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llvm-project/polly/lib/Transform/ForwardOpTree.cpp

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//===- ForwardOpTree.h ------------------------------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Move instructions between statements.
//
//===----------------------------------------------------------------------===//
#include "polly/ForwardOpTree.h"
#include "polly/Options.h"
#include "polly/ScopBuilder.h"
#include "polly/ScopInfo.h"
#include "polly/ScopPass.h"
#include "polly/Support/GICHelper.h"
#include "polly/Support/ISLOStream.h"
#include "polly/Support/ISLTools.h"
#include "polly/Support/VirtualInstruction.h"
#include "polly/ZoneAlgo.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "isl/ctx.h"
#include "isl/isl-noexceptions.h"
#include <cassert>
#include <memory>
#define DEBUG_TYPE "polly-optree"
using namespace llvm;
using namespace polly;
static cl::opt<bool>
AnalyzeKnown("polly-optree-analyze-known",
cl::desc("Analyze array contents for load forwarding"),
cl::cat(PollyCategory), cl::init(true), cl::Hidden);
static cl::opt<bool>
NormalizePHIs("polly-optree-normalize-phi",
cl::desc("Replace PHIs by their incoming values"),
cl::cat(PollyCategory), cl::init(false), cl::Hidden);
static cl::opt<unsigned>
MaxOps("polly-optree-max-ops",
cl::desc("Maximum number of ISL operations to invest for known "
"analysis; 0=no limit"),
cl::init(1000000), cl::cat(PollyCategory), cl::Hidden);
STATISTIC(KnownAnalyzed, "Number of successfully analyzed SCoPs");
STATISTIC(KnownOutOfQuota,
"Analyses aborted because max_operations was reached");
STATISTIC(TotalInstructionsCopied, "Number of copied instructions");
STATISTIC(TotalKnownLoadsForwarded,
"Number of forwarded loads because their value was known");
STATISTIC(TotalReloads, "Number of reloaded values");
STATISTIC(TotalReadOnlyCopied, "Number of copied read-only accesses");
STATISTIC(TotalForwardedTrees, "Number of forwarded operand trees");
STATISTIC(TotalModifiedStmts,
"Number of statements with at least one forwarded tree");
STATISTIC(ScopsModified, "Number of SCoPs with at least one forwarded tree");
STATISTIC(NumValueWrites, "Number of scalar value writes after OpTree");
STATISTIC(NumValueWritesInLoops,
"Number of scalar value writes nested in affine loops after OpTree");
STATISTIC(NumPHIWrites, "Number of scalar phi writes after OpTree");
STATISTIC(NumPHIWritesInLoops,
"Number of scalar phi writes nested in affine loops after OpTree");
STATISTIC(NumSingletonWrites, "Number of singleton writes after OpTree");
STATISTIC(NumSingletonWritesInLoops,
"Number of singleton writes nested in affine loops after OpTree");
namespace {
/// The state of whether an operand tree was/can be forwarded.
///
/// The items apply to an instructions and its operand tree with the instruction
/// as the root element. If the value in question is not an instruction in the
/// SCoP, it can be a leaf of an instruction's operand tree.
enum ForwardingDecision {
/// The root instruction or value cannot be forwarded at all.
FD_CannotForward,
/// The root instruction or value can be forwarded as a leaf of a larger
/// operand tree.
/// It does not make sense to move the value itself, it would just replace it
/// by a use of itself. For instance, a constant "5" used in a statement can
/// be forwarded, but it would just replace it by the same constant "5".
/// However, it makes sense to move as an operand of
///
/// %add = add 5, 5
///
/// where "5" is moved as part of a larger operand tree. "5" would be placed
/// (disregarding for a moment that literal constants don't have a location
/// and can be used anywhere) into the same statement as %add would.
FD_CanForwardLeaf,
/// The root instruction can be forwarded and doing so avoids a scalar
/// dependency.
///
/// This can be either because the operand tree can be moved to the target
/// statement, or a memory access is redirected to read from a different
/// location.
FD_CanForwardProfitably,
/// Used to indicate that a forwarding has be carried out successfully, and
/// the forwarded memory access can be deleted.
FD_DidForwardTree,
/// Used to indicate that a forwarding has be carried out successfully, and
/// the forwarded memory access is being reused.
FD_DidForwardLeaf,
/// A forwarding method cannot be applied to the operand tree.
/// The difference to FD_CannotForward is that there might be other methods
/// that can handle it.
/// The conditions that make an operand tree applicable must be checked even
/// with DoIt==true because a method following the one that returned
/// FD_NotApplicable might have returned FD_CanForwardTree.
FD_NotApplicable
};
/// Implementation of operand tree forwarding for a specific SCoP.
///
/// For a statement that requires a scalar value (through a value read
/// MemoryAccess), see if its operand can be moved into the statement. If so,
/// the MemoryAccess is removed and the all the operand tree instructions are
/// moved into the statement. All original instructions are left in the source
/// statements. The simplification pass can clean these up.
class ForwardOpTreeImpl : ZoneAlgorithm {
private:
/// Scope guard to limit the number of isl operations for this pass.
IslMaxOperationsGuard &MaxOpGuard;
/// How many instructions have been copied to other statements.
int NumInstructionsCopied = 0;
/// Number of loads forwarded because their value was known.
int NumKnownLoadsForwarded = 0;
/// Number of values reloaded from known array elements.
int NumReloads = 0;
/// How many read-only accesses have been copied.
int NumReadOnlyCopied = 0;
/// How many operand trees have been forwarded.
int NumForwardedTrees = 0;
/// Number of statements with at least one forwarded operand tree.
int NumModifiedStmts = 0;
/// Whether we carried out at least one change to the SCoP.
bool Modified = false;
/// Contains the zones where array elements are known to contain a specific
/// value.
/// { [Element[] -> Zone[]] -> ValInst[] }
/// @see computeKnown()
isl::union_map Known;
/// Translator for newly introduced ValInsts to already existing ValInsts such
/// that new introduced load instructions can reuse the Known analysis of its
/// original load. { ValInst[] -> ValInst[] }
isl::union_map Translator;
/// Get list of array elements that do contain the same ValInst[] at Domain[].
///
/// @param ValInst { Domain[] -> ValInst[] }
/// The values for which we search for alternative locations,
/// per statement instance.
///
/// @return { Domain[] -> Element[] }
/// For each statement instance, the array elements that contain the
/// same ValInst.
isl::union_map findSameContentElements(isl::union_map ValInst) {
assert(!ValInst.is_single_valued().is_false());
// { Domain[] }
isl::union_set Domain = ValInst.domain();
// { Domain[] -> Scatter[] }
isl::union_map Schedule = getScatterFor(Domain);
// { Element[] -> [Scatter[] -> ValInst[]] }
isl::union_map MustKnownCurried =
convertZoneToTimepoints(Known, isl::dim::in, false, true).curry();
// { [Domain[] -> ValInst[]] -> Scatter[] }
isl::union_map DomValSched = ValInst.domain_map().apply_range(Schedule);
// { [Scatter[] -> ValInst[]] -> [Domain[] -> ValInst[]] }
isl::union_map SchedValDomVal =
DomValSched.range_product(ValInst.range_map()).reverse();
// { Element[] -> [Domain[] -> ValInst[]] }
isl::union_map MustKnownInst = MustKnownCurried.apply_range(SchedValDomVal);
// { Domain[] -> Element[] }
isl::union_map MustKnownMap =
MustKnownInst.uncurry().domain().unwrap().reverse();
simplify(MustKnownMap);
return MustKnownMap;
}
/// Find a single array element for each statement instance, within a single
/// array.
///
/// @param MustKnown { Domain[] -> Element[] }
/// Set of candidate array elements.
/// @param Domain { Domain[] }
/// The statement instance for which we need elements for.
///
/// @return { Domain[] -> Element[] }
/// For each statement instance, an array element out of @p MustKnown.
/// All array elements must be in the same array (Polly does not yet
/// support reading from different accesses using the same
/// MemoryAccess). If no mapping for all of @p Domain exists, returns
/// null.
isl::map singleLocation(isl::union_map MustKnown, isl::set Domain) {
// { Domain[] -> Element[] }
isl::map Result;
// MemoryAccesses can read only elements from a single array
// (i.e. not: { Dom[0] -> A[0]; Dom[1] -> B[1] }).
// Look through all spaces until we find one that contains at least the
// wanted statement instance.s
for (isl::map Map : MustKnown.get_map_list()) {
// Get the array this is accessing.
isl::id ArrayId = Map.get_tuple_id(isl::dim::out);
ScopArrayInfo *SAI = static_cast<ScopArrayInfo *>(ArrayId.get_user());
// No support for generation of indirect array accesses.
if (SAI->getBasePtrOriginSAI())
continue;
// Determine whether this map contains all wanted values.
isl::set MapDom = Map.domain();
if (!Domain.is_subset(MapDom).is_true())
continue;
// There might be multiple array elements that contain the same value, but
// choose only one of them. lexmin is used because it returns a one-value
// mapping, we do not care about which one.
// TODO: Get the simplest access function.
Result = Map.lexmin();
break;
}
return Result;
}
public:
ForwardOpTreeImpl(Scop *S, LoopInfo *LI, IslMaxOperationsGuard &MaxOpGuard)
: ZoneAlgorithm("polly-optree", S, LI), MaxOpGuard(MaxOpGuard) {}
/// Compute the zones of known array element contents.
///
/// @return True if the computed #Known is usable.
bool computeKnownValues() {
isl::union_map MustKnown, KnownFromLoad, KnownFromInit;
// Check that nothing strange occurs.
collectCompatibleElts();
{
IslQuotaScope QuotaScope = MaxOpGuard.enter();
computeCommon();
if (NormalizePHIs)
computeNormalizedPHIs();
Known = computeKnown(true, true);
// Preexisting ValInsts use the known content analysis of themselves.
Translator = makeIdentityMap(Known.range(), false);
}
if (!Known || !Translator || !NormalizeMap) {
assert(isl_ctx_last_error(IslCtx.get()) == isl_error_quota);
Known = nullptr;
Translator = nullptr;
NormalizeMap = nullptr;
LLVM_DEBUG(dbgs() << "Known analysis exceeded max_operations\n");
return false;
}
KnownAnalyzed++;
LLVM_DEBUG(dbgs() << "All known: " << Known << "\n");
return true;
}
void printStatistics(raw_ostream &OS, int Indent = 0) {
OS.indent(Indent) << "Statistics {\n";
OS.indent(Indent + 4) << "Instructions copied: " << NumInstructionsCopied
<< '\n';
OS.indent(Indent + 4) << "Known loads forwarded: " << NumKnownLoadsForwarded
<< '\n';
OS.indent(Indent + 4) << "Reloads: " << NumReloads << '\n';
OS.indent(Indent + 4) << "Read-only accesses copied: " << NumReadOnlyCopied
<< '\n';
OS.indent(Indent + 4) << "Operand trees forwarded: " << NumForwardedTrees
<< '\n';
OS.indent(Indent + 4) << "Statements with forwarded operand trees: "
<< NumModifiedStmts << '\n';
OS.indent(Indent) << "}\n";
}
void printStatements(raw_ostream &OS, int Indent = 0) const {
OS.indent(Indent) << "After statements {\n";
for (auto &Stmt : *S) {
OS.indent(Indent + 4) << Stmt.getBaseName() << "\n";
for (auto *MA : Stmt)
MA->print(OS);
OS.indent(Indent + 12);
Stmt.printInstructions(OS);
}
OS.indent(Indent) << "}\n";
}
/// Create a new MemoryAccess of type read and MemoryKind::Array.
///
/// @param Stmt The statement in which the access occurs.
/// @param LI The instruction that does the access.
/// @param AccessRelation The array element that each statement instance
/// accesses.
///
/// @param The newly created access.
MemoryAccess *makeReadArrayAccess(ScopStmt *Stmt, LoadInst *LI,
isl::map AccessRelation) {
isl::id ArrayId = AccessRelation.get_tuple_id(isl::dim::out);
ScopArrayInfo *SAI = reinterpret_cast<ScopArrayInfo *>(ArrayId.get_user());
// Create a dummy SCEV access, to be replaced anyway.
SmallVector<const SCEV *, 4> Sizes;
Sizes.reserve(SAI->getNumberOfDimensions());
SmallVector<const SCEV *, 4> Subscripts;
Subscripts.reserve(SAI->getNumberOfDimensions());
for (unsigned i = 0; i < SAI->getNumberOfDimensions(); i += 1) {
Sizes.push_back(SAI->getDimensionSize(i));
Subscripts.push_back(nullptr);
}
MemoryAccess *Access =
new MemoryAccess(Stmt, LI, MemoryAccess::READ, SAI->getBasePtr(),
LI->getType(), true, {}, Sizes, LI, MemoryKind::Array);
S->addAccessFunction(Access);
Stmt->addAccess(Access, true);
Access->setNewAccessRelation(AccessRelation);
return Access;
}
/// Forward a load by reading from an array element that contains the same
/// value. Typically the location it was loaded from.
///
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param Inst The (possibly speculatable) instruction to forward.
/// @param UseStmt The statement that uses @p Inst.
/// @param UseLoop The loop @p Inst is used in.
/// @param DefStmt The statement @p Inst is defined in.
/// @param DefLoop The loop which contains @p Inst.
/// @param DoIt If false, only determine whether an operand tree can be
/// forwarded. If true, carry out the forwarding. Do not
/// use DoIt==true if an operand tree is not known to be
/// forwardable.
///
/// @return FD_NotApplicable if @p Inst cannot be forwarded by creating a new
/// load.
/// FD_CannotForward if the pointer operand cannot be forwarded.
/// FD_CanForwardProfitably if @p Inst is forwardable.
/// FD_DidForwardTree if @p DoIt was true.
ForwardingDecision forwardKnownLoad(ScopStmt *TargetStmt, Instruction *Inst,
ScopStmt *UseStmt, Loop *UseLoop,
ScopStmt *DefStmt, Loop *DefLoop,
bool DoIt) {
// Cannot do anything without successful known analysis.
if (Known.is_null() || Translator.is_null() ||
MaxOpGuard.hasQuotaExceeded())
return FD_NotApplicable;
LoadInst *LI = dyn_cast<LoadInst>(Inst);
if (!LI)
return FD_NotApplicable;
// If the load is already in the statement, no forwarding is necessary.
// However, it might happen that the LoadInst is already present in the
// statement's instruction list. In that case we do as follows:
// - For the evaluation (DoIt==false), we can trivially forward it as it is
// benefit of forwarding an already present instruction.
// - For the execution (DoIt==true), prepend the instruction (to make it
// available to all instructions following in the instruction list), but
// do not add another MemoryAccess.
MemoryAccess *Access = TargetStmt->getArrayAccessOrNULLFor(LI);
if (Access && !DoIt)
return FD_CanForwardProfitably;
ForwardingDecision OpDecision = forwardTree(
TargetStmt, LI->getPointerOperand(), DefStmt, DefLoop, DoIt);
switch (OpDecision) {
case FD_CannotForward:
assert(!DoIt);
return OpDecision;
case FD_CanForwardLeaf:
case FD_CanForwardProfitably:
assert(!DoIt);
break;
case FD_DidForwardLeaf:
case FD_DidForwardTree:
assert(DoIt);
break;
default:
llvm_unreachable("Shouldn't return this");
}
IslQuotaScope QuotaScope = MaxOpGuard.enter(!DoIt);
// { DomainDef[] -> ValInst[] }
isl::map ExpectedVal = makeValInst(Inst, UseStmt, UseLoop);
assert(!isNormalized(ExpectedVal).is_false() &&
"LoadInsts are always normalized");
// { DomainUse[] -> DomainTarget[] }
isl::map UseToTarget = getDefToTarget(UseStmt, TargetStmt);
// { DomainTarget[] -> ValInst[] }
isl::map TargetExpectedVal = ExpectedVal.apply_domain(UseToTarget);
isl::union_map TranslatedExpectedVal =
isl::union_map(TargetExpectedVal).apply_range(Translator);
// { DomainTarget[] -> Element[] }
isl::union_map Candidates = findSameContentElements(TranslatedExpectedVal);
isl::map SameVal = singleLocation(Candidates, getDomainFor(TargetStmt));
if (!SameVal)
return FD_NotApplicable;
if (DoIt)
TargetStmt->prependInstruction(LI);
if (!DoIt)
return FD_CanForwardProfitably;
if (Access) {
LLVM_DEBUG(
dbgs() << " forwarded known load with preexisting MemoryAccess"
<< Access << "\n");
} else {
Access = makeReadArrayAccess(TargetStmt, LI, SameVal);
LLVM_DEBUG(dbgs() << " forwarded known load with new MemoryAccess"
<< Access << "\n");
// { ValInst[] }
isl::space ValInstSpace = ExpectedVal.get_space().range();
// After adding a new load to the SCoP, also update the Known content
// about it. The new load will have a known ValInst of
// { [DomainTarget[] -> Value[]] }
// but which -- because it is a copy of it -- has same value as the
// { [DomainDef[] -> Value[]] }
// that it replicates. Instead of cloning the known content of
// [DomainDef[] -> Value[]]
// for DomainTarget[], we add a 'translator' that maps
// [DomainTarget[] -> Value[]] to [DomainDef[] -> Value[]]
// before comparing to the known content.
// TODO: 'Translator' could also be used to map PHINodes to their incoming
// ValInsts.
if (ValInstSpace.is_wrapping()) {
// { DefDomain[] -> Value[] }
isl::map ValInsts = ExpectedVal.range().unwrap();
// { DefDomain[] }
isl::set DefDomain = ValInsts.domain();
// { Value[] }
isl::space ValSpace = ValInstSpace.unwrap().range();
// { Value[] -> Value[] }
isl::map ValToVal =
isl::map::identity(ValSpace.map_from_domain_and_range(ValSpace));
// { DomainDef[] -> DomainTarget[] }
isl::map DefToTarget = getDefToTarget(DefStmt, TargetStmt);
// { [TargetDomain[] -> Value[]] -> [DefDomain[] -> Value] }
isl::map LocalTranslator = DefToTarget.reverse().product(ValToVal);
Translator = Translator.add_map(LocalTranslator);
LLVM_DEBUG(dbgs() << " local translator is " << LocalTranslator
<< "\n");
}
}
LLVM_DEBUG(dbgs() << " expected values where " << TargetExpectedVal
<< "\n");
LLVM_DEBUG(dbgs() << " candidate elements where " << Candidates
<< "\n");
assert(Access);
NumKnownLoadsForwarded++;
TotalKnownLoadsForwarded++;
return FD_DidForwardTree;
}
/// Forward a scalar by redirecting the access to an array element that stores
/// the same value.
///
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param Inst The scalar to forward.
/// @param UseStmt The statement that uses @p Inst.
/// @param UseLoop The loop @p Inst is used in.
/// @param DefStmt The statement @p Inst is defined in.
/// @param DefLoop The loop which contains @p Inst.
/// @param DoIt If false, only determine whether an operand tree can be
/// forwarded. If true, carry out the forwarding. Do not
/// use DoIt==true if an operand tree is not known to be
/// forwardable.
///
/// @return FD_NotApplicable if @p Inst cannot be reloaded.
/// FD_CanForwardLeaf if @p Inst can be reloaded.
/// FD_CanForwardProfitably if @p Inst has been reloaded.
/// FD_DidForwardLeaf if @p DoIt was true.
ForwardingDecision reloadKnownContent(ScopStmt *TargetStmt, Instruction *Inst,
ScopStmt *UseStmt, Loop *UseLoop,
ScopStmt *DefStmt, Loop *DefLoop,
bool DoIt) {
// Cannot do anything without successful known analysis.
if (Known.is_null() || Translator.is_null() ||
MaxOpGuard.hasQuotaExceeded())
return FD_NotApplicable;
MemoryAccess *Access = TargetStmt->lookupInputAccessOf(Inst);
if (Access && Access->isLatestArrayKind()) {
if (DoIt)
return FD_DidForwardLeaf;
return FD_CanForwardLeaf;
}
// Don't spend too much time analyzing whether it can be reloaded. When
// carrying-out the forwarding, we cannot bail-out in the middle of the
// transformation. It also shouldn't take as long because some results are
// cached.
IslQuotaScope QuotaScope = MaxOpGuard.enter(!DoIt);
// { DomainDef[] -> ValInst[] }
isl::union_map ExpectedVal = makeNormalizedValInst(Inst, UseStmt, UseLoop);
// { DomainUse[] -> DomainTarget[] }
isl::map UseToTarget = getDefToTarget(UseStmt, TargetStmt);
// { DomainTarget[] -> ValInst[] }
isl::union_map TargetExpectedVal = ExpectedVal.apply_domain(UseToTarget);
isl::union_map TranslatedExpectedVal =
TargetExpectedVal.apply_range(Translator);
// { DomainTarget[] -> Element[] }
isl::union_map Candidates = findSameContentElements(TranslatedExpectedVal);
isl::map SameVal = singleLocation(Candidates, getDomainFor(TargetStmt));
if (!SameVal)
return FD_NotApplicable;
if (!DoIt)
return FD_CanForwardProfitably;
if (!Access)
Access = TargetStmt->ensureValueRead(Inst);
simplify(SameVal);
Access->setNewAccessRelation(SameVal);
TotalReloads++;
NumReloads++;
return FD_DidForwardLeaf;
}
/// Forwards a speculatively executable instruction.
///
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param UseInst The (possibly speculatable) instruction to forward.
/// @param DefStmt The statement @p UseInst is defined in.
/// @param DefLoop The loop which contains @p UseInst.
/// @param DoIt If false, only determine whether an operand tree can be
/// forwarded. If true, carry out the forwarding. Do not
/// use DoIt==true if an operand tree is not known to be
/// forwardable.
///
/// @return FD_NotApplicable if @p UseInst is not speculatable.
/// FD_CannotForward if one of @p UseInst's operands is not
/// forwardable.
/// FD_CanForwardTree if @p UseInst is forwardable.
/// FD_DidForward if @p DoIt was true.
ForwardingDecision forwardSpeculatable(ScopStmt *TargetStmt,
Instruction *UseInst,
ScopStmt *DefStmt, Loop *DefLoop,
bool DoIt) {
// PHIs, unless synthesizable, are not yet supported.
if (isa<PHINode>(UseInst))
return FD_NotApplicable;
// Compatible instructions must satisfy the following conditions:
// 1. Idempotent (instruction will be copied, not moved; although its
// original instance might be removed by simplification)
// 2. Not access memory (There might be memory writes between)
// 3. Not cause undefined behaviour (we might copy to a location when the
// original instruction was no executed; this is currently not possible
// because we do not forward PHINodes)
// 4. Not leak memory if executed multiple times (i.e. malloc)
//
// Instruction::mayHaveSideEffects is not sufficient because it considers
// malloc to not have side-effects. llvm::isSafeToSpeculativelyExecute is
// not sufficient because it allows memory accesses.
if (mayBeMemoryDependent(*UseInst))
return FD_NotApplicable;
if (DoIt) {
// To ensure the right order, prepend this instruction before its
// operands. This ensures that its operands are inserted before the
// instruction using them.
// TODO: The operand tree is not really a tree, but a DAG. We should be
// able to handle DAGs without duplication.
TargetStmt->prependInstruction(UseInst);
NumInstructionsCopied++;
TotalInstructionsCopied++;
}
for (Value *OpVal : UseInst->operand_values()) {
ForwardingDecision OpDecision =
forwardTree(TargetStmt, OpVal, DefStmt, DefLoop, DoIt);
switch (OpDecision) {
case FD_CannotForward:
assert(!DoIt);
return FD_CannotForward;
case FD_CanForwardLeaf:
case FD_CanForwardProfitably:
assert(!DoIt);
break;
case FD_DidForwardLeaf:
case FD_DidForwardTree:
assert(DoIt);
break;
case FD_NotApplicable:
llvm_unreachable("forwardTree should never return FD_NotApplicable");
}
}
if (DoIt)
return FD_DidForwardTree;
return FD_CanForwardProfitably;
}
/// Determines whether an operand tree can be forwarded or carries out a
/// forwarding, depending on the @p DoIt flag.
///
/// @param TargetStmt The statement the operand tree will be copied to.
/// @param UseVal The value (usually an instruction) which is root of an
/// operand tree.
/// @param UseStmt The statement that uses @p UseVal.
/// @param UseLoop The loop @p UseVal is used in.
/// @param DoIt If false, only determine whether an operand tree can be
/// forwarded. If true, carry out the forwarding. Do not
/// use DoIt==true if an operand tree is not known to be
/// forwardable.
///
/// @return If DoIt==false, return whether the operand tree can be forwarded.
/// If DoIt==true, return FD_DidForward.
ForwardingDecision forwardTree(ScopStmt *TargetStmt, Value *UseVal,
ScopStmt *UseStmt, Loop *UseLoop, bool DoIt) {
ScopStmt *DefStmt = nullptr;
Loop *DefLoop = nullptr;
// { DefDomain[] -> TargetDomain[] }
isl::map DefToTarget;
VirtualUse VUse = VirtualUse::create(UseStmt, UseLoop, UseVal, true);
switch (VUse.getKind()) {
case VirtualUse::Constant:
case VirtualUse::Block:
case VirtualUse::Hoisted:
// These can be used anywhere without special considerations.
if (DoIt)
return FD_DidForwardTree;
return FD_CanForwardLeaf;
case VirtualUse::Synthesizable: {
// ScopExpander will take care for of generating the code at the new
// location.
if (DoIt)
return FD_DidForwardTree;
// Check if the value is synthesizable at the new location as well. This
// might be possible when leaving a loop for which ScalarEvolution is
// unable to derive the exit value for.
// TODO: If there is a LCSSA PHI at the loop exit, use that one.
// If the SCEV contains a SCEVAddRecExpr, we currently depend on that we
// do not forward past its loop header. This would require us to use a
// previous loop induction variable instead the current one. We currently
// do not allow forwarding PHI nodes, thus this should never occur (the
// only exception where no phi is necessary being an unreachable loop
// without edge from the outside).
VirtualUse TargetUse = VirtualUse::create(
S, TargetStmt, TargetStmt->getSurroundingLoop(), UseVal, true);
if (TargetUse.getKind() == VirtualUse::Synthesizable)
return FD_CanForwardLeaf;
LLVM_DEBUG(
dbgs() << " Synthesizable would not be synthesizable anymore: "
<< *UseVal << "\n");
return FD_CannotForward;
}
case VirtualUse::ReadOnly:
// Note that we cannot return FD_CanForwardTree here. With a operand tree
// depth of 0, UseVal is the use in TargetStmt that we try to replace.
// With -polly-analyze-read-only-scalars=true we would ensure the
// existence of a MemoryAccess (which already exists for a leaf) and be
// removed again by tryForwardTree because it's goal is to remove this
// scalar MemoryAccess. It interprets FD_CanForwardTree as the permission
// to do so.
if (!DoIt)
return FD_CanForwardLeaf;
// If we model read-only scalars, we need to create a MemoryAccess for it.
if (ModelReadOnlyScalars)
TargetStmt->ensureValueRead(UseVal);
NumReadOnlyCopied++;
TotalReadOnlyCopied++;
return FD_DidForwardLeaf;
case VirtualUse::Intra:
// Knowing that UseStmt and DefStmt are the same statement instance, just
// reuse the information about UseStmt for DefStmt
DefStmt = UseStmt;
LLVM_FALLTHROUGH;
case VirtualUse::Inter:
Instruction *Inst = cast<Instruction>(UseVal);
if (!DefStmt) {
DefStmt = S->getStmtFor(Inst);
if (!DefStmt)
return FD_CannotForward;
}
DefLoop = LI->getLoopFor(Inst->getParent());
ForwardingDecision SpeculativeResult =
forwardSpeculatable(TargetStmt, Inst, DefStmt, DefLoop, DoIt);
if (SpeculativeResult != FD_NotApplicable)
return SpeculativeResult;
ForwardingDecision KnownResult = forwardKnownLoad(
TargetStmt, Inst, UseStmt, UseLoop, DefStmt, DefLoop, DoIt);
if (KnownResult != FD_NotApplicable)
return KnownResult;
ForwardingDecision ReloadResult = reloadKnownContent(
TargetStmt, Inst, UseStmt, UseLoop, DefStmt, DefLoop, DoIt);
if (ReloadResult != FD_NotApplicable)
return ReloadResult;
// When no method is found to forward the operand tree, we effectively
// cannot handle it.
LLVM_DEBUG(dbgs() << " Cannot forward instruction: " << *Inst << "\n");
return FD_CannotForward;
}
llvm_unreachable("Case unhandled");
}
/// Try to forward an operand tree rooted in @p RA.
bool tryForwardTree(MemoryAccess *RA) {
assert(RA->isLatestScalarKind());
LLVM_DEBUG(dbgs() << "Trying to forward operand tree " << RA << "...\n");
ScopStmt *Stmt = RA->getStatement();
Loop *InLoop = Stmt->getSurroundingLoop();
isl::map TargetToUse;
if (!Known.is_null()) {
isl::space DomSpace = Stmt->getDomainSpace();
TargetToUse =
isl::map::identity(DomSpace.map_from_domain_and_range(DomSpace));
}
ForwardingDecision Assessment =
forwardTree(Stmt, RA->getAccessValue(), Stmt, InLoop, false);
assert(Assessment != FD_DidForwardTree && Assessment != FD_DidForwardLeaf);
if (Assessment != FD_CanForwardProfitably)
return false;
ForwardingDecision Execution =
forwardTree(Stmt, RA->getAccessValue(), Stmt, InLoop, true);
assert(((Execution == FD_DidForwardTree) ||
(Execution == FD_DidForwardLeaf)) &&
"A previous positive assessment must also be executable");
if (Execution == FD_DidForwardTree)
Stmt->removeSingleMemoryAccess(RA);
return true;
}
/// Return which SCoP this instance is processing.
Scop *getScop() const { return S; }
/// Run the algorithm: Use value read accesses as operand tree roots and try
/// to forward them into the statement.
bool forwardOperandTrees() {
for (ScopStmt &Stmt : *S) {
bool StmtModified = false;
// Because we are modifying the MemoryAccess list, collect them first to
// avoid iterator invalidation.
SmallVector<MemoryAccess *, 16> Accs;
for (MemoryAccess *RA : Stmt) {
if (!RA->isRead())
continue;
if (!RA->isLatestScalarKind())
continue;
Accs.push_back(RA);
}
for (MemoryAccess *RA : Accs) {
if (tryForwardTree(RA)) {
Modified = true;
StmtModified = true;
NumForwardedTrees++;
TotalForwardedTrees++;
}
}
if (StmtModified) {
NumModifiedStmts++;
TotalModifiedStmts++;
}
}
if (Modified)
ScopsModified++;
return Modified;
}
/// Print the pass result, performed transformations and the SCoP after the
/// transformation.
void print(raw_ostream &OS, int Indent = 0) {
printStatistics(OS, Indent);
if (!Modified) {
// This line can easily be checked in regression tests.
OS << "ForwardOpTree executed, but did not modify anything\n";
return;
}
printStatements(OS, Indent);
}
};
/// Pass that redirects scalar reads to array elements that are known to contain
/// the same value.
///
/// This reduces the number of scalar accesses and therefore potentially
/// increases the freedom of the scheduler. In the ideal case, all reads of a
/// scalar definition are redirected (We currently do not care about removing
/// the write in this case). This is also useful for the main DeLICM pass as
/// there are less scalars to be mapped.
class ForwardOpTree : public ScopPass {
private:
/// The pass implementation, also holding per-scop data.
std::unique_ptr<ForwardOpTreeImpl> Impl;
public:
static char ID;
explicit ForwardOpTree() : ScopPass(ID) {}
ForwardOpTree(const ForwardOpTree &) = delete;
ForwardOpTree &operator=(const ForwardOpTree &) = delete;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequiredTransitive<ScopInfoRegionPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.setPreservesAll();
}
bool runOnScop(Scop &S) override {
// Free resources for previous SCoP's computation, if not yet done.
releaseMemory();
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
{
IslMaxOperationsGuard MaxOpGuard(S.getIslCtx().get(), MaxOps, false);
Impl = llvm::make_unique<ForwardOpTreeImpl>(&S, &LI, MaxOpGuard);
if (AnalyzeKnown) {
LLVM_DEBUG(dbgs() << "Prepare forwarders...\n");
Impl->computeKnownValues();
}
LLVM_DEBUG(dbgs() << "Forwarding operand trees...\n");
Impl->forwardOperandTrees();
if (MaxOpGuard.hasQuotaExceeded()) {
LLVM_DEBUG(dbgs() << "Not all operations completed because of "
"max_operations exceeded\n");
KnownOutOfQuota++;
}
}
LLVM_DEBUG(dbgs() << "\nFinal Scop:\n");
LLVM_DEBUG(dbgs() << S);
// Update statistics
auto ScopStats = S.getStatistics();
NumValueWrites += ScopStats.NumValueWrites;
NumValueWritesInLoops += ScopStats.NumValueWritesInLoops;
NumPHIWrites += ScopStats.NumPHIWrites;
NumPHIWritesInLoops += ScopStats.NumPHIWritesInLoops;
NumSingletonWrites += ScopStats.NumSingletonWrites;
NumSingletonWritesInLoops += ScopStats.NumSingletonWritesInLoops;
return false;
}
void printScop(raw_ostream &OS, Scop &S) const override {
if (!Impl)
return;
assert(Impl->getScop() == &S);
Impl->print(OS);
}
void releaseMemory() override { Impl.reset(); }
}; // class ForwardOpTree
char ForwardOpTree::ID;
} // namespace
ScopPass *polly::createForwardOpTreePass() { return new ForwardOpTree(); }
INITIALIZE_PASS_BEGIN(ForwardOpTree, "polly-optree",
"Polly - Forward operand tree", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(ForwardOpTree, "polly-optree",
"Polly - Forward operand tree", false, false)
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