
/// by adding run-time checks (overflow checks) if necessary.
static bool hasComputableBounds(PredicatedScalarEvolution &PSE,
const ValueToValueMap &Strides, Value *Ptr,
Loop *L, bool Assume) {
const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, Strides, Ptr);
// The bounds for loop-invariant pointer is trivial.
if (PSE.getSE()->isLoopInvariant(PtrScev, L))
return true;
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
if (!AR && Assume)
AR = PSE.getAsAddRec(Ptr);
if (!AR)
return false;
return AR->isAffine();
}
/// Check whether a pointer address cannot wrap.
static bool isNoWrap(PredicatedScalarEvolution &PSE,
const ValueToValueMap &Strides, Value *Ptr, Loop *L) {
const SCEV *PtrScev = PSE.getSCEV(Ptr);
if (PSE.getSE()->isLoopInvariant(PtrScev, L))
return true;
int64_t Stride = getPtrStride(PSE, Ptr, L, Strides);
if (Stride == 1 || PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW))
return true;
return false;
}
bool AccessAnalysis::createCheckForAccess(RuntimePointerChecking &RtCheck,
MemAccessInfo Access,
const ValueToValueMap &StridesMap,
DenseMap<Value *, unsigned> &DepSetId,
Loop *TheLoop, unsigned &RunningDepId,
unsigned ASId, bool ShouldCheckWrap,
bool Assume) {
Value *Ptr = Access.getPointer();
if (!hasComputableBounds(PSE, StridesMap, Ptr, TheLoop, Assume))
return false;
// When we run after a failing dependency check we have to make sure
// we don't have wrapping pointers.
if (ShouldCheckWrap && !isNoWrap(PSE, StridesMap, Ptr, TheLoop)) {
auto *Expr = PSE.getSCEV(Ptr);
if (!Assume || !isa<SCEVAddRecExpr>(Expr))
return false;
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
}
// The id of the dependence set.
unsigned DepId;
if (isDependencyCheckNeeded()) {
Value *Leader = DepCands.getLeaderValue(Access).getPointer();
unsigned &LeaderId = DepSetId[Leader];
if (!LeaderId)
LeaderId = RunningDepId++;
DepId = LeaderId;
} else
// Each access has its own dependence set.
DepId = RunningDepId++;
bool IsWrite = Access.getInt();
RtCheck.insert(TheLoop, Ptr, IsWrite, DepId, ASId, StridesMap, PSE);
LLVM_DEBUG(dbgs() << "LAA: Found a runtime check ptr:" << *Ptr << '\n');
return true;
}
bool AccessAnalysis::canCheckPtrAtRT(RuntimePointerChecking &RtCheck,
ScalarEvolution *SE, Loop *TheLoop,
const ValueToValueMap &StridesMap,
bool ShouldCheckWrap) {
// Find pointers with computable bounds. We are going to use this information
// to place a runtime bound check.
bool CanDoRT = true;
bool NeedRTCheck = false;
if (!IsRTCheckAnalysisNeeded) return true;
bool IsDepCheckNeeded = isDependencyCheckNeeded();
// We assign a consecutive id to access from different alias sets.
// Accesses between different groups doesn't need to be checked.
unsigned ASId = 1;
for (auto &AS : AST) {
int NumReadPtrChecks = 0;
int NumWritePtrChecks = 0;
bool CanDoAliasSetRT = true;
// We assign consecutive id to access from different dependence sets.
// Accesses within the same set don't need a runtime check.
unsigned RunningDepId = 1;
DenseMap<Value *, unsigned> DepSetId;
SmallVector<MemAccessInfo, 4> Retries;
for (auto A : AS) {
Value *Ptr = A.getValue();
bool IsWrite = Accesses.count(MemAccessInfo(Ptr, true));
MemAccessInfo Access(Ptr, IsWrite);
if (IsWrite)
++NumWritePtrChecks;
else
++NumReadPtrChecks;
if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId, TheLoop,
RunningDepId, ASId, ShouldCheckWrap, false)) {
LLVM_DEBUG(dbgs() << "LAA: Can't find bounds for ptr:" << *Ptr << '\n');
Retries.push_back(Access);
CanDoAliasSetRT = false;
}
}
// If we have at least two writes or one write and a read then we need to
// check them. But there is no need to checks if there is only one
// dependence set for this alias set.
//
// Note that this function computes CanDoRT and NeedRTCheck independently.
// For example CanDoRT=false, NeedRTCheck=false means that we have a pointer
// for which we couldn't find the bounds but we don't actually need to emit
// any checks so it does not matter.
bool NeedsAliasSetRTCheck = false;
if (!(IsDepCheckNeeded && CanDoAliasSetRT && RunningDepId == 2))
NeedsAliasSetRTCheck = (NumWritePtrChecks >= 2 ||
(NumReadPtrChecks >= 1 && NumWritePtrChecks >= 1));
// We need to perform run-time alias checks, but some pointers had bounds
// that couldn't be checked.
if (NeedsAliasSetRTCheck && !CanDoAliasSetRT) {
// Reset the CanDoSetRt flag and retry all accesses that have failed.
// We know that we need these checks, so we can now be more aggressive
// and add further checks if required (overflow checks).
CanDoAliasSetRT = true;
for (auto Access : Retries)
if (!createCheckForAccess(RtCheck, Access, StridesMap, DepSetId,
TheLoop, RunningDepId, ASId,
ShouldCheckWrap, /*Assume=*/true)) {
CanDoAliasSetRT = false;
break;
}
}
CanDoRT &= CanDoAliasSetRT;
NeedRTCheck |= NeedsAliasSetRTCheck;
++ASId;
}
// If the pointers that we would use for the bounds comparison have different
// address spaces, assume the values aren't directly comparable, so we can't
// use them for the runtime check. We also have to assume they could
// overlap. In the future there should be metadata for whether address spaces
// are disjoint.
unsigned NumPointers = RtCheck.Pointers.size();
for (unsigned i = 0; i < NumPointers; ++i) {
for (unsigned j = i + 1; j < NumPointers; ++j) {
// Only need to check pointers between two different dependency sets.
if (RtCheck.Pointers[i].DependencySetId ==
RtCheck.Pointers[j].DependencySetId)
continue;
// Only need to check pointers in the same alias set.
if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
continue;
Value *PtrI = RtCheck.Pointers[i].PointerValue;
Value *PtrJ = RtCheck.Pointers[j].PointerValue;
unsigned ASi = PtrI->getType()->getPointerAddressSpace();
unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
if (ASi != ASj) {
LLVM_DEBUG(
dbgs() << "LAA: Runtime check would require comparison between"
" different address spaces\n");
return false;
}
}
}
if (NeedRTCheck && CanDoRT)
RtCheck.generateChecks(DepCands, IsDepCheckNeeded);
LLVM_DEBUG(dbgs() << "LAA: We need to do " << RtCheck.getNumberOfChecks()
<< " pointer comparisons.\n");
RtCheck.Need = NeedRTCheck;
bool CanDoRTIfNeeded = !NeedRTCheck || CanDoRT;
if (!CanDoRTIfNeeded)
RtCheck.reset();
return CanDoRTIfNeeded;
}
void AccessAnalysis::processMemAccesses() {
// We process the set twice: first we process read-write pointers, last we
// process read-only pointers. This allows us to skip dependence tests for
// read-only pointers.
LLVM_DEBUG(dbgs() << "LAA: Processing memory accesses...\n");
LLVM_DEBUG(dbgs() << " AST: "; AST.dump());
LLVM_DEBUG(dbgs() << "LAA: Accesses(" << Accesses.size() << "):\n");
LLVM_DEBUG({
for (auto A : Accesses)
dbgs() << "\t" << *A.getPointer() << " (" <<
(A.getInt() ? "write" : (ReadOnlyPtr.count(A.getPointer()) ?
"read-only" : "read")) << ")\n";
});
// The AliasSetTracker has nicely partitioned our pointers by metadata
// compatibility and potential for underlying-object overlap. As a result, we
// only need to check for potential pointer dependencies within each alias
// set.
for (auto &AS : AST) {
// Note that both the alias-set tracker and the alias sets themselves used
// linked lists internally and so the iteration order here is deterministic
// (matching the original instruction order within each set).
bool SetHasWrite = false;
// Map of pointers to last access encountered.
typedef DenseMap<const Value*, MemAccessInfo> UnderlyingObjToAccessMap;
UnderlyingObjToAccessMap ObjToLastAccess;
// Set of access to check after all writes have been processed.
PtrAccessSet DeferredAccesses;
// Iterate over each alias set twice, once to process read/write pointers,
// and then to process read-only pointers.
for (int SetIteration = 0; SetIteration < 2; ++SetIteration) {
bool UseDeferred = SetIteration > 0;
PtrAccessSet &S = UseDeferred ? DeferredAccesses : Accesses;
for (auto AV : AS) {
Value *Ptr = AV.getValue();
// For a single memory access in AliasSetTracker, Accesses may contain
// both read and write, and they both need to be handled for CheckDeps.
for (auto AC : S) {
if (AC.getPointer() != Ptr)
continue;
bool IsWrite = AC.getInt();
// If we're using the deferred access set, then it contains only
// reads.
bool IsReadOnlyPtr = ReadOnlyPtr.count(Ptr) && !IsWrite;
if (UseDeferred && !IsReadOnlyPtr)
continue;
// Otherwise, the pointer must be in the PtrAccessSet, either as a
// read or a write.
assert(((IsReadOnlyPtr && UseDeferred) || IsWrite ||
S.count(MemAccessInfo(Ptr, false))) &&
"Alias-set pointer not in the access set?");
MemAccessInfo Access(Ptr, IsWrite);
DepCands.insert(Access);
// Memorize read-only pointers for later processing and skip them in
// the first round (they need to be checked after we have seen all
// write pointers). Note: we also mark pointer that are not
// consecutive as "read-only" pointers (so that we check
// "a[b[i]] +="). Hence, we need the second check for "!IsWrite".
if (!UseDeferred && IsReadOnlyPtr) {
DeferredAccesses.insert(Access);
continue;
}
// If this is a write - check other reads and writes for conflicts. If
// this is a read only check other writes for conflicts (but only if
// there is no other write to the ptr - this is an optimization to
// catch "a[i] = a[i] + " without having to do a dependence check).
if ((IsWrite || IsReadOnlyPtr) && SetHasWrite) {
CheckDeps.push_back(Access);
IsRTCheckAnalysisNeeded = true;
}
if (IsWrite)
SetHasWrite = true;
// Create sets of pointers connected by a shared alias set and
// underlying object.
typedef SmallVector<const Value *, 16> ValueVector;
ValueVector TempObjects;
GetUnderlyingObjects(Ptr, TempObjects, DL, LI);
LLVM_DEBUG(dbgs()
<< "Underlying objects for pointer " << *Ptr << "\n");
for (const Value *UnderlyingObj : TempObjects) {
// nullptr never alias, don't join sets for pointer that have "null"
// in their UnderlyingObjects list.
if (isa<ConstantPointerNull>(UnderlyingObj) &&
!NullPointerIsDefined(
TheLoop->getHeader()->getParent(),
UnderlyingObj->getType()->getPointerAddressSpace()))
continue;
UnderlyingObjToAccessMap::iterator Prev =
ObjToLastAccess.find(UnderlyingObj);
if (Prev != ObjToLastAccess.end())
DepCands.unionSets(Access, Prev->second);
ObjToLastAccess[UnderlyingObj] = Access;
LLVM_DEBUG(dbgs() << " " << *UnderlyingObj << "\n");
}
}
}
}
}
}
static bool isInBoundsGep(Value *Ptr) {
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr))
return GEP->isInBounds();
return false;
}
/// Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
/// i.e. monotonically increasing/decreasing.
static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
PredicatedScalarEvolution &PSE, const Loop *L) {
// FIXME: This should probably only return true for NUW.
if (AR->getNoWrapFlags(SCEV::NoWrapMask))
return true;
// Scalar evolution does not propagate the non-wrapping flags to values that
// are derived from a non-wrapping induction variable because non-wrapping
// could be flow-sensitive.
//
// Look through the potentially overflowing instruction to try to prove
// non-wrapping for the *specific* value of Ptr.
// The arithmetic implied by an inbounds GEP can't overflow.
auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
if (!GEP || !GEP->isInBounds())
return false;
// Make sure there is only one non-const index and analyze that.
Value *NonConstIndex = nullptr;
for (Value *Index : make_range(GEP->idx_begin(), GEP->idx_end()))
if (!isa<ConstantInt>(Index)) {
if (NonConstIndex)
return false;
NonConstIndex = Index;
}
if (!NonConstIndex)
// The recurrence is on the pointer, ignore for now.
return false;
// The index in GEP is signed. It is non-wrapping if it's derived from a NSW
// AddRec using a NSW operation.
if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
if (OBO->hasNoSignedWrap() &&
// Assume constant for other the operand so that the AddRec can be
// easily found.
isa<ConstantInt>(OBO->getOperand(1))) {
auto *OpScev = PSE.getSCEV(OBO->getOperand(0));
if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
}
return false;
}
/// Check whether the access through \p Ptr has a constant stride.
int64_t llvm::getPtrStride(PredicatedScalarEvolution &PSE, Value *Ptr,
const Loop *Lp, const ValueToValueMap &StridesMap,
bool Assume, bool ShouldCheckWrap) {
Type *Ty = Ptr->getType();
assert(Ty->isPointerTy() && "Unexpected non-ptr");
// Make sure that the pointer does not point to aggregate types.
auto *PtrTy = cast<PointerType>(Ty);
if (PtrTy->getElementType()->isAggregateType()) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a pointer to a scalar type"
<< *Ptr << "\n");
return 0;
}
const SCEV *PtrScev = replaceSymbolicStrideSCEV(PSE, StridesMap, Ptr);
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrScev);
if (Assume && !AR)
AR = PSE.getAsAddRec(Ptr);
if (!AR) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not an AddRecExpr pointer " << *Ptr
<< " SCEV: " << *PtrScev << "\n");
return 0;
}
// The access function must stride over the innermost loop.
if (Lp != AR->getLoop()) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
<< *Ptr << " SCEV: " << *AR << "\n");
return 0;
}
// The address calculation must not wrap. Otherwise, a dependence could be
// inverted.
// An inbounds getelementptr that is a AddRec with a unit stride
// cannot wrap per definition. The unit stride requirement is checked later.
// An getelementptr without an inbounds attribute and unit stride would have
// to access the pointer value "0" which is undefined behavior in address
// space 0, therefore we can also vectorize this case.
bool IsInBoundsGEP = isInBoundsGep(Ptr);
bool IsNoWrapAddRec = !ShouldCheckWrap ||
PSE.hasNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW) ||
isNoWrapAddRec(Ptr, AR, PSE, Lp);
if (!IsNoWrapAddRec && !IsInBoundsGEP &&
NullPointerIsDefined(Lp->getHeader()->getParent(),
PtrTy->getAddressSpace())) {
if (Assume) {
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
IsNoWrapAddRec = true;
LLVM_DEBUG(dbgs() << "LAA: Pointer may wrap in the address space:\n"
<< "LAA: Pointer: " << *Ptr << "\n"
<< "LAA: SCEV: " << *AR << "\n"
<< "LAA: Added an overflow assumption\n");
} else {
LLVM_DEBUG(
dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "
<< *Ptr << " SCEV: " << *AR << "\n");
return 0;
}
}
// Check the step is constant.
const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
// Calculate the pointer stride and check if it is constant.
const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
if (!C) {
LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
<< " SCEV: " << *AR << "\n");
return 0;
}
auto &DL = Lp->getHeader()->getModule()->getDataLayout();
int64_t Size = DL.getTypeAllocSize(PtrTy->getElementType());
const APInt &APStepVal = C->getAPInt();
// Huge step value - give up.
if (APStepVal.getBitWidth() > 64)
return 0;
int64_t StepVal = APStepVal.getSExtValue();
// Strided access.
int64_t Stride = StepVal / Size;
int64_t Rem = StepVal % Size;
if (Rem)
return 0;
// If the SCEV could wrap but we have an inbounds gep with a unit stride we
// know we can't "wrap around the address space". In case of address space
// zero we know that this won't happen without triggering undefined behavior.
if (!IsNoWrapAddRec && Stride != 1 && Stride != -1 &&
(IsInBoundsGEP || !NullPointerIsDefined(Lp->getHeader()->getParent(),
PtrTy->getAddressSpace()))) {
if (Assume) {
// We can avoid this case by adding a run-time check.
LLVM_DEBUG(dbgs() << "LAA: Non unit strided pointer which is not either "
<< "inbounds or in address space 0 may wrap:\n"
<< "LAA: Pointer: " << *Ptr << "\n"
<< "LAA: SCEV: " << *AR << "\n"
<< "LAA: Added an overflow assumption\n");
PSE.setNoOverflow(Ptr, SCEVWrapPredicate::IncrementNUSW);
} else
return 0;
}
return Stride;
}
bool llvm::sortPtrAccesses(ArrayRef<Value *> VL, const DataLayout &DL,
ScalarEvolution &SE,
SmallVectorImpl<unsigned> &SortedIndices) {
assert(llvm::all_of(
VL, [](const Value *V) { return V->getType()->isPointerTy(); }) &&
"Expected list of pointer operands.");
SmallVector<std::pair<int64_t, Value *>, 4> OffValPairs;
OffValPairs.reserve(VL.size());
// Walk over the pointers, and map each of them to an offset relative to
// first pointer in the array.
Value *Ptr0 = VL[0];
const SCEV *Scev0 = SE.getSCEV(Ptr0);
Value *Obj0 = GetUnderlyingObject(Ptr0, DL);
llvm::SmallSet<int64_t, 4> Offsets;
for (auto *Ptr : VL) {
// TODO: Outline this code as a special, more time consuming, version of
// computeConstantDifference() function.
if (Ptr->getType()->getPointerAddressSpace() !=
Ptr0->getType()->getPointerAddressSpace())
return false;
// If a pointer refers to a different underlying object, bail - the
// pointers are by definition incomparable.
Value *CurrObj = GetUnderlyingObject(Ptr, DL);
if (CurrObj != Obj0)
return false;
const SCEV *Scev = SE.getSCEV(Ptr);
const auto *Diff = dyn_cast<SCEVConstant>(SE.getMinusSCEV(Scev, Scev0));
// The pointers may not have a constant offset from each other, or SCEV
// may just not be smart enough to figure out they do. Regardless,
// there's nothing we can do.
if (!Diff)
return false;
// Check if the pointer with the same offset is found.
int64_t Offset = Diff->getAPInt().getSExtValue();
if (!Offsets.insert(Offset).second)
return false;
OffValPairs.emplace_back(Offset, Ptr);
}
SortedIndices.clear();
SortedIndices.resize(VL.size());
std::iota(SortedIndices.begin(), SortedIndices.end(), 0);
// Sort the memory accesses and keep the order of their uses in UseOrder.
llvm::stable_sort(SortedIndices, [&](unsigned Left, unsigned Right) {
return OffValPairs[Left].first < OffValPairs[Right].first;
});
// Check if the order is consecutive already.
if (llvm::all_of(SortedIndices, [&SortedIndices](const unsigned I) {
return I == SortedIndices[I];
}))
SortedIndices.clear();
return true;
}
/// Take the address space operand from the Load/Store instruction.
/// Returns -1 if this is not a valid Load/Store instruction.
static unsigned getAddressSpaceOperand(Value *I) {
if (LoadInst *L = dyn_cast<LoadInst>(I))
return L->getPointerAddressSpace();
if (StoreInst *S = dyn_cast<StoreInst>(I))
return S->getPointerAddressSpace();
return -1;
}
/// Returns true if the memory operations \p A and \p B are consecutive.
bool llvm::isConsecutiveAccess(Value *A, Value *B, const DataLayout &DL,
ScalarEvolution &SE, bool CheckType) {
Value *PtrA = getLoadStorePointerOperand(A);
Value *PtrB = getLoadStorePointerOperand(B);
unsigned ASA = getAddressSpaceOperand(A);
unsigned ASB = getAddressSpaceOperand(B);
// Check that the address spaces match and that the pointers are valid.
if (!PtrA || !PtrB || (ASA != ASB))
return false;
// Make sure that A and B are different pointers.
if (PtrA == PtrB)
return false;
// Make sure that A and B have the same type if required.
if (CheckType && PtrA->getType() != PtrB->getType())
return false;
unsigned IdxWidth = DL.getIndexSizeInBits(ASA);
Type *Ty = cast<PointerType>(PtrA->getType())->getElementType();
APInt OffsetA(IdxWidth, 0), OffsetB(IdxWidth, 0);
PtrA = PtrA->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetA);
PtrB = PtrB->stripAndAccumulateInBoundsConstantOffsets(DL, OffsetB);
// Retrieve the address space again as pointer stripping now tracks through
// `addrspacecast`.
ASA = cast<PointerType>(PtrA->getType())->getAddressSpace();
ASB = cast<PointerType>(PtrB->getType())->getAddressSpace();
// Check that the address spaces match and that the pointers are valid.
if (ASA != ASB)
return false;
IdxWidth = DL.getIndexSizeInBits(ASA);
OffsetA = OffsetA.sextOrTrunc(IdxWidth);
OffsetB = OffsetB.sextOrTrunc(IdxWidth);
APInt Size(IdxWidth, DL.getTypeStoreSize(Ty));
// OffsetDelta = OffsetB - OffsetA;
const SCEV *OffsetSCEVA = SE.getConstant(OffsetA);
const SCEV *OffsetSCEVB = SE.getConstant(OffsetB);
const SCEV *OffsetDeltaSCEV = SE.getMinusSCEV(OffsetSCEVB, OffsetSCEVA);
const APInt &OffsetDelta = cast<SCEVConstant>(OffsetDeltaSCEV)->getAPInt();
// Check if they are based on the same pointer. That makes the offsets
// sufficient.
if (PtrA == PtrB)
return OffsetDelta == Size;
// Compute the necessary base pointer delta to have the necessary final delta
// equal to the size.
// BaseDelta = Size - OffsetDelta;
const SCEV *SizeSCEV = SE.getConstant(Size);
const SCEV *BaseDelta = SE.getMinusSCEV(SizeSCEV, OffsetDeltaSCEV);
// Otherwise compute the distance with SCEV between the base pointers.
const SCEV *PtrSCEVA = SE.getSCEV(PtrA);
const SCEV *PtrSCEVB = SE.getSCEV(PtrB);
const SCEV *X = SE.getAddExpr(PtrSCEVA, BaseDelta);
return X == PtrSCEVB;
}
MemoryDepChecker::VectorizationSafetyStatus
MemoryDepChecker::Dependence::isSafeForVectorization(DepType Type) {
switch (Type) {
case NoDep:
case Forward:
case BackwardVectorizable:
return VectorizationSafetyStatus::Safe;
case Unknown:
return VectorizationSafetyStatus::PossiblySafeWithRtChecks;
case ForwardButPreventsForwarding:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return VectorizationSafetyStatus::Unsafe;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::Dependence::isBackward() const {
switch (Type) {
case NoDep:
case Forward:
case ForwardButPreventsForwarding:
case Unknown:
return false;
case BackwardVectorizable:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return true;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::Dependence::isPossiblyBackward() const {
return isBackward() || Type == Unknown;
}
bool MemoryDepChecker::Dependence::isForward() const {
switch (Type) {
case Forward:
case ForwardButPreventsForwarding:
return true;
case NoDep:
case Unknown:
case BackwardVectorizable:
case Backward:
case BackwardVectorizableButPreventsForwarding:
return false;
}
llvm_unreachable("unexpected DepType!");
}
bool MemoryDepChecker::couldPreventStoreLoadForward(uint64_t Distance,
uint64_t TypeByteSize) {
// If loads occur at a distance that is not a multiple of a feasible vector
// factor store-load forwarding does not take place.
// Positive dependences might cause troubles because vectorizing them might
// prevent store-load forwarding making vectorized code run a lot slower.
// a[i] = a[i-3] ^ a[i-8];
// The stores to a[i:i+1] don't align with the stores to a[i-3:i-2] and
// hence on your typical architecture store-load forwarding does not take
// place. Vectorizing in such cases does not make sense.
// Store-load forwarding distance.
// After this many iterations store-to-load forwarding conflicts should not
// cause any slowdowns.
const uint64_t NumItersForStoreLoadThroughMemory = 8 * TypeByteSize;
// Maximum vector factor.
uint64_t MaxVFWithoutSLForwardIssues = std::min(
VectorizerParams::MaxVectorWidth * TypeByteSize, MaxSafeDepDistBytes);
// Compute the smallest VF at which the store and load would be misaligned.
for (uint64_t VF = 2 * TypeByteSize; VF <= MaxVFWithoutSLForwardIssues;
VF *= 2) {
// If the number of vector iteration between the store and the load are
// small we could incur conflicts.
if (Distance % VF && Distance / VF < NumItersForStoreLoadThroughMemory) {
MaxVFWithoutSLForwardIssues = (VF >>= 1);
break;
}
}
if (MaxVFWithoutSLForwardIssues < 2 * TypeByteSize) {
LLVM_DEBUG(
dbgs() << "LAA: Distance " << Distance
<< " that could cause a store-load forwarding conflict\n");
return true;
}
if (MaxVFWithoutSLForwardIssues < MaxSafeDepDistBytes &&
MaxVFWithoutSLForwardIssues !=
VectorizerParams::MaxVectorWidth * TypeByteSize)
MaxSafeDepDistBytes = MaxVFWithoutSLForwardIssues;
return false;
}
void MemoryDepChecker::mergeInStatus(VectorizationSafetyStatus S) {
if (Status < S)
Status = S;
}
/// Given a non-constant (unknown) dependence-distance \p Dist between two
/// memory accesses, that have the same stride whose absolute value is given
/// in \p Stride, and that have the same type size \p TypeByteSize,
/// in a loop whose takenCount is \p BackedgeTakenCount, check if it is
/// possible to prove statically that the dependence distance is larger
/// than the range that the accesses will travel through the execution of
/// the loop. If so, return true; false otherwise. This is useful for
/// example in loops such as the following (PR31098):
/// for (i = 0; i < D; ++i) {
/// = out[i];
/// out[i+D] =
/// }
static bool isSafeDependenceDistance(const DataLayout &DL, ScalarEvolution &SE,
const SCEV &BackedgeTakenCount,
const SCEV &Dist, uint64_t Stride,
uint64_t TypeByteSize) {
// If we can prove that
// (**) |Dist| > BackedgeTakenCount * Step
// where Step is the absolute stride of the memory accesses in bytes,
// then there is no dependence.
//
// Rationale:
// We basically want to check if the absolute distance (|Dist/Step|)
// is >= the loop iteration count (or > BackedgeTakenCount).
// This is equivalent to the Strong SIV Test (Practical Dependence Testing,
// Section 4.2.1); Note, that for vectorization it is sufficient to prove
// that the dependence distance is >= VF; This is checked elsewhere.
// But in some cases we can prune unknown dependence distances early, and
// even before selecting the VF, and without a runtime test, by comparing
// the distance against the loop iteration count. Since the vectorized code
// will be executed only if LoopCount >= VF, proving distance >= LoopCount
// also guarantees that distance >= VF.
//
const uint64_t ByteStride = Stride * TypeByteSize;
const SCEV *Step = SE.getConstant(BackedgeTakenCount.getType(), ByteStride);
const SCEV *Product = SE.getMulExpr(&BackedgeTakenCount, Step);
const SCEV *CastedDist = &Dist;
const SCEV *CastedProduct = Product;
uint64_t DistTypeSize = DL.getTypeAllocSize(Dist.getType());
uint64_t ProductTypeSize = DL.getTypeAllocSize(Product->getType());
// The dependence distance can be positive/negative, so we sign extend Dist;
// The multiplication of the absolute stride in bytes and the
// backedgeTakenCount is non-negative, so we zero extend Product.
if (DistTypeSize > ProductTypeSize)
CastedProduct = SE.getZeroExtendExpr(Product, Dist.getType());
else
CastedDist = SE.getNoopOrSignExtend(&Dist, Product->getType());
// Is Dist - (BackedgeTakenCount * Step) > 0 ?
// (If so, then we have proven (**) because |Dist| >= Dist)
const SCEV *Minus = SE.getMinusSCEV(CastedDist, CastedProduct);
if (SE.isKnownPositive(Minus))
return true;
// Second try: Is -Dist - (BackedgeTakenCount * Step) > 0 ?
// (If so, then we have proven (**) because |Dist| >= -1*Dist)
const SCEV *NegDist = SE.getNegativeSCEV(CastedDist);
Minus = SE.getMinusSCEV(NegDist, CastedProduct);
if (SE.isKnownPositive(Minus))
return true;
return false;
}
/// Check the dependence for two accesses with the same stride \p Stride.
/// \p Distance is the positive distance and \p TypeByteSize is type size in
/// bytes.
///
/// \returns true if they are independent.
static bool areStridedAccessesIndependent(uint64_t Distance, uint64_t Stride,
uint64_t TypeByteSize) {
assert(Stride > 1 && "The stride must be greater than 1");
assert(TypeByteSize > 0 && "The type size in byte must be non-zero");
assert(Distance > 0 && "The distance must be non-zero");
// Skip if the distance is not multiple of type byte size.
if (Distance % TypeByteSize)
return false;
uint64_t ScaledDist = Distance / TypeByteSize;
// No dependence if the scaled distance is not multiple of the stride.
// E.g.
// for (i = 0; i < 1024 ; i += 4)
// A[i+2] = A[i] + 1;
//
// Two accesses in memory (scaled distance is 2, stride is 4):
// | A[0] | | | | A[4] | | | |
// | | | A[2] | | | | A[6] | |
//
// E.g.
// for (i = 0; i < 1024 ; i += 3)
// A[i+4] = A[i] + 1;
//
// Two accesses in memory (scaled distance is 4, stride is 3):
// | A[0] | | | A[3] | | | A[6] | | |
// | | | | | A[4] | | | A[7] | |
return ScaledDist % Stride;
}
MemoryDepChecker::Dependence::DepType
MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
const MemAccessInfo &B, unsigned BIdx,
const ValueToValueMap &Strides) {
assert (AIdx < BIdx && "Must pass arguments in program order");
Value *APtr = A.getPointer();
Value *BPtr = B.getPointer();
bool AIsWrite = A.getInt();
bool BIsWrite = B.getInt();
// Two reads are independent.
if (!AIsWrite && !BIsWrite)
return Dependence::NoDep;
// We cannot check pointers in different address spaces.
if (APtr->getType()->getPointerAddressSpace() !=
BPtr->getType()->getPointerAddressSpace())
return Dependence::Unknown;
int64_t StrideAPtr = getPtrStride(PSE, APtr, InnermostLoop, Strides, true);
int64_t StrideBPtr = getPtrStride(PSE, BPtr, InnermostLoop, Strides, true);
const SCEV *Src = PSE.getSCEV(APtr);
const SCEV *Sink = PSE.getSCEV(BPtr);
// If the induction step is negative we have to invert source and sink of the
// dependence.
if (StrideAPtr < 0) {
std::swap(APtr, BPtr);
std::swap(Src, Sink);
std::swap(AIsWrite, BIsWrite);
std::swap(AIdx, BIdx);
std::swap(StrideAPtr, StrideBPtr);
}
const SCEV *Dist = PSE.getSE()->getMinusSCEV(Sink, Src);
LLVM_DEBUG(dbgs() << "LAA: Src Scev: " << *Src << "Sink Scev: " << *Sink
<< "(Induction step: " << StrideAPtr << ")\n");
LLVM_DEBUG(dbgs() << "LAA: Distance for " << *InstMap[AIdx] << " to "
<< *InstMap[BIdx] << ": " << *Dist << "\n");
// Need accesses with constant stride. We don't want to vectorize
// "A[B[i]] += ..." and similar code or pointer arithmetic that could wrap in
// the address space.
if (!StrideAPtr || !StrideBPtr || StrideAPtr != StrideBPtr){
LLVM_DEBUG(dbgs() << "Pointer access with non-constant stride\n");
return Dependence::Unknown;
}
Type *ATy = APtr->getType()->getPointerElementType();
Type *BTy = BPtr->getType()->getPointerElementType();
auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
uint64_t TypeByteSize = DL.getTypeAllocSize(ATy);
uint64_t Stride = std::abs(StrideAPtr);
const SCEVConstant *C = dyn_cast<SCEVConstant>(Dist);
if (!C) {
if (TypeByteSize == DL.getTypeAllocSize(BTy) &&
isSafeDependenceDistance(DL, *(PSE.getSE()),
*(PSE.getBackedgeTakenCount()), *Dist, Stride,
TypeByteSize))
return Dependence::NoDep;
LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
FoundNonConstantDistanceDependence = true;
return Dependence::Unknown;
}
const APInt &Val = C->getAPInt();
int64_t Distance = Val.getSExtValue();
// Attempt to prove strided accesses independent.
if (std::abs(Distance) > 0 && Stride > 1 && ATy == BTy &&
areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
return Dependence::NoDep;
}
// Negative distances are not plausible dependencies.
if (Val.isNegative()) {
bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
if (IsTrueDataDependence && EnableForwardingConflictDetection &&
(couldPreventStoreLoadForward(Val.abs().getZExtValue(), TypeByteSize) ||
ATy != BTy)) {
LLVM_DEBUG(dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
return Dependence::ForwardButPreventsForwarding;
}
LLVM_DEBUG(dbgs() << "LAA: Dependence is negative\n");
return Dependence::Forward;
}
// Write to the same location with the same size.
// Could be improved to assert type sizes are the same (i32 == float, etc).
if (Val == 0) {
if (ATy == BTy)
return Dependence::Forward;
LLVM_DEBUG(
dbgs() << "LAA: Zero dependence difference but different types\n");
return Dependence::Unknown;
}
assert(Val.isStrictlyPositive() && "Expect a positive value");
if (ATy != BTy) {
LLVM_DEBUG(
dbgs()
<< "LAA: ReadWrite-Write positive dependency with different types\n");
return Dependence::Unknown;
}
// Bail out early if passed-in parameters make vectorization not feasible.
unsigned ForcedFactor = (VectorizerParams::VectorizationFactor ?
VectorizerParams::VectorizationFactor : 1);
unsigned ForcedUnroll = (VectorizerParams::VectorizationInterleave ?
VectorizerParams::VectorizationInterleave : 1);
// The minimum number of iterations for a vectorized/unrolled version.
unsigned MinNumIter = std::max(ForcedFactor * ForcedUnroll, 2U);
// It's not vectorizable if the distance is smaller than the minimum distance
// needed for a vectroized/unrolled version. Vectorizing one iteration in
// front needs TypeByteSize * Stride. Vectorizing the last iteration needs
// TypeByteSize (No need to plus the last gap distance).
//
// E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
// foo(int *A) {
// int *B = (int *)((char *)A + 14);
// for (i = 0 ; i < 1024 ; i += 2)
// B[i] = A[i] + 1;
// }
//
// Two accesses in memory (stride is 2):
// | A[0] | | A[2] | | A[4] | | A[6] | |
// | B[0] | | B[2] | | B[4] |
//
// Distance needs for vectorizing iterations except the last iteration:
// 4 * 2 * (MinNumIter - 1). Distance needs for the last iteration: 4.
// So the minimum distance needed is: 4 * 2 * (MinNumIter - 1) + 4.
//
// If MinNumIter is 2, it is vectorizable as the minimum distance needed is
// 12, which is less than distance.
//
// If MinNumIter is 4 (Say if a user forces the vectorization factor to be 4),
// the minimum distance needed is 28, which is greater than distance. It is
// not safe to do vectorization.
uint64_t MinDistanceNeeded =
TypeByteSize * Stride * (MinNumIter - 1) + TypeByteSize;
if (MinDistanceNeeded > static_cast<uint64_t>(Distance)) {
LLVM_DEBUG(dbgs() << "LAA: Failure because of positive distance "
<< Distance << '\n');
return Dependence::Backward;
}
// Unsafe if the minimum distance needed is greater than max safe distance.
if (MinDistanceNeeded > MaxSafeDepDistBytes) {
LLVM_DEBUG(dbgs() << "LAA: Failure because it needs at least "
<< MinDistanceNeeded << " size in bytes");
return Dependence::Backward;
}
// Positive distance bigger than max vectorization factor.
// FIXME: Should use max factor instead of max distance in bytes, which could
// not handle different types.
// E.g. Assume one char is 1 byte in memory and one int is 4 bytes.
// void foo (int *A, char *B) {
// for (unsigned i = 0; i < 1024; i++) {
// A[i+2] = A[i] + 1;
// B[i+2] = B[i] + 1;
// }
// }
//
// This case is currently unsafe according to the max safe distance. If we
// analyze the two accesses on array B, the max safe dependence distance
// is 2. Then we analyze the accesses on array A, the minimum distance needed
// is 8, which is less than 2 and forbidden vectorization, But actually
// both A and B could be vectorized by 2 iterations.
MaxSafeDepDistBytes =
std::min(static_cast<uint64_t>(Distance), MaxSafeDepDistBytes);
bool IsTrueDataDependence = (!AIsWrite && BIsWrite);
if (IsTrueDataDependence && EnableForwardingConflictDetection &&
couldPreventStoreLoadForward(Distance, TypeByteSize))
return Dependence::BackwardVectorizableButPreventsForwarding;
uint64_t MaxVF = MaxSafeDepDistBytes / (TypeByteSize * Stride);
LLVM_DEBUG(dbgs() << "LAA: Positive distance " << Val.getSExtValue()
<< " with max VF = " << MaxVF << '\n');
uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
MaxSafeRegisterWidth = std::min(MaxSafeRegisterWidth, MaxVFInBits);
return Dependence::BackwardVectorizable;
}
bool MemoryDepChecker::areDepsSafe(DepCandidates &AccessSets,
MemAccessInfoList &CheckDeps,
const ValueToValueMap &Strides) {
MaxSafeDepDistBytes = -1;
SmallPtrSet<MemAccessInfo, 8> Visited;
for (MemAccessInfo CurAccess : CheckDeps) {
if (Visited.count(CurAccess))
continue;
// Get the relevant memory access set.
EquivalenceClasses<MemAccessInfo>::iterator I =
AccessSets.findValue(AccessSets.getLeaderValue(CurAccess));
// Check accesses within this set.
EquivalenceClasses<MemAccessInfo>::member_iterator AI =
AccessSets.member_begin(I);
EquivalenceClasses<MemAccessInfo>::member_iterator AE =
AccessSets.member_end();
// Check every access pair.
while (AI != AE) {
Visited.insert(*AI);
bool AIIsWrite = AI->getInt();
// Check loads only against next equivalent class, but stores also against
// other stores in the same equivalence class - to the same address.
EquivalenceClasses<MemAccessInfo>::member_iterator OI =
(AIIsWrite ? AI : std::next(AI));
while (OI != AE) {
// Check every accessing instruction pair in program order.
for (std::vector<unsigned>::iterator I1 = Accesses[*AI].begin(),
I1E = Accesses[*AI].end(); I1 != I1E; ++I1)
// Scan all accesses of another equivalence class, but only the next
// accesses of the same equivalent class.
for (std::vector<unsigned>::iterator
I2 = (OI == AI ? std::next(I1) : Accesses[*OI].begin()),
I2E = (OI == AI ? I1E : Accesses[*OI].end());
I2 != I2E; ++I2) {
auto A = std::make_pair(&*AI, *I1);
auto B = std::make_pair(&*OI, *I2);
assert(*I1 != *I2);
if (*I1 > *I2)
std::swap(A, B);
Dependence::DepType Type =
isDependent(*A.first, A.second, *B.first, B.second, Strides);
mergeInStatus(Dependence::isSafeForVectorization(Type));
// Gather dependences unless we accumulated MaxDependences
// dependences. In that case return as soon as we find the first
// unsafe dependence. This puts a limit on this quadratic
// algorithm.
if (RecordDependences) {
if (Type != Dependence::NoDep)
Dependences.push_back(Dependence(A.second, B.second, Type));
if (Dependences.size() >= MaxDependences) {
RecordDependences = false;
Dependences.clear();
LLVM_DEBUG(dbgs()
<< "Too many dependences, stopped recording\n");
}
}
if (!RecordDependences && !isSafeForVectorization())
return false;
}
++OI;
}
AI++;
}
}
LLVM_DEBUG(dbgs() << "Total Dependences: " << Dependences.size() << "\n");
return isSafeForVectorization();
}
SmallVector<Instruction *, 4>
MemoryDepChecker::getInstructionsForAccess(Value *Ptr, bool isWrite) const {
MemAccessInfo Access(Ptr, isWrite);
auto &IndexVector = Accesses.find(Access)->second;
SmallVector<Instruction *, 4> Insts;
transform(IndexVector,
std::back_inserter(Insts),
[&](unsigned Idx) { return this->InstMap[Idx]; });
return Insts;
}
const char *MemoryDepChecker::Dependence::DepName[] = {
"NoDep", "Unknown", "Forward", "ForwardButPreventsForwarding", "Backward",
"BackwardVectorizable", "BackwardVectorizableButPreventsForwarding"};
void MemoryDepChecker::Dependence::print(
raw_ostream &OS, unsigned Depth,
const SmallVectorImpl<Instruction *> &Instrs) const {
OS.indent(Depth) << DepName[Type] << ":\n";
OS.indent(Depth + 2) << *Instrs[Source] << " -> \n";
OS.indent(Depth + 2) << *Instrs[Destination] << "\n";
}
bool LoopAccessInfo::canAnalyzeLoop() {
// We need to have a loop header.
LLVM_DEBUG(dbgs() << "LAA: Found a loop in "
<< TheLoop->getHeader()->getParent()->getName() << ": "
<< TheLoop->getHeader()->getName() << '\n');
// We can only analyze innermost loops.
if (!TheLoop->empty()) {
LLVM_DEBUG(dbgs() << "LAA: loop is not the innermost loop\n");
recordAnalysis("NotInnerMostLoop") << "loop is not the innermost loop";
return false;
}
// We must have a single backedge.
if (TheLoop->getNumBackEdges() != 1) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// We must have a single exiting block.
if (!TheLoop->getExitingBlock()) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// We only handle bottom-tested loops, i.e. loop in which the condition is
// checked at the end of each iteration. With that we can assume that all
// instructions in the loop are executed the same number of times.
if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
LLVM_DEBUG(
dbgs() << "LAA: loop control flow is not understood by analyzer\n");
recordAnalysis("CFGNotUnderstood")
<< "loop control flow is not understood by analyzer";
return false;
}
// ScalarEvolution needs to be able to find the exit count.
const SCEV *ExitCount = PSE->getBackedgeTakenCount();
if (ExitCount == PSE->getSE()->getCouldNotCompute()) {
recordAnalysis("CantComputeNumberOfIterations")
<< "could not determine number of loop iterations";
LLVM_DEBUG(dbgs() << "LAA: SCEV could not compute the loop exit count.\n");
return false;
}
return true;
}
void LoopAccessInfo::analyzeLoop(AliasAnalysis *AA, LoopInfo *LI,
const TargetLibraryInfo *TLI,
DominatorTree *DT) {
typedef SmallPtrSet<Value*, 16> ValueSet;
// Holds the Load and Store instructions.
SmallVector<LoadInst *, 16> Loads;
SmallVector<StoreInst *, 16> Stores;
// Holds all the different accesses in the loop.
unsigned NumReads = 0;
unsigned NumReadWrites = 0;
bool HasComplexMemInst = false;
// A runtime check is only legal to insert if there are no convergent calls.
HasConvergentOp = false;
PtrRtChecking->Pointers.clear();
PtrRtChecking->Need = false;
const bool IsAnnotatedParallel = TheLoop->isAnnotatedParallel();
// For each block.
for (BasicBlock *BB : TheLoop->blocks()) {
// Scan the BB and collect legal loads and stores. Also detect any
// convergent instructions.
for (Instruction &I : *BB) {
if (auto *Call = dyn_cast<CallBase>(&I)) {
if (Call->isConvergent())
HasConvergentOp = true;
}
// With both a non-vectorizable memory instruction and a convergent
// operation, found in this loop, no reason to continue the search.
if (HasComplexMemInst && HasConvergentOp) {
CanVecMem = false;
return;
}
// Avoid hitting recordAnalysis multiple times.
if (HasComplexMemInst)
continue;
// If this is a load, save it. If this instruction can read from memory
// but is not a load, then we quit. Notice that we don't handle function
// calls that read or write.
if (I.mayReadFromMemory()) {
// Many math library functions read the rounding mode. We will only
// vectorize a loop if it contains known function calls that don't set
// the flag. Therefore, it is safe to ignore this read from memory.
auto *Call = dyn_cast<CallInst>(&I);
if (Call && getVectorIntrinsicIDForCall(Call, TLI))
continue;
// If the function has an explicit vectorized counterpart, we can safely
// assume that it can be vectorized.
if (Call && !Call->isNoBuiltin() && Call->getCalledFunction() &&
!VFDatabase::getMappings(*Call).empty())
continue;
auto *Ld = dyn_cast<LoadInst>(&I);
if (!Ld) {
recordAnalysis("CantVectorizeInstruction", Ld)
<< "instruction cannot be vectorized";
HasComplexMemInst = true;
continue;
}
if (!Ld->isSimple() && !IsAnnotatedParallel) {
recordAnalysis("NonSimpleLoad", Ld)
<< "read with atomic ordering or volatile read";
LLVM_DEBUG(dbgs() << "LAA: Found a non-simple load.\n");
HasComplexMemInst = true;
continue;
}
NumLoads++;
Loads.push_back(Ld);
DepChecker->addAccess(Ld);
if (EnableMemAccessVersioning)
collectStridedAccess(Ld);
continue;
}
// Save 'store' instructions. Abort if other instructions write to memory.
if (I.mayWriteToMemory()) {
auto *St = dyn_cast<StoreInst>(&I);
if (!St) {
recordAnalysis("CantVectorizeInstruction", St)
<< "instruction cannot be vectorized";
HasComplexMemInst = true;
continue;
}
if (!St->isSimple() && !IsAnnotatedParallel) {
recordAnalysis("NonSimpleStore", St)
<< "write with atomic ordering or volatile write";
LLVM_DEBUG(dbgs() << "LAA: Found a non-simple store.\n");
HasComplexMemInst = true;
continue;
}
NumStores++;
Stores.push_back(St);
DepChecker->addAccess(St);
if (EnableMemAccessVersioning)
collectStridedAccess(St);
}
} // Next instr.
} // Next block.
if (HasComplexMemInst) {
CanVecMem = false;
return;
}
// Now we have two lists that hold the loads and the stores.
// Next, we find the pointers that they use.
// Check if we see any stores. If there are no stores, then we don't
// care if the pointers are *restrict*.
if (!Stores.size()) {
LLVM_DEBUG(dbgs() << "LAA: Found a read-only loop!\n");
CanVecMem = true;
return;
}
MemoryDepChecker::DepCandidates DependentAccesses;
AccessAnalysis Accesses(TheLoop->getHeader()->getModule()->getDataLayout(),
TheLoop, AA, LI, DependentAccesses, *PSE);
// Holds the analyzed pointers. We don't want to call GetUnderlyingObjects
// multiple times on the same object. If the ptr is accessed twice, once
// for read and once for write, it will only appear once (on the write
// list). This is okay, since we are going to check for conflicts between
// writes and between reads and writes, but not between reads and reads.
ValueSet Seen;
// Record uniform store addresses to identify if we have multiple stores
// to the same address.
ValueSet UniformStores;
for (StoreInst *ST : Stores) {
Value *Ptr = ST->getPointerOperand();
if (isUniform(Ptr))
HasDependenceInvolvingLoopInvariantAddress |=
!UniformStores.insert(Ptr).second;
// If we did *not* see this pointer before, insert it to the read-write
// list. At this phase it is only a 'write' list.
if (Seen.insert(Ptr).second) {
++NumReadWrites;
MemoryLocation Loc = MemoryLocation::get(ST);
// The TBAA metadata could have a control dependency on the predication
// condition, so we cannot rely on it when determining whether or not we
// need runtime pointer checks.
if (blockNeedsPredication(ST->getParent(), TheLoop, DT))
Loc.AATags.TBAA = nullptr;
Accesses.addStore(Loc);
}
}
if (IsAnnotatedParallel) {
LLVM_DEBUG(
dbgs() << "LAA: A loop annotated parallel, ignore memory dependency "
<< "checks.\n");
CanVecMem = true;
return;
}
for (LoadInst *LD : Loads) {
Value *Ptr = LD->getPointerOperand();
// If we did *not* see this pointer before, insert it to the
// read list. If we *did* see it before, then it is already in
// the read-write list. This allows us to vectorize expressions
// such as A[i] += x; Because the address of A[i] is a read-write
// pointer. This only works if the index of A[i] is consecutive.
// If the address of i is unknown (for example A[B[i]]) then we may
// read a few words, modify, and write a few words, and some of the
// words may be written to the same address.
bool IsReadOnlyPtr = false;
if (Seen.insert(Ptr).second ||
!getPtrStride(*PSE, Ptr, TheLoop, SymbolicStrides)) {
++NumReads;
IsReadOnlyPtr = true;
}
// See if there is an unsafe dependency between a load to a uniform address and
// store to the same uniform address.
if (UniformStores.count(Ptr)) {
LLVM_DEBUG(dbgs() << "LAA: Found an unsafe dependency between a uniform "
"load and uniform store to the same address!\n");
HasDependenceInvolvingLoopInvariantAddress = true;
}
MemoryLocation Loc = MemoryLocation::get(LD);
// The TBAA metadata could have a control dependency on the predication
// condition, so we cannot rely on it when determining whether or not we
// need runtime pointer checks.
if (blockNeedsPredication(LD->getParent(), TheLoop, DT))
Loc.AATags.TBAA = nullptr;
Accesses.addLoad(Loc, IsReadOnlyPtr);
}
// If we write (or read-write) to a single destination and there are no
// other reads in this loop then is it safe to vectorize.
if (NumReadWrites == 1 && NumReads == 0) {
LLVM_DEBUG(dbgs() << "LAA: Found a write-only loop!\n");
CanVecMem = true;
return;
}
// Build dependence sets and check whether we need a runtime pointer bounds
// check.
Accesses.buildDependenceSets();
// Find pointers with computable bounds. We are going to use this information
// to place a runtime bound check.
bool CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, PSE->getSE(),
TheLoop, SymbolicStrides);
if (!CanDoRTIfNeeded) {
recordAnalysis("CantIdentifyArrayBounds") << "cannot identify array bounds";
LLVM_DEBUG(dbgs() << "LAA: We can't vectorize because we can't find "
<< "the array bounds.\n");
CanVecMem = false;
return;
}
LLVM_DEBUG(
dbgs() << "LAA: May be able to perform a memory runtime check if needed.\n");
CanVecMem = true;
if (Accesses.isDependencyCheckNeeded()) {
LLVM_DEBUG(dbgs() << "LAA: Checking memory dependencies\n");
CanVecMem = DepChecker->areDepsSafe(
DependentAccesses, Accesses.getDependenciesToCheck(), SymbolicStrides);
MaxSafeDepDistBytes = DepChecker->getMaxSafeDepDistBytes();
if (!CanVecMem && DepChecker->shouldRetryWithRuntimeCheck()) {
LLVM_DEBUG(dbgs() << "LAA: Retrying with memory checks\n");
// Clear the dependency checks. We assume they are not needed.
Accesses.resetDepChecks(*DepChecker);
PtrRtChecking->reset();
PtrRtChecking->Need = true;
auto *SE = PSE->getSE();
CanDoRTIfNeeded = Accesses.canCheckPtrAtRT(*PtrRtChecking, SE, TheLoop,
SymbolicStrides, true);
// Check that we found the bounds for the pointer.
if (!CanDoRTIfNeeded) {
recordAnalysis("CantCheckMemDepsAtRunTime")
<< "cannot check memory dependencies at runtime";
LLVM_DEBUG(dbgs() << "LAA: Can't vectorize with memory checks\n");
CanVecMem = false;
return;
}
CanVecMem = true;
}
}
if (HasConvergentOp) {
recordAnalysis("CantInsertRuntimeCheckWithConvergent")
<< "cannot add control dependency to convergent operation";
LLVM_DEBUG(dbgs() << "LAA: We can't vectorize because a runtime check "
"would be needed with a convergent operation\n");
CanVecMem = false;
return;
}
if (CanVecMem)
LLVM_DEBUG(
dbgs() << "LAA: No unsafe dependent memory operations in loop. We"
<< (PtrRtChecking->Need ? "" : " don't")
<< " need runtime memory checks.\n");
else {
recordAnalysis("UnsafeMemDep")
<< "unsafe dependent memory operations in loop. Use "
"#pragma loop distribute(enable) to allow loop distribution "
"to attempt to isolate the offending operations into a separate "
"loop";
LLVM_DEBUG(dbgs() << "LAA: unsafe dependent memory operations in loop\n");
}
}
bool LoopAccessInfo::blockNeedsPredication(BasicBlock *BB, Loop *TheLoop,
DominatorTree *DT) {
assert(TheLoop->contains(BB) && "Unknown block used");
// Blocks that do not dominate the latch need predication.
BasicBlock* Latch = TheLoop->getLoopLatch();
return !DT->dominates(BB, Latch);
}
OptimizationRemarkAnalysis &LoopAccessInfo::recordAnalysis(StringRef RemarkName,
Instruction *I) {
assert(!Report && "Multiple reports generated");
Value *CodeRegion = TheLoop->getHeader();
DebugLoc DL = TheLoop->getStartLoc();
if (I) {
CodeRegion = I->getParent();
// If there is no debug location attached to the instruction, revert back to
// using the loop's.
if (I->getDebugLoc())
DL = I->getDebugLoc();
}
Report = std::make_unique<OptimizationRemarkAnalysis>(DEBUG_TYPE, RemarkName, DL,
CodeRegion);
return *Report;
}
bool LoopAccessInfo::isUniform(Value *V) const {
auto *SE = PSE->getSE();
// Since we rely on SCEV for uniformity, if the type is not SCEVable, it is
// never considered uniform.
// TODO: Is this really what we want? Even without FP SCEV, we may want some
// trivially loop-invariant FP values to be considered uniform.
if (!SE->isSCEVable(V->getType()))
return false;
return (SE->isLoopInvariant(SE->getSCEV(V), TheLoop));
}
// FIXME: this function is currently a duplicate of the one in
// LoopVectorize.cpp.
static Instruction *getFirstInst(Instruction *FirstInst, Value *V,
Instruction *Loc) {
if (FirstInst)
return FirstInst;
if (Instruction *I = dyn_cast<Instruction>(V))
return I->getParent() == Loc->getParent() ? I : nullptr;
return nullptr;
}
namespace {
/// IR Values for the lower and upper bounds of a pointer evolution. We
/// need to use value-handles because SCEV expansion can invalidate previously
/// expanded values. Thus expansion of a pointer can invalidate the bounds for
/// a previous one.
struct PointerBounds {
TrackingVH<Value> Start;
TrackingVH<Value> End;
};
} // end anonymous namespace
/// Expand code for the lower and upper bound of the pointer group \p CG
/// in \p TheLoop. \return the values for the bounds.
static PointerBounds
expandBounds(const RuntimePointerChecking::CheckingPtrGroup *CG, Loop *TheLoop,
Instruction *Loc, SCEVExpander &Exp, ScalarEvolution *SE,
const RuntimePointerChecking &PtrRtChecking) {
Value *Ptr = PtrRtChecking.Pointers[CG->Members[0]].PointerValue;
const SCEV *Sc = SE->getSCEV(Ptr);
unsigned AS = Ptr->getType()->getPointerAddressSpace();
LLVMContext &Ctx = Loc->getContext();
// Use this type for pointer arithmetic.
Type *PtrArithTy = Type::getInt8PtrTy(Ctx, AS);
if (SE->isLoopInvariant(Sc, TheLoop)) {
LLVM_DEBUG(dbgs() << "LAA: Adding RT check for a loop invariant ptr:"
<< *Ptr << "\n");
// Ptr could be in the loop body. If so, expand a new one at the correct
// location.
Instruction *Inst = dyn_cast<Instruction>(Ptr);
Value *NewPtr = (Inst && TheLoop->contains(Inst))
? Exp.expandCodeFor(Sc, PtrArithTy, Loc)
: Ptr;
// We must return a half-open range, which means incrementing Sc.
const SCEV *ScPlusOne = SE->getAddExpr(Sc, SE->getOne(PtrArithTy));
Value *NewPtrPlusOne = Exp.expandCodeFor(ScPlusOne, PtrArithTy, Loc);
return {NewPtr, NewPtrPlusOne};
} else {
Value *Start = nullptr, *End = nullptr;
LLVM_DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
Start = Exp.expandCodeFor(CG->Low, PtrArithTy, Loc);
End = Exp.expandCodeFor(CG->High, PtrArithTy, Loc);
LLVM_DEBUG(dbgs() << "Start: " << *CG->Low << " End: " << *CG->High
<< "\n");
return {Start, End};
}
}
/// Turns a collection of checks into a collection of expanded upper and
/// lower bounds for both pointers in the check.
static SmallVector<std::pair<PointerBounds, PointerBounds>, 4> expandBounds(
const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks,
Loop *L, Instruction *Loc, ScalarEvolution *SE, SCEVExpander &Exp,
const RuntimePointerChecking &PtrRtChecking) {
SmallVector<std::pair<PointerBounds, PointerBounds>, 4> ChecksWithBounds;
// Here we're relying on the SCEV Expander's cache to only emit code for the
// same bounds once.
transform(
PointerChecks, std::back_inserter(ChecksWithBounds),
[&](const RuntimePointerChecking::PointerCheck &Check) {
PointerBounds
First = expandBounds(Check.first, L, Loc, Exp, SE, PtrRtChecking),
Second = expandBounds(Check.second, L, Loc, Exp, SE, PtrRtChecking);
return std::make_pair(First, Second);
});
return ChecksWithBounds;
}
std::pair<Instruction *, Instruction *> LoopAccessInfo::addRuntimeChecks(
Instruction *Loc,
const SmallVectorImpl<RuntimePointerChecking::PointerCheck> &PointerChecks)
const {
const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
auto *SE = PSE->getSE();
SCEVExpander Exp(*SE, DL, "induction");
auto ExpandedChecks =
expandBounds(PointerChecks, TheLoop, Loc, SE, Exp, *PtrRtChecking);
LLVMContext &Ctx = Loc->getContext();
Instruction *FirstInst = nullptr;
IRBuilder<> ChkBuilder(Loc);
// Our instructions might fold to a constant.
Value *MemoryRuntimeCheck = nullptr;
for (const auto &Check : ExpandedChecks) {
const PointerBounds &A = Check.first, &B = Check.second;
// Check if two pointers (A and B) conflict where conflict is computed as:
// start(A) <= end(B) && start(B) <= end(A)
unsigned AS0 = A.Start->getType()->getPointerAddressSpace();
unsigned AS1 = B.Start->getType()->getPointerAddressSpace();
assert((AS0 == B.End->getType()->getPointerAddressSpace()) &&
(AS1 == A.End->getType()->getPointerAddressSpace()) &&
"Trying to bounds check pointers with different address spaces");
Type *PtrArithTy0 = Type::getInt8PtrTy(Ctx, AS0);
Type *PtrArithTy1 = Type::getInt8PtrTy(Ctx, AS1);
Value *Start0 = ChkBuilder.CreateBitCast(A.Start, PtrArithTy0, "bc");
Value *Start1 = ChkBuilder.CreateBitCast(B.Start, PtrArithTy1, "bc");
Value *End0 = ChkBuilder.CreateBitCast(A.End, PtrArithTy1, "bc");
Value *End1 = ChkBuilder.CreateBitCast(B.End, PtrArithTy0, "bc");
// [A|B].Start points to the first accessed byte under base [A|B].
// [A|B].End points to the last accessed byte, plus one.
// There is no conflict when the intervals are disjoint:
// NoConflict = (B.Start >= A.End) || (A.Start >= B.End)
//
// bound0 = (B.Start < A.End)
// bound1 = (A.Start < B.End)
// IsConflict = bound0 & bound1
Value *Cmp0 = ChkBuilder.CreateICmpULT(Start0, End1, "bound0");
FirstInst = getFirstInst(FirstInst, Cmp0, Loc);
Value *Cmp1 = ChkBuilder.CreateICmpULT(Start1, End0, "bound1");
FirstInst = getFirstInst(FirstInst, Cmp1, Loc);
Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
if (MemoryRuntimeCheck) {
IsConflict =
ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
FirstInst = getFirstInst(FirstInst, IsConflict, Loc);
}
MemoryRuntimeCheck = IsConflict;
}
if (!MemoryRuntimeCheck)
return std::make_pair(nullptr, nullptr);
// We have to do this trickery because the IRBuilder might fold the check to a
// constant expression in which case there is no Instruction anchored in a
// the block.
Instruction *Check = BinaryOperator::CreateAnd(MemoryRuntimeCheck,
ConstantInt::getTrue(Ctx));
ChkBuilder.Insert(Check, "memcheck.conflict");
FirstInst = getFirstInst(FirstInst, Check, Loc);
return std::make_pair(FirstInst, Check);
}
std::pair<Instruction *, Instruction *>
LoopAccessInfo::addRuntimeChecks(Instruction *Loc) const {
if (!PtrRtChecking->Need)
return std::make_pair(nullptr, nullptr);
return addRuntimeChecks(Loc, PtrRtChecking->getChecks());
}
void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
Value *Ptr = nullptr;
if (LoadInst *LI = dyn_cast<LoadInst>(MemAccess))
Ptr = LI->getPointerOperand();
else if (StoreInst *SI = dyn_cast<StoreInst>(MemAccess))
Ptr = SI->getPointerOperand();
else
return;
Value *Stride = getStrideFromPointer(Ptr, PSE->getSE(), TheLoop);
if (!Stride)
return;
LLVM_DEBUG(dbgs() << "LAA: Found a strided access that is a candidate for "
"versioning:");
LLVM_DEBUG(dbgs() << " Ptr: " << *Ptr << " Stride: " << *Stride << "\n");
// Avoid adding the "Stride == 1" predicate when we know that
// Stride >= Trip-Count. Such a predicate will effectively optimize a single
// or zero iteration loop, as Trip-Count <= Stride == 1.
//
// TODO: We are currently not making a very informed decision on when it is
// beneficial to apply stride versioning. It might make more sense that the
// users of this analysis (such as the vectorizer) will trigger it, based on
// their specific cost considerations; For example, in cases where stride
// versioning does not help resolving memory accesses/dependences, the
// vectorizer should evaluate the cost of the runtime test, and the benefit
// of various possible stride specializations, considering the alternatives
// of using gather/scatters (if available).
const SCEV *StrideExpr = PSE->getSCEV(Stride);
const SCEV *BETakenCount = PSE->getBackedgeTakenCount();
// Match the types so we can compare the stride and the BETakenCount.
// The Stride can be positive/negative, so we sign extend Stride;
// The backedgeTakenCount is non-negative, so we zero extend BETakenCount.
const DataLayout &DL = TheLoop->getHeader()->getModule()->getDataLayout();
uint64_t StrideTypeSize = DL.getTypeAllocSize(StrideExpr->getType());
uint64_t BETypeSize = DL.getTypeAllocSize(BETakenCount->getType());
const SCEV *CastedStride = StrideExpr;
const SCEV *CastedBECount = BETakenCount;
ScalarEvolution *SE = PSE->getSE();
if (BETypeSize >= StrideTypeSize)
CastedStride = SE->getNoopOrSignExtend(StrideExpr, BETakenCount->getType());
else
CastedBECount = SE->getZeroExtendExpr(BETakenCount, StrideExpr->getType());
const SCEV *StrideMinusBETaken = SE->getMinusSCEV(CastedStride, CastedBECount);
// Since TripCount == BackEdgeTakenCount + 1, checking:
// "Stride >= TripCount" is equivalent to checking:
// Stride - BETakenCount > 0
if (SE->isKnownPositive(StrideMinusBETaken)) {
LLVM_DEBUG(
dbgs() << "LAA: Stride>=TripCount; No point in versioning as the "
"Stride==1 predicate will imply that the loop executes "
"at most once.\n");
return;
}
LLVM_DEBUG(dbgs() << "LAA: Found a strided access that we can version.");
SymbolicStrides[Ptr] = Stride;
StrideSet.insert(Stride);
}
LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
const TargetLibraryInfo *TLI, AliasAnalysis *AA,
DominatorTree *DT, LoopInfo *LI)
: PSE(std::make_unique<PredicatedScalarEvolution>(*SE, *L)),
PtrRtChecking(std::make_unique<RuntimePointerChecking>(SE)),
DepChecker(std::make_unique<MemoryDepChecker>(*PSE, L)), TheLoop(L),
NumLoads(0), NumStores(0), MaxSafeDepDistBytes(-1), CanVecMem(false),
HasConvergentOp(false),
HasDependenceInvolvingLoopInvariantAddress(false) {
if (canAnalyzeLoop())
analyzeLoop(AA, LI, TLI, DT);
}
void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
if (CanVecMem) {
OS.indent(Depth) << "Memory dependences are safe";
if (MaxSafeDepDistBytes != -1ULL)
OS << " with a maximum dependence distance of " << MaxSafeDepDistBytes
<< " bytes";
if (PtrRtChecking->Need)
OS << " with run-time checks";
OS << "\n";
}
if (HasConvergentOp)
OS.indent(Depth) << "Has convergent operation in loop\n";
if (Report)
OS.indent(Depth) << "Report: " << Report->getMsg() << "\n";
if (auto *Dependences = DepChecker->getDependences()) {
OS.indent(Depth) << "Dependences:\n";
for (auto &Dep : *Dependences) {
Dep.print(OS, Depth + 2, DepChecker->getMemoryInstructions());
OS << "\n";
}
} else
OS.indent(Depth) << "Too many dependences, not recorded\n";
// List the pair of accesses need run-time checks to prove independence.
PtrRtChecking->print(OS, Depth);
OS << "\n";
OS.indent(Depth) << "Non vectorizable stores to invariant address were "
<< (HasDependenceInvolvingLoopInvariantAddress ? "" : "not ")
<< "found in loop.\n";
OS.indent(Depth) << "SCEV assumptions:\n";
PSE->getUnionPredicate().print(OS, Depth);
OS << "\n";
OS.indent(Depth) << "Expressions re-written:\n";
PSE->print(OS, Depth);
}
LoopAccessLegacyAnalysis::LoopAccessLegacyAnalysis() : FunctionPass(ID) {
initializeLoopAccessLegacyAnalysisPass(*PassRegistry::getPassRegistry());
}
const LoopAccessInfo &LoopAccessLegacyAnalysis::getInfo(Loop *L) {
auto &LAI = LoopAccessInfoMap[L];
if (!LAI)
LAI = std::make_unique<LoopAccessInfo>(L, SE, TLI, AA, DT, LI);
return *LAI.get();
}
void LoopAccessLegacyAnalysis::print(raw_ostream &OS, const Module *M) const {
LoopAccessLegacyAnalysis &LAA = *const_cast<LoopAccessLegacyAnalysis *>(this);
for (Loop *TopLevelLoop : *LI)
for (Loop *L : depth_first(TopLevelLoop)) {
OS.indent(2) << L->getHeader()->getName() << ":\n";
auto &LAI = LAA.getInfo(L);
LAI.print(OS, 4);
}
}
bool LoopAccessLegacyAnalysis::runOnFunction(Function &F) {
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
TLI = TLIP ? &TLIP->getTLI(F) : nullptr;
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
return false;
}
void LoopAccessLegacyAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.setPreservesAll();
}
char LoopAccessLegacyAnalysis::ID = 0;
static const char laa_name[] = "Loop Access Analysis";
#define LAA_NAME "loop-accesses"
INITIALIZE_PASS_BEGIN(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(LoopAccessLegacyAnalysis, LAA_NAME, laa_name, false, true)
AnalysisKey LoopAccessAnalysis::Key;
LoopAccessInfo LoopAccessAnalysis::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR) {
return LoopAccessInfo(&L, &AR.SE, &AR.TLI, &AR.AA, &AR.DT, &AR.LI);
}
namespace llvm {
Pass *createLAAPass() {
return new LoopAccessLegacyAnalysis();
}
} // end namespace llvm
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