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llvm-project/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

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//===- DeadStoreElimination.cpp - MemorySSA Backed Dead Store Elimination -===//
//
// 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 code below implements dead store elimination using MemorySSA. It uses
// the following general approach: given a MemoryDef, walk upwards to find
// clobbering MemoryDefs that may be killed by the starting def. Then check
// that there are no uses that may read the location of the original MemoryDef
// in between both MemoryDefs. A bit more concretely:
//
// For all MemoryDefs StartDef:
// 1. Get the next dominating clobbering MemoryDef (EarlierAccess) by walking
// upwards.
// 2. Check that there are no reads between EarlierAccess and the StartDef by
// checking all uses starting at EarlierAccess and walking until we see
// StartDef.
// 3. For each found CurrentDef, check that:
// 1. There are no barrier instructions between CurrentDef and StartDef (like
// throws or stores with ordering constraints).
// 2. StartDef is executed whenever CurrentDef is executed.
// 3. StartDef completely overwrites CurrentDef.
// 4. Erase CurrentDef from the function and MemorySSA.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Value.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/DebugCounter.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#include <utility>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "dse"
STATISTIC(NumRemainingStores, "Number of stores remaining after DSE");
STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther, "Number of other instrs removed");
STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
STATISTIC(NumModifiedStores, "Number of stores modified");
STATISTIC(NumCFGChecks, "Number of stores modified");
STATISTIC(NumCFGTries, "Number of stores modified");
STATISTIC(NumCFGSuccess, "Number of stores modified");
STATISTIC(NumGetDomMemoryDefPassed,
"Number of times a valid candidate is returned from getDomMemoryDef");
STATISTIC(NumDomMemDefChecks,
"Number iterations check for reads in getDomMemoryDef");
DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa",
"Controls which MemoryDefs are eliminated.");
static cl::opt<bool>
EnablePartialOverwriteTracking("enable-dse-partial-overwrite-tracking",
cl::init(true), cl::Hidden,
cl::desc("Enable partial-overwrite tracking in DSE"));
static cl::opt<bool>
EnablePartialStoreMerging("enable-dse-partial-store-merging",
cl::init(true), cl::Hidden,
cl::desc("Enable partial store merging in DSE"));
static cl::opt<unsigned>
MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden,
cl::desc("The number of memory instructions to scan for "
"dead store elimination (default = 100)"));
static cl::opt<unsigned> MemorySSAUpwardsStepLimit(
"dse-memoryssa-walklimit", cl::init(90), cl::Hidden,
cl::desc("The maximum number of steps while walking upwards to find "
"MemoryDefs that may be killed (default = 90)"));
static cl::opt<unsigned> MemorySSAPartialStoreLimit(
"dse-memoryssa-partial-store-limit", cl::init(5), cl::Hidden,
cl::desc("The maximum number candidates that only partially overwrite the "
"killing MemoryDef to consider"
" (default = 5)"));
static cl::opt<unsigned> MemorySSADefsPerBlockLimit(
"dse-memoryssa-defs-per-block-limit", cl::init(5000), cl::Hidden,
cl::desc("The number of MemoryDefs we consider as candidates to eliminated "
"other stores per basic block (default = 5000)"));
static cl::opt<unsigned> MemorySSASameBBStepCost(
"dse-memoryssa-samebb-cost", cl::init(1), cl::Hidden,
cl::desc(
"The cost of a step in the same basic block as the killing MemoryDef"
"(default = 1)"));
static cl::opt<unsigned>
MemorySSAOtherBBStepCost("dse-memoryssa-otherbb-cost", cl::init(5),
cl::Hidden,
cl::desc("The cost of a step in a different basic "
"block than the killing MemoryDef"
"(default = 5)"));
static cl::opt<unsigned> MemorySSAPathCheckLimit(
"dse-memoryssa-path-check-limit", cl::init(50), cl::Hidden,
cl::desc("The maximum number of blocks to check when trying to prove that "
"all paths to an exit go through a killing block (default = 50)"));
//===----------------------------------------------------------------------===//
// Helper functions
//===----------------------------------------------------------------------===//
using OverlapIntervalsTy = std::map<int64_t, int64_t>;
using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>;
/// Does this instruction write some memory? This only returns true for things
/// that we can analyze with other helpers below.
static bool hasAnalyzableMemoryWrite(Instruction *I,
const TargetLibraryInfo &TLI) {
if (isa<StoreInst>(I))
return true;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default:
return false;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
case Intrinsic::memcpy_inline:
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memmove_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
case Intrinsic::init_trampoline:
case Intrinsic::lifetime_end:
case Intrinsic::masked_store:
return true;
}
}
if (auto *CB = dyn_cast<CallBase>(I)) {
LibFunc LF;
if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
switch (LF) {
case LibFunc_strcpy:
case LibFunc_strncpy:
case LibFunc_strcat:
case LibFunc_strncat:
return true;
default:
return false;
}
}
}
return false;
}
/// Return a Location stored to by the specified instruction. If isRemovable
/// returns true, this function and getLocForRead completely describe the memory
/// operations for this instruction.
static MemoryLocation getLocForWrite(Instruction *Inst,
const TargetLibraryInfo &TLI) {
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
return MemoryLocation::get(SI);
// memcpy/memmove/memset.
if (auto *MI = dyn_cast<AnyMemIntrinsic>(Inst))
return MemoryLocation::getForDest(MI);
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
switch (II->getIntrinsicID()) {
default:
return MemoryLocation(); // Unhandled intrinsic.
case Intrinsic::init_trampoline:
return MemoryLocation::getAfter(II->getArgOperand(0));
case Intrinsic::masked_store:
return MemoryLocation::getForArgument(II, 1, TLI);
case Intrinsic::lifetime_end: {
uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
return MemoryLocation(II->getArgOperand(1), Len);
}
}
}
if (auto *CB = dyn_cast<CallBase>(Inst))
// All the supported TLI functions so far happen to have dest as their
// first argument.
return MemoryLocation::getAfter(CB->getArgOperand(0));
return MemoryLocation();
}
/// If the value of this instruction and the memory it writes to is unused, may
/// we delete this instruction?
static bool isRemovable(Instruction *I) {
// Don't remove volatile/atomic stores.
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: llvm_unreachable("doesn't pass 'hasAnalyzableMemoryWrite' predicate");
case Intrinsic::lifetime_end:
// Never remove dead lifetime_end's, e.g. because it is followed by a
// free.
return false;
case Intrinsic::init_trampoline:
// Always safe to remove init_trampoline.
return true;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
case Intrinsic::memcpy_inline:
// Don't remove volatile memory intrinsics.
return !cast<MemIntrinsic>(II)->isVolatile();
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memmove_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
case Intrinsic::masked_store:
return true;
}
}
// note: only get here for calls with analyzable writes - i.e. libcalls
if (auto *CB = dyn_cast<CallBase>(I))
return CB->use_empty();
return false;
}
/// Returns true if the end of this instruction can be safely shortened in
/// length.
static bool isShortenableAtTheEnd(Instruction *I) {
// Don't shorten stores for now
if (isa<StoreInst>(I))
return false;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: return false;
case Intrinsic::memset:
case Intrinsic::memcpy:
case Intrinsic::memcpy_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
// Do shorten memory intrinsics.
// FIXME: Add memmove if it's also safe to transform.
return true;
}
}
// Don't shorten libcalls calls for now.
return false;
}
/// Returns true if the beginning of this instruction can be safely shortened
/// in length.
static bool isShortenableAtTheBeginning(Instruction *I) {
// FIXME: Handle only memset for now. Supporting memcpy/memmove should be
// easily done by offsetting the source address.
return isa<AnyMemSetInst>(I);
}
static uint64_t getPointerSize(const Value *V, const DataLayout &DL,
const TargetLibraryInfo &TLI,
const Function *F) {
uint64_t Size;
ObjectSizeOpts Opts;
Opts.NullIsUnknownSize = NullPointerIsDefined(F);
if (getObjectSize(V, Size, DL, &TLI, Opts))
return Size;
return MemoryLocation::UnknownSize;
}
namespace {
enum OverwriteResult {
OW_Begin,
OW_Complete,
OW_End,
OW_PartialEarlierWithFullLater,
OW_MaybePartial,
OW_Unknown
};
} // end anonymous namespace
/// Check if two instruction are masked stores that completely
/// overwrite one another. More specifically, \p Later has to
/// overwrite \p Earlier.
static OverwriteResult isMaskedStoreOverwrite(const Instruction *Later,
const Instruction *Earlier,
BatchAAResults &AA) {
const auto *IIL = dyn_cast<IntrinsicInst>(Later);
const auto *IIE = dyn_cast<IntrinsicInst>(Earlier);
if (IIL == nullptr || IIE == nullptr)
return OW_Unknown;
if (IIL->getIntrinsicID() != Intrinsic::masked_store ||
IIE->getIntrinsicID() != Intrinsic::masked_store)
return OW_Unknown;
// Pointers.
Value *LP = IIL->getArgOperand(1)->stripPointerCasts();
Value *EP = IIE->getArgOperand(1)->stripPointerCasts();
if (LP != EP && !AA.isMustAlias(LP, EP))
return OW_Unknown;
// Masks.
// TODO: check that Later's mask is a superset of the Earlier's mask.
if (IIL->getArgOperand(3) != IIE->getArgOperand(3))
return OW_Unknown;
return OW_Complete;
}
/// Return 'OW_Complete' if a store to the 'Later' location (by \p LaterI
/// instruction) completely overwrites a store to the 'Earlier' location.
/// (by \p EarlierI instruction).
/// Return OW_MaybePartial if \p Later does not completely overwrite
/// \p Earlier, but they both write to the same underlying object. In that
/// case, use isPartialOverwrite to check if \p Later partially overwrites
/// \p Earlier. Returns 'OW_Unknown' if nothing can be determined.
static OverwriteResult
isOverwrite(const Instruction *LaterI, const Instruction *EarlierI,
const MemoryLocation &Later, const MemoryLocation &Earlier,
const DataLayout &DL, const TargetLibraryInfo &TLI,
int64_t &EarlierOff, int64_t &LaterOff, BatchAAResults &AA,
const Function *F) {
// FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
// get imprecise values here, though (except for unknown sizes).
if (!Later.Size.isPrecise() || !Earlier.Size.isPrecise()) {
// In case no constant size is known, try to an IR values for the number
// of bytes written and check if they match.
const auto *LaterMemI = dyn_cast<MemIntrinsic>(LaterI);
const auto *EarlierMemI = dyn_cast<MemIntrinsic>(EarlierI);
if (LaterMemI && EarlierMemI) {
const Value *LaterV = LaterMemI->getLength();
const Value *EarlierV = EarlierMemI->getLength();
if (LaterV == EarlierV && AA.isMustAlias(Earlier, Later))
return OW_Complete;
}
// Masked stores have imprecise locations, but we can reason about them
// to some extent.
return isMaskedStoreOverwrite(LaterI, EarlierI, AA);
}
const uint64_t LaterSize = Later.Size.getValue();
const uint64_t EarlierSize = Earlier.Size.getValue();
// Query the alias information
AliasResult AAR = AA.alias(Later, Earlier);
// If the start pointers are the same, we just have to compare sizes to see if
// the later store was larger than the earlier store.
if (AAR == AliasResult::MustAlias) {
// Make sure that the Later size is >= the Earlier size.
if (LaterSize >= EarlierSize)
return OW_Complete;
}
// If we hit a partial alias we may have a full overwrite
if (AAR == AliasResult::PartialAlias) {
int64_t Off = AA.getClobberOffset(Later, Earlier).getValueOr(0);
if (Off > 0 && (uint64_t)Off + EarlierSize <= LaterSize)
return OW_Complete;
}
// Check to see if the later store is to the entire object (either a global,
// an alloca, or a byval/inalloca argument). If so, then it clearly
// overwrites any other store to the same object.
const Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
const Value *UO1 = getUnderlyingObject(P1), *UO2 = getUnderlyingObject(P2);
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (UO1 != UO2)
return OW_Unknown;
// If the "Later" store is to a recognizable object, get its size.
uint64_t ObjectSize = getPointerSize(UO2, DL, TLI, F);
if (ObjectSize != MemoryLocation::UnknownSize)
if (ObjectSize == LaterSize && ObjectSize >= EarlierSize)
return OW_Complete;
// Okay, we have stores to two completely different pointers. Try to
// decompose the pointer into a "base + constant_offset" form. If the base
// pointers are equal, then we can reason about the two stores.
EarlierOff = 0;
LaterOff = 0;
const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
// If the base pointers still differ, we have two completely different stores.
if (BP1 != BP2)
return OW_Unknown;
// The later access completely overlaps the earlier store if and only if
// both start and end of the earlier one is "inside" the later one:
// |<->|--earlier--|<->|
// |-------later-------|
// Accesses may overlap if and only if start of one of them is "inside"
// another one:
// |<->|--earlier--|<----->|
// |-------later-------|
// OR
// |----- earlier -----|
// |<->|---later---|<----->|
//
// We have to be careful here as *Off is signed while *.Size is unsigned.
// Check if the earlier access starts "not before" the later one.
if (EarlierOff >= LaterOff) {
// If the earlier access ends "not after" the later access then the earlier
// one is completely overwritten by the later one.
if (uint64_t(EarlierOff - LaterOff) + EarlierSize <= LaterSize)
return OW_Complete;
// If start of the earlier access is "before" end of the later access then
// accesses overlap.
else if ((uint64_t)(EarlierOff - LaterOff) < LaterSize)
return OW_MaybePartial;
}
// If start of the later access is "before" end of the earlier access then
// accesses overlap.
else if ((uint64_t)(LaterOff - EarlierOff) < EarlierSize) {
return OW_MaybePartial;
}
// Can reach here only if accesses are known not to overlap. There is no
// dedicated code to indicate no overlap so signal "unknown".
return OW_Unknown;
}
/// Return 'OW_Complete' if a store to the 'Later' location completely
/// overwrites a store to the 'Earlier' location, 'OW_End' if the end of the
/// 'Earlier' location is completely overwritten by 'Later', 'OW_Begin' if the
/// beginning of the 'Earlier' location is overwritten by 'Later'.
/// 'OW_PartialEarlierWithFullLater' means that an earlier (big) store was
/// overwritten by a latter (smaller) store which doesn't write outside the big
/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
/// NOTE: This function must only be called if both \p Later and \p Earlier
/// write to the same underlying object with valid \p EarlierOff and \p
/// LaterOff.
static OverwriteResult isPartialOverwrite(const MemoryLocation &Later,
const MemoryLocation &Earlier,
int64_t EarlierOff, int64_t LaterOff,
Instruction *DepWrite,
InstOverlapIntervalsTy &IOL) {
const uint64_t LaterSize = Later.Size.getValue();
const uint64_t EarlierSize = Earlier.Size.getValue();
// We may now overlap, although the overlap is not complete. There might also
// be other incomplete overlaps, and together, they might cover the complete
// earlier write.
// Note: The correctness of this logic depends on the fact that this function
// is not even called providing DepWrite when there are any intervening reads.
if (EnablePartialOverwriteTracking &&
LaterOff < int64_t(EarlierOff + EarlierSize) &&
int64_t(LaterOff + LaterSize) >= EarlierOff) {
// Insert our part of the overlap into the map.
auto &IM = IOL[DepWrite];
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: Earlier [" << EarlierOff
<< ", " << int64_t(EarlierOff + EarlierSize)
<< ") Later [" << LaterOff << ", "
<< int64_t(LaterOff + LaterSize) << ")\n");
// Make sure that we only insert non-overlapping intervals and combine
// adjacent intervals. The intervals are stored in the map with the ending
// offset as the key (in the half-open sense) and the starting offset as
// the value.
int64_t LaterIntStart = LaterOff, LaterIntEnd = LaterOff + LaterSize;
// Find any intervals ending at, or after, LaterIntStart which start
// before LaterIntEnd.
auto ILI = IM.lower_bound(LaterIntStart);
if (ILI != IM.end() && ILI->second <= LaterIntEnd) {
// This existing interval is overlapped with the current store somewhere
// in [LaterIntStart, LaterIntEnd]. Merge them by erasing the existing
// intervals and adjusting our start and end.
LaterIntStart = std::min(LaterIntStart, ILI->second);
LaterIntEnd = std::max(LaterIntEnd, ILI->first);
ILI = IM.erase(ILI);
// Continue erasing and adjusting our end in case other previous
// intervals are also overlapped with the current store.
//
// |--- ealier 1 ---| |--- ealier 2 ---|
// |------- later---------|
//
while (ILI != IM.end() && ILI->second <= LaterIntEnd) {
assert(ILI->second > LaterIntStart && "Unexpected interval");
LaterIntEnd = std::max(LaterIntEnd, ILI->first);
ILI = IM.erase(ILI);
}
}
IM[LaterIntEnd] = LaterIntStart;
ILI = IM.begin();
if (ILI->second <= EarlierOff &&
ILI->first >= int64_t(EarlierOff + EarlierSize)) {
LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: Earlier ["
<< EarlierOff << ", "
<< int64_t(EarlierOff + EarlierSize)
<< ") Composite Later [" << ILI->second << ", "
<< ILI->first << ")\n");
++NumCompletePartials;
return OW_Complete;
}
}
// Check for an earlier store which writes to all the memory locations that
// the later store writes to.
if (EnablePartialStoreMerging && LaterOff >= EarlierOff &&
int64_t(EarlierOff + EarlierSize) > LaterOff &&
uint64_t(LaterOff - EarlierOff) + LaterSize <= EarlierSize) {
LLVM_DEBUG(dbgs() << "DSE: Partial overwrite an earlier load ["
<< EarlierOff << ", "
<< int64_t(EarlierOff + EarlierSize)
<< ") by a later store [" << LaterOff << ", "
<< int64_t(LaterOff + LaterSize) << ")\n");
// TODO: Maybe come up with a better name?
return OW_PartialEarlierWithFullLater;
}
// Another interesting case is if the later store overwrites the end of the
// earlier store.
//
// |--earlier--|
// |-- later --|
//
// In this case we may want to trim the size of earlier to avoid generating
// writes to addresses which will definitely be overwritten later
if (!EnablePartialOverwriteTracking &&
(LaterOff > EarlierOff && LaterOff < int64_t(EarlierOff + EarlierSize) &&
int64_t(LaterOff + LaterSize) >= int64_t(EarlierOff + EarlierSize)))
return OW_End;
// Finally, we also need to check if the later store overwrites the beginning
// of the earlier store.
//
// |--earlier--|
// |-- later --|
//
// In this case we may want to move the destination address and trim the size
// of earlier to avoid generating writes to addresses which will definitely
// be overwritten later.
if (!EnablePartialOverwriteTracking &&
(LaterOff <= EarlierOff && int64_t(LaterOff + LaterSize) > EarlierOff)) {
assert(int64_t(LaterOff + LaterSize) < int64_t(EarlierOff + EarlierSize) &&
"Expect to be handled as OW_Complete");
return OW_Begin;
}
// Otherwise, they don't completely overlap.
return OW_Unknown;
}
/// Returns true if the memory which is accessed by the second instruction is not
/// modified between the first and the second instruction.
/// Precondition: Second instruction must be dominated by the first
/// instruction.
static bool
memoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI,
BatchAAResults &AA, const DataLayout &DL,
DominatorTree *DT) {
// Do a backwards scan through the CFG from SecondI to FirstI. Look for
// instructions which can modify the memory location accessed by SecondI.
//
// While doing the walk keep track of the address to check. It might be
// different in different basic blocks due to PHI translation.
using BlockAddressPair = std::pair<BasicBlock *, PHITransAddr>;
SmallVector<BlockAddressPair, 16> WorkList;
// Keep track of the address we visited each block with. Bail out if we
// visit a block with different addresses.
DenseMap<BasicBlock *, Value *> Visited;
BasicBlock::iterator FirstBBI(FirstI);
++FirstBBI;
BasicBlock::iterator SecondBBI(SecondI);
BasicBlock *FirstBB = FirstI->getParent();
BasicBlock *SecondBB = SecondI->getParent();
MemoryLocation MemLoc = MemoryLocation::get(SecondI);
auto *MemLocPtr = const_cast<Value *>(MemLoc.Ptr);
// Start checking the SecondBB.
WorkList.push_back(
std::make_pair(SecondBB, PHITransAddr(MemLocPtr, DL, nullptr)));
bool isFirstBlock = true;
// Check all blocks going backward until we reach the FirstBB.
while (!WorkList.empty()) {
BlockAddressPair Current = WorkList.pop_back_val();
BasicBlock *B = Current.first;
PHITransAddr &Addr = Current.second;
Value *Ptr = Addr.getAddr();
// Ignore instructions before FirstI if this is the FirstBB.
BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
BasicBlock::iterator EI;
if (isFirstBlock) {
// Ignore instructions after SecondI if this is the first visit of SecondBB.
assert(B == SecondBB && "first block is not the store block");
EI = SecondBBI;
isFirstBlock = false;
} else {
// It's not SecondBB or (in case of a loop) the second visit of SecondBB.
// In this case we also have to look at instructions after SecondI.
EI = B->end();
}
for (; BI != EI; ++BI) {
Instruction *I = &*BI;
if (I->mayWriteToMemory() && I != SecondI)
if (isModSet(AA.getModRefInfo(I, MemLoc.getWithNewPtr(Ptr))))
return false;
}
if (B != FirstBB) {
assert(B != &FirstBB->getParent()->getEntryBlock() &&
"Should not hit the entry block because SI must be dominated by LI");
for (BasicBlock *Pred : predecessors(B)) {
PHITransAddr PredAddr = Addr;
if (PredAddr.NeedsPHITranslationFromBlock(B)) {
if (!PredAddr.IsPotentiallyPHITranslatable())
return false;
if (PredAddr.PHITranslateValue(B, Pred, DT, false))
return false;
}
Value *TranslatedPtr = PredAddr.getAddr();
auto Inserted = Visited.insert(std::make_pair(Pred, TranslatedPtr));
if (!Inserted.second) {
// We already visited this block before. If it was with a different
// address - bail out!
if (TranslatedPtr != Inserted.first->second)
return false;
// ... otherwise just skip it.
continue;
}
WorkList.push_back(std::make_pair(Pred, PredAddr));
}
}
}
return true;
}
static bool tryToShorten(Instruction *EarlierWrite, int64_t &EarlierStart,
uint64_t &EarlierSize, int64_t LaterStart,
uint64_t LaterSize, bool IsOverwriteEnd) {
auto *EarlierIntrinsic = cast<AnyMemIntrinsic>(EarlierWrite);
Align PrefAlign = EarlierIntrinsic->getDestAlign().valueOrOne();
// We assume that memet/memcpy operates in chunks of the "largest" native
// type size and aligned on the same value. That means optimal start and size
// of memset/memcpy should be modulo of preferred alignment of that type. That
// is it there is no any sense in trying to reduce store size any further
// since any "extra" stores comes for free anyway.
// On the other hand, maximum alignment we can achieve is limited by alignment
// of initial store.
// TODO: Limit maximum alignment by preferred (or abi?) alignment of the
// "largest" native type.
// Note: What is the proper way to get that value?
// Should TargetTransformInfo::getRegisterBitWidth be used or anything else?
// PrefAlign = std::min(DL.getPrefTypeAlign(LargestType), PrefAlign);
int64_t ToRemoveStart = 0;
uint64_t ToRemoveSize = 0;
// Compute start and size of the region to remove. Make sure 'PrefAlign' is
// maintained on the remaining store.
if (IsOverwriteEnd) {
// Calculate required adjustment for 'LaterStart'in order to keep remaining
// store size aligned on 'PerfAlign'.
uint64_t Off =
offsetToAlignment(uint64_t(LaterStart - EarlierStart), PrefAlign);
ToRemoveStart = LaterStart + Off;
if (EarlierSize <= uint64_t(ToRemoveStart - EarlierStart))
return false;
ToRemoveSize = EarlierSize - uint64_t(ToRemoveStart - EarlierStart);
} else {
ToRemoveStart = EarlierStart;
assert(LaterSize >= uint64_t(EarlierStart - LaterStart) &&
"Not overlapping accesses?");
ToRemoveSize = LaterSize - uint64_t(EarlierStart - LaterStart);
// Calculate required adjustment for 'ToRemoveSize'in order to keep
// start of the remaining store aligned on 'PerfAlign'.
uint64_t Off = offsetToAlignment(ToRemoveSize, PrefAlign);
if (Off != 0) {
if (ToRemoveSize <= (PrefAlign.value() - Off))
return false;
ToRemoveSize -= PrefAlign.value() - Off;
}
assert(isAligned(PrefAlign, ToRemoveSize) &&
"Should preserve selected alignment");
}
assert(ToRemoveSize > 0 && "Shouldn't reach here if nothing to remove");
assert(EarlierSize > ToRemoveSize && "Can't remove more than original size");
uint64_t NewSize = EarlierSize - ToRemoveSize;
if (auto *AMI = dyn_cast<AtomicMemIntrinsic>(EarlierWrite)) {
// When shortening an atomic memory intrinsic, the newly shortened
// length must remain an integer multiple of the element size.
const uint32_t ElementSize = AMI->getElementSizeInBytes();
if (0 != NewSize % ElementSize)
return false;
}
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
<< (IsOverwriteEnd ? "END" : "BEGIN") << ": "
<< *EarlierWrite << "\n KILLER [" << ToRemoveStart << ", "
<< int64_t(ToRemoveStart + ToRemoveSize) << ")\n");
Value *EarlierWriteLength = EarlierIntrinsic->getLength();
Value *TrimmedLength =
ConstantInt::get(EarlierWriteLength->getType(), NewSize);
EarlierIntrinsic->setLength(TrimmedLength);
EarlierIntrinsic->setDestAlignment(PrefAlign);
if (!IsOverwriteEnd) {
Value *Indices[1] = {
ConstantInt::get(EarlierWriteLength->getType(), ToRemoveSize)};
GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds(
EarlierIntrinsic->getRawDest()->getType()->getPointerElementType(),
EarlierIntrinsic->getRawDest(), Indices, "", EarlierWrite);
NewDestGEP->setDebugLoc(EarlierIntrinsic->getDebugLoc());
EarlierIntrinsic->setDest(NewDestGEP);
}
// Finally update start and size of earlier access.
if (!IsOverwriteEnd)
EarlierStart += ToRemoveSize;
EarlierSize = NewSize;
return true;
}
static bool tryToShortenEnd(Instruction *EarlierWrite,
OverlapIntervalsTy &IntervalMap,
int64_t &EarlierStart, uint64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheEnd(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = --IntervalMap.end();
int64_t LaterStart = OII->second;
uint64_t LaterSize = OII->first - LaterStart;
assert(OII->first - LaterStart >= 0 && "Size expected to be positive");
if (LaterStart > EarlierStart &&
// Note: "LaterStart - EarlierStart" is known to be positive due to
// preceding check.
(uint64_t)(LaterStart - EarlierStart) < EarlierSize &&
// Note: "EarlierSize - (uint64_t)(LaterStart - EarlierStart)" is known to
// be non negative due to preceding checks.
LaterSize >= EarlierSize - (uint64_t)(LaterStart - EarlierStart)) {
if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
LaterSize, true)) {
IntervalMap.erase(OII);
return true;
}
}
return false;
}
static bool tryToShortenBegin(Instruction *EarlierWrite,
OverlapIntervalsTy &IntervalMap,
int64_t &EarlierStart, uint64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheBeginning(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = IntervalMap.begin();
int64_t LaterStart = OII->second;
uint64_t LaterSize = OII->first - LaterStart;
assert(OII->first - LaterStart >= 0 && "Size expected to be positive");
if (LaterStart <= EarlierStart &&
// Note: "EarlierStart - LaterStart" is known to be non negative due to
// preceding check.
LaterSize > (uint64_t)(EarlierStart - LaterStart)) {
// Note: "LaterSize - (uint64_t)(EarlierStart - LaterStart)" is known to be
// positive due to preceding checks.
assert(LaterSize - (uint64_t)(EarlierStart - LaterStart) < EarlierSize &&
"Should have been handled as OW_Complete");
if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart,
LaterSize, false)) {
IntervalMap.erase(OII);
return true;
}
}
return false;
}
static bool removePartiallyOverlappedStores(const DataLayout &DL,
InstOverlapIntervalsTy &IOL,
const TargetLibraryInfo &TLI) {
bool Changed = false;
for (auto OI : IOL) {
Instruction *EarlierWrite = OI.first;
MemoryLocation Loc = getLocForWrite(EarlierWrite, TLI);
assert(isRemovable(EarlierWrite) && "Expect only removable instruction");
const Value *Ptr = Loc.Ptr->stripPointerCasts();
int64_t EarlierStart = 0;
uint64_t EarlierSize = Loc.Size.getValue();
GetPointerBaseWithConstantOffset(Ptr, EarlierStart, DL);
OverlapIntervalsTy &IntervalMap = OI.second;
Changed |=
tryToShortenEnd(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
if (IntervalMap.empty())
continue;
Changed |=
tryToShortenBegin(EarlierWrite, IntervalMap, EarlierStart, EarlierSize);
}
return Changed;
}
static Constant *tryToMergePartialOverlappingStores(
StoreInst *Earlier, StoreInst *Later, int64_t InstWriteOffset,
int64_t DepWriteOffset, const DataLayout &DL, BatchAAResults &AA,
DominatorTree *DT) {
if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
DL.typeSizeEqualsStoreSize(Earlier->getValueOperand()->getType()) &&
Later && isa<ConstantInt>(Later->getValueOperand()) &&
DL.typeSizeEqualsStoreSize(Later->getValueOperand()->getType()) &&
memoryIsNotModifiedBetween(Earlier, Later, AA, DL, DT)) {
// If the store we find is:
// a) partially overwritten by the store to 'Loc'
// b) the later store is fully contained in the earlier one and
// c) they both have a constant value
// d) none of the two stores need padding
// Merge the two stores, replacing the earlier store's value with a
// merge of both values.
// TODO: Deal with other constant types (vectors, etc), and probably
// some mem intrinsics (if needed)
APInt EarlierValue =
cast<ConstantInt>(Earlier->getValueOperand())->getValue();
APInt LaterValue = cast<ConstantInt>(Later->getValueOperand())->getValue();
unsigned LaterBits = LaterValue.getBitWidth();
assert(EarlierValue.getBitWidth() > LaterValue.getBitWidth());
LaterValue = LaterValue.zext(EarlierValue.getBitWidth());
// Offset of the smaller store inside the larger store
unsigned BitOffsetDiff = (InstWriteOffset - DepWriteOffset) * 8;
unsigned LShiftAmount = DL.isBigEndian() ? EarlierValue.getBitWidth() -
BitOffsetDiff - LaterBits
: BitOffsetDiff;
APInt Mask = APInt::getBitsSet(EarlierValue.getBitWidth(), LShiftAmount,
LShiftAmount + LaterBits);
// Clear the bits we'll be replacing, then OR with the smaller
// store, shifted appropriately.
APInt Merged = (EarlierValue & ~Mask) | (LaterValue << LShiftAmount);
LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n Earlier: " << *Earlier
<< "\n Later: " << *Later
<< "\n Merged Value: " << Merged << '\n');
return ConstantInt::get(Earlier->getValueOperand()->getType(), Merged);
}
return nullptr;
}
namespace {
// Returns true if \p I is an intrisnic that does not read or write memory.
bool isNoopIntrinsic(Instruction *I) {
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
case Intrinsic::lifetime_start:
case Intrinsic::lifetime_end:
case Intrinsic::invariant_end:
case Intrinsic::launder_invariant_group:
case Intrinsic::assume:
return true;
case Intrinsic::dbg_addr:
case Intrinsic::dbg_declare:
case Intrinsic::dbg_label:
case Intrinsic::dbg_value:
llvm_unreachable("Intrinsic should not be modeled in MemorySSA");
default:
return false;
}
}
return false;
}
// Check if we can ignore \p D for DSE.
bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) {
Instruction *DI = D->getMemoryInst();
// Calls that only access inaccessible memory cannot read or write any memory
// locations we consider for elimination.
if (auto *CB = dyn_cast<CallBase>(DI))
if (CB->onlyAccessesInaccessibleMemory())
return true;
// We can eliminate stores to locations not visible to the caller across
// throwing instructions.
if (DI->mayThrow() && !DefVisibleToCaller)
return true;
// We can remove the dead stores, irrespective of the fence and its ordering
// (release/acquire/seq_cst). Fences only constraints the ordering of
// already visible stores, it does not make a store visible to other
// threads. So, skipping over a fence does not change a store from being
// dead.
if (isa<FenceInst>(DI))
return true;
// Skip intrinsics that do not really read or modify memory.
if (isNoopIntrinsic(D->getMemoryInst()))
return true;
return false;
}
struct DSEState {
Function &F;
AliasAnalysis &AA;
/// The single BatchAA instance that is used to cache AA queries. It will
/// not be invalidated over the whole run. This is safe, because:
/// 1. Only memory writes are removed, so the alias cache for memory
/// locations remains valid.
/// 2. No new instructions are added (only instructions removed), so cached
/// information for a deleted value cannot be accessed by a re-used new
/// value pointer.
BatchAAResults BatchAA;
MemorySSA &MSSA;
DominatorTree &DT;
PostDominatorTree &PDT;
const TargetLibraryInfo &TLI;
const DataLayout &DL;
// All MemoryDefs that potentially could kill other MemDefs.
SmallVector<MemoryDef *, 64> MemDefs;
// Any that should be skipped as they are already deleted
SmallPtrSet<MemoryAccess *, 4> SkipStores;
// Keep track of all of the objects that are invisible to the caller before
// the function returns.
// SmallPtrSet<const Value *, 16> InvisibleToCallerBeforeRet;
DenseMap<const Value *, bool> InvisibleToCallerBeforeRet;
// Keep track of all of the objects that are invisible to the caller after
// the function returns.
DenseMap<const Value *, bool> InvisibleToCallerAfterRet;
// Keep track of blocks with throwing instructions not modeled in MemorySSA.
SmallPtrSet<BasicBlock *, 16> ThrowingBlocks;
// Post-order numbers for each basic block. Used to figure out if memory
// accesses are executed before another access.
DenseMap<BasicBlock *, unsigned> PostOrderNumbers;
/// Keep track of instructions (partly) overlapping with killing MemoryDefs per
/// basic block.
DenseMap<BasicBlock *, InstOverlapIntervalsTy> IOLs;
DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
PostDominatorTree &PDT, const TargetLibraryInfo &TLI)
: F(F), AA(AA), BatchAA(AA, /*CacheOffsets =*/true), MSSA(MSSA), DT(DT),
PDT(PDT), TLI(TLI), DL(F.getParent()->getDataLayout()) {}
static DSEState get(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
DominatorTree &DT, PostDominatorTree &PDT,
const TargetLibraryInfo &TLI) {
DSEState State(F, AA, MSSA, DT, PDT, TLI);
// Collect blocks with throwing instructions not modeled in MemorySSA and
// alloc-like objects.
unsigned PO = 0;
for (BasicBlock *BB : post_order(&F)) {
State.PostOrderNumbers[BB] = PO++;
for (Instruction &I : *BB) {
MemoryAccess *MA = MSSA.getMemoryAccess(&I);
if (I.mayThrow() && !MA)
State.ThrowingBlocks.insert(I.getParent());
auto *MD = dyn_cast_or_null<MemoryDef>(MA);
if (MD && State.MemDefs.size() < MemorySSADefsPerBlockLimit &&
(State.getLocForWriteEx(&I) || State.isMemTerminatorInst(&I)))
State.MemDefs.push_back(MD);
}
}
// Treat byval or inalloca arguments the same as Allocas, stores to them are
// dead at the end of the function.
for (Argument &AI : F.args())
if (AI.hasPassPointeeByValueCopyAttr()) {
// For byval, the caller doesn't know the address of the allocation.
if (AI.hasByValAttr())
State.InvisibleToCallerBeforeRet.insert({&AI, true});
State.InvisibleToCallerAfterRet.insert({&AI, true});
}
return State;
}
bool isInvisibleToCallerAfterRet(const Value *V) {
if (isa<AllocaInst>(V))
return true;
auto I = InvisibleToCallerAfterRet.insert({V, false});
if (I.second) {
if (!isInvisibleToCallerBeforeRet(V)) {
I.first->second = false;
} else {
auto *Inst = dyn_cast<Instruction>(V);
if (Inst && isAllocLikeFn(Inst, &TLI))
I.first->second = !PointerMayBeCaptured(V, true, false);
}
}
return I.first->second;
}
bool isInvisibleToCallerBeforeRet(const Value *V) {
if (isa<AllocaInst>(V))
return true;
auto I = InvisibleToCallerBeforeRet.insert({V, false});
if (I.second) {
auto *Inst = dyn_cast<Instruction>(V);
if (Inst && isAllocLikeFn(Inst, &TLI))
// NOTE: This could be made more precise by PointerMayBeCapturedBefore
// with the killing MemoryDef. But we refrain from doing so for now to
// limit compile-time and this does not cause any changes to the number
// of stores removed on a large test set in practice.
I.first->second = !PointerMayBeCaptured(V, false, true);
}
return I.first->second;
}
Optional<MemoryLocation> getLocForWriteEx(Instruction *I) const {
if (!I->mayWriteToMemory())
return None;
if (auto *MTI = dyn_cast<AnyMemIntrinsic>(I))
return {MemoryLocation::getForDest(MTI)};
if (auto *CB = dyn_cast<CallBase>(I)) {
// If the functions may write to memory we do not know about, bail out.
if (!CB->onlyAccessesArgMemory() &&
!CB->onlyAccessesInaccessibleMemOrArgMem())
return None;
LibFunc LF;
if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
switch (LF) {
case LibFunc_strcpy:
case LibFunc_strncpy:
case LibFunc_strcat:
case LibFunc_strncat:
return {MemoryLocation::getAfter(CB->getArgOperand(0))};
default:
break;
}
}
switch (CB->getIntrinsicID()) {
case Intrinsic::init_trampoline:
return {MemoryLocation::getAfter(CB->getArgOperand(0))};
case Intrinsic::masked_store:
return {MemoryLocation::getForArgument(CB, 1, TLI)};
default:
break;
}
return None;
}
return MemoryLocation::getOrNone(I);
}
/// Returns true if \p UseInst completely overwrites \p DefLoc
/// (stored by \p DefInst).
bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,
Instruction *UseInst) {
// UseInst has a MemoryDef associated in MemorySSA. It's possible for a
// MemoryDef to not write to memory, e.g. a volatile load is modeled as a
// MemoryDef.
if (!UseInst->mayWriteToMemory())
return false;
if (auto *CB = dyn_cast<CallBase>(UseInst))
if (CB->onlyAccessesInaccessibleMemory())
return false;
int64_t InstWriteOffset, DepWriteOffset;
if (auto CC = getLocForWriteEx(UseInst))
return isOverwrite(UseInst, DefInst, *CC, DefLoc, DL, TLI, DepWriteOffset,
InstWriteOffset, BatchAA, &F) == OW_Complete;
return false;
}
/// Returns true if \p Def is not read before returning from the function.
bool isWriteAtEndOfFunction(MemoryDef *Def) {
LLVM_DEBUG(dbgs() << " Check if def " << *Def << " ("
<< *Def->getMemoryInst()
<< ") is at the end the function \n");
auto MaybeLoc = getLocForWriteEx(Def->getMemoryInst());
if (!MaybeLoc) {
LLVM_DEBUG(dbgs() << " ... could not get location for write.\n");
return false;
}
SmallVector<MemoryAccess *, 4> WorkList;
SmallPtrSet<MemoryAccess *, 8> Visited;
auto PushMemUses = [&WorkList, &Visited](MemoryAccess *Acc) {
if (!Visited.insert(Acc).second)
return;
for (Use &U : Acc->uses())
WorkList.push_back(cast<MemoryAccess>(U.getUser()));
};
PushMemUses(Def);
for (unsigned I = 0; I < WorkList.size(); I++) {
if (WorkList.size() >= MemorySSAScanLimit) {
LLVM_DEBUG(dbgs() << " ... hit exploration limit.\n");
return false;
}
MemoryAccess *UseAccess = WorkList[I];
// Simply adding the users of MemoryPhi to the worklist is not enough,
// because we might miss read clobbers in different iterations of a loop,
// for example.
// TODO: Add support for phi translation to handle the loop case.
if (isa<MemoryPhi>(UseAccess))
return false;
// TODO: Checking for aliasing is expensive. Consider reducing the amount
// of times this is called and/or caching it.
Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
if (isReadClobber(*MaybeLoc, UseInst)) {
LLVM_DEBUG(dbgs() << " ... hit read clobber " << *UseInst << ".\n");
return false;
}
if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess))
PushMemUses(UseDef);
}
return true;
}
/// If \p I is a memory terminator like llvm.lifetime.end or free, return a
/// pair with the MemoryLocation terminated by \p I and a boolean flag
/// indicating whether \p I is a free-like call.
Optional<std::pair<MemoryLocation, bool>>
getLocForTerminator(Instruction *I) const {
uint64_t Len;
Value *Ptr;
if (match(I, m_Intrinsic<Intrinsic::lifetime_end>(m_ConstantInt(Len),
m_Value(Ptr))))
return {std::make_pair(MemoryLocation(Ptr, Len), false)};
if (auto *CB = dyn_cast<CallBase>(I)) {
if (isFreeCall(I, &TLI))
return {std::make_pair(MemoryLocation::getAfter(CB->getArgOperand(0)),
true)};
}
return None;
}
/// Returns true if \p I is a memory terminator instruction like
/// llvm.lifetime.end or free.
bool isMemTerminatorInst(Instruction *I) const {
IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
return (II && II->getIntrinsicID() == Intrinsic::lifetime_end) ||
isFreeCall(I, &TLI);
}
/// Returns true if \p MaybeTerm is a memory terminator for \p Loc from
/// instruction \p AccessI.
bool isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
Instruction *MaybeTerm) {
Optional<std::pair<MemoryLocation, bool>> MaybeTermLoc =
getLocForTerminator(MaybeTerm);
if (!MaybeTermLoc)
return false;
// If the terminator is a free-like call, all accesses to the underlying
// object can be considered terminated.
if (getUnderlyingObject(Loc.Ptr) !=
getUnderlyingObject(MaybeTermLoc->first.Ptr))
return false;
auto TermLoc = MaybeTermLoc->first;
if (MaybeTermLoc->second) {
const Value *LocUO = getUnderlyingObject(Loc.Ptr);
return BatchAA.isMustAlias(TermLoc.Ptr, LocUO);
}
int64_t InstWriteOffset, DepWriteOffset;
return isOverwrite(MaybeTerm, AccessI, TermLoc, Loc, DL, TLI,
DepWriteOffset, InstWriteOffset, BatchAA,
&F) == OW_Complete;
}
// Returns true if \p Use may read from \p DefLoc.
bool isReadClobber(const MemoryLocation &DefLoc, Instruction *UseInst) {
if (isNoopIntrinsic(UseInst))
return false;
// Monotonic or weaker atomic stores can be re-ordered and do not need to be
// treated as read clobber.
if (auto SI = dyn_cast<StoreInst>(UseInst))
return isStrongerThan(SI->getOrdering(), AtomicOrdering::Monotonic);
if (!UseInst->mayReadFromMemory())
return false;
if (auto *CB = dyn_cast<CallBase>(UseInst))
if (CB->onlyAccessesInaccessibleMemory())
return false;
// NOTE: For calls, the number of stores removed could be slightly improved
// by using AA.callCapturesBefore(UseInst, DefLoc, &DT), but that showed to
// be expensive compared to the benefits in practice. For now, avoid more
// expensive analysis to limit compile-time.
return isRefSet(BatchAA.getModRefInfo(UseInst, DefLoc));
}
/// Returns true if \p Ptr is guaranteed to be loop invariant for any possible
/// loop. In particular, this guarantees that it only references a single
/// MemoryLocation during execution of the containing function.
bool IsGuaranteedLoopInvariant(Value *Ptr) {
auto IsGuaranteedLoopInvariantBase = [this](Value *Ptr) {
Ptr = Ptr->stripPointerCasts();
if (auto *I = dyn_cast<Instruction>(Ptr)) {
if (isa<AllocaInst>(Ptr))
return true;
if (isAllocLikeFn(I, &TLI))
return true;
return false;
}
return true;
};
Ptr = Ptr->stripPointerCasts();
if (auto *I = dyn_cast<Instruction>(Ptr)) {
if (I->getParent() == &I->getFunction()->getEntryBlock()) {
return true;
}
}
if (auto *GEP = dyn_cast<GEPOperator>(Ptr)) {
return IsGuaranteedLoopInvariantBase(GEP->getPointerOperand()) &&
GEP->hasAllConstantIndices();
}
return IsGuaranteedLoopInvariantBase(Ptr);
}
// Find a MemoryDef writing to \p DefLoc and dominating \p StartAccess, with
// no read access between them or on any other path to a function exit block
// if \p DefLoc is not accessible after the function returns. If there is no
// such MemoryDef, return None. The returned value may not (completely)
// overwrite \p DefLoc. Currently we bail out when we encounter an aliasing
// MemoryUse (read).
Optional<MemoryAccess *>
getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
const MemoryLocation &DefLoc, const Value *DefUO,
unsigned &ScanLimit, unsigned &WalkerStepLimit,
bool IsMemTerm, unsigned &PartialLimit) {
if (ScanLimit == 0 || WalkerStepLimit == 0) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return None;
}
MemoryAccess *Current = StartAccess;
Instruction *KillingI = KillingDef->getMemoryInst();
bool StepAgain;
LLVM_DEBUG(dbgs() << " trying to get dominating access\n");
// Find the next clobbering Mod access for DefLoc, starting at StartAccess.
Optional<MemoryLocation> CurrentLoc;
do {
StepAgain = false;
LLVM_DEBUG({
dbgs() << " visiting " << *Current;
if (!MSSA.isLiveOnEntryDef(Current) && isa<MemoryUseOrDef>(Current))
dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst()
<< ")";
dbgs() << "\n";
});
// Reached TOP.
if (MSSA.isLiveOnEntryDef(Current)) {
LLVM_DEBUG(dbgs() << " ... found LiveOnEntryDef\n");
return None;
}
// Cost of a step. Accesses in the same block are more likely to be valid
// candidates for elimination, hence consider them cheaper.
unsigned StepCost = KillingDef->getBlock() == Current->getBlock()
? MemorySSASameBBStepCost
: MemorySSAOtherBBStepCost;
if (WalkerStepLimit <= StepCost) {
LLVM_DEBUG(dbgs() << " ... hit walker step limit\n");
return None;
}
WalkerStepLimit -= StepCost;
// Return for MemoryPhis. They cannot be eliminated directly and the
// caller is responsible for traversing them.
if (isa<MemoryPhi>(Current)) {
LLVM_DEBUG(dbgs() << " ... found MemoryPhi\n");
return Current;
}
// Below, check if CurrentDef is a valid candidate to be eliminated by
// KillingDef. If it is not, check the next candidate.
MemoryDef *CurrentDef = cast<MemoryDef>(Current);
Instruction *CurrentI = CurrentDef->getMemoryInst();
if (canSkipDef(CurrentDef, !isInvisibleToCallerBeforeRet(DefUO))) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
continue;
}
// Before we try to remove anything, check for any extra throwing
// instructions that block us from DSEing
if (mayThrowBetween(KillingI, CurrentI, DefUO)) {
LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
return None;
}
// Check for anything that looks like it will be a barrier to further
// removal
if (isDSEBarrier(DefUO, CurrentI)) {
LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
return None;
}
// If Current is known to be on path that reads DefLoc or is a read
// clobber, bail out, as the path is not profitable. We skip this check
// for intrinsic calls, because the code knows how to handle memcpy
// intrinsics.
if (!isa<IntrinsicInst>(CurrentI) && isReadClobber(DefLoc, CurrentI))
return None;
// Quick check if there are direct uses that are read-clobbers.
if (any_of(Current->uses(), [this, &DefLoc, StartAccess](Use &U) {
if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(U.getUser()))
return !MSSA.dominates(StartAccess, UseOrDef) &&
isReadClobber(DefLoc, UseOrDef->getMemoryInst());
return false;
})) {
LLVM_DEBUG(dbgs() << " ... found a read clobber\n");
return None;
}
// If Current cannot be analyzed or is not removable, check the next
// candidate.
if (!hasAnalyzableMemoryWrite(CurrentI, TLI) || !isRemovable(CurrentI)) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
continue;
}
// If Current does not have an analyzable write location, skip it
CurrentLoc = getLocForWriteEx(CurrentI);
if (!CurrentLoc) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
continue;
}
// AliasAnalysis does not account for loops. Limit elimination to
// candidates for which we can guarantee they always store to the same
// memory location and not multiple locations in a loop.
if (Current->getBlock() != KillingDef->getBlock() &&
!IsGuaranteedLoopInvariant(const_cast<Value *>(CurrentLoc->Ptr))) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
WalkerStepLimit -= 1;
continue;
}
if (IsMemTerm) {
// If the killing def is a memory terminator (e.g. lifetime.end), check
// the next candidate if the current Current does not write the same
// underlying object as the terminator.
if (!isMemTerminator(*CurrentLoc, CurrentI, KillingI)) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
}
continue;
} else {
int64_t InstWriteOffset, DepWriteOffset;
auto OR = isOverwrite(KillingI, CurrentI, DefLoc, *CurrentLoc, DL, TLI,
DepWriteOffset, InstWriteOffset, BatchAA, &F);
// If Current does not write to the same object as KillingDef, check
// the next candidate.
if (OR == OW_Unknown) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
} else if (OR == OW_MaybePartial) {
// If KillingDef only partially overwrites Current, check the next
// candidate if the partial step limit is exceeded. This aggressively
// limits the number of candidates for partial store elimination,
// which are less likely to be removable in the end.
if (PartialLimit <= 1) {
StepAgain = true;
Current = CurrentDef->getDefiningAccess();
WalkerStepLimit -= 1;
continue;
}
PartialLimit -= 1;
}
}
} while (StepAgain);
// Accesses to objects accessible after the function returns can only be
// eliminated if the access is killed along all paths to the exit. Collect
// the blocks with killing (=completely overwriting MemoryDefs) and check if
// they cover all paths from EarlierAccess to any function exit.
SmallPtrSet<Instruction *, 16> KillingDefs;
KillingDefs.insert(KillingDef->getMemoryInst());
MemoryAccess *EarlierAccess = Current;
Instruction *EarlierMemInst =
cast<MemoryDef>(EarlierAccess)->getMemoryInst();
LLVM_DEBUG(dbgs() << " Checking for reads of " << *EarlierAccess << " ("
<< *EarlierMemInst << ")\n");
SmallSetVector<MemoryAccess *, 32> WorkList;
auto PushMemUses = [&WorkList](MemoryAccess *Acc) {
for (Use &U : Acc->uses())
WorkList.insert(cast<MemoryAccess>(U.getUser()));
};
PushMemUses(EarlierAccess);
// Optimistically collect all accesses for reads. If we do not find any
// read clobbers, add them to the cache.
SmallPtrSet<MemoryAccess *, 16> KnownNoReads;
if (!EarlierMemInst->mayReadFromMemory())
KnownNoReads.insert(EarlierAccess);
// Check if EarlierDef may be read.
for (unsigned I = 0; I < WorkList.size(); I++) {
MemoryAccess *UseAccess = WorkList[I];
LLVM_DEBUG(dbgs() << " " << *UseAccess);
// Bail out if the number of accesses to check exceeds the scan limit.
if (ScanLimit < (WorkList.size() - I)) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return None;
}
--ScanLimit;
NumDomMemDefChecks++;
KnownNoReads.insert(UseAccess);
if (isa<MemoryPhi>(UseAccess)) {
if (any_of(KillingDefs, [this, UseAccess](Instruction *KI) {
return DT.properlyDominates(KI->getParent(),
UseAccess->getBlock());
})) {
LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n");
continue;
}
LLVM_DEBUG(dbgs() << "\n ... adding PHI uses\n");
PushMemUses(UseAccess);
continue;
}
Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");
if (any_of(KillingDefs, [this, UseInst](Instruction *KI) {
return DT.dominates(KI, UseInst);
})) {
LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing def\n");
continue;
}
// A memory terminator kills all preceeding MemoryDefs and all succeeding
// MemoryAccesses. We do not have to check it's users.
if (isMemTerminator(*CurrentLoc, EarlierMemInst, UseInst)) {
LLVM_DEBUG(
dbgs()
<< " ... skipping, memterminator invalidates following accesses\n");
continue;
}
if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess)->getMemoryInst())) {
LLVM_DEBUG(dbgs() << " ... adding uses of intrinsic\n");
PushMemUses(UseAccess);
continue;
}
if (UseInst->mayThrow() && !isInvisibleToCallerBeforeRet(DefUO)) {
LLVM_DEBUG(dbgs() << " ... found throwing instruction\n");
return None;
}
// Uses which may read the original MemoryDef mean we cannot eliminate the
// original MD. Stop walk.
if (isReadClobber(*CurrentLoc, UseInst)) {
LLVM_DEBUG(dbgs() << " ... found read clobber\n");
return None;
}
// For the KillingDef and EarlierAccess we only have to check if it reads
// the memory location.
// TODO: It would probably be better to check for self-reads before
// calling the function.
if (KillingDef == UseAccess || EarlierAccess == UseAccess) {
LLVM_DEBUG(dbgs() << " ... skipping killing def/dom access\n");
continue;
}
// Check all uses for MemoryDefs, except for defs completely overwriting
// the original location. Otherwise we have to check uses of *all*
// MemoryDefs we discover, including non-aliasing ones. Otherwise we might
// miss cases like the following
// 1 = Def(LoE) ; <----- EarlierDef stores [0,1]
// 2 = Def(1) ; (2, 1) = NoAlias, stores [2,3]
// Use(2) ; MayAlias 2 *and* 1, loads [0, 3].
// (The Use points to the *first* Def it may alias)
// 3 = Def(1) ; <---- Current (3, 2) = NoAlias, (3,1) = MayAlias,
// stores [0,1]
if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) {
if (isCompleteOverwrite(*CurrentLoc, EarlierMemInst, UseInst)) {
if (!isInvisibleToCallerAfterRet(DefUO) &&
UseAccess != EarlierAccess) {
BasicBlock *MaybeKillingBlock = UseInst->getParent();
if (PostOrderNumbers.find(MaybeKillingBlock)->second <
PostOrderNumbers.find(EarlierAccess->getBlock())->second) {
LLVM_DEBUG(dbgs()
<< " ... found killing def " << *UseInst << "\n");
KillingDefs.insert(UseInst);
}
}
} else
PushMemUses(UseDef);
}
}
// For accesses to locations visible after the function returns, make sure
// that the location is killed (=overwritten) along all paths from
// EarlierAccess to the exit.
if (!isInvisibleToCallerAfterRet(DefUO)) {
SmallPtrSet<BasicBlock *, 16> KillingBlocks;
for (Instruction *KD : KillingDefs)
KillingBlocks.insert(KD->getParent());
assert(!KillingBlocks.empty() &&
"Expected at least a single killing block");
// Find the common post-dominator of all killing blocks.
BasicBlock *CommonPred = *KillingBlocks.begin();
for (auto I = std::next(KillingBlocks.begin()), E = KillingBlocks.end();
I != E; I++) {
if (!CommonPred)
break;
CommonPred = PDT.findNearestCommonDominator(CommonPred, *I);
}
// If CommonPred is in the set of killing blocks, just check if it
// post-dominates EarlierAccess.
if (KillingBlocks.count(CommonPred)) {
if (PDT.dominates(CommonPred, EarlierAccess->getBlock()))
return {EarlierAccess};
return None;
}
// If the common post-dominator does not post-dominate EarlierAccess,
// there is a path from EarlierAccess to an exit not going through a
// killing block.
if (PDT.dominates(CommonPred, EarlierAccess->getBlock())) {
SetVector<BasicBlock *> WorkList;
// If CommonPred is null, there are multiple exits from the function.
// They all have to be added to the worklist.
if (CommonPred)
WorkList.insert(CommonPred);
else
for (BasicBlock *R : PDT.roots())
WorkList.insert(R);
NumCFGTries++;
// Check if all paths starting from an exit node go through one of the
// killing blocks before reaching EarlierAccess.
for (unsigned I = 0; I < WorkList.size(); I++) {
NumCFGChecks++;
BasicBlock *Current = WorkList[I];
if (KillingBlocks.count(Current))
continue;
if (Current == EarlierAccess->getBlock())
return None;
// EarlierAccess is reachable from the entry, so we don't have to
// explore unreachable blocks further.
if (!DT.isReachableFromEntry(Current))
continue;
for (BasicBlock *Pred : predecessors(Current))
WorkList.insert(Pred);
if (WorkList.size() >= MemorySSAPathCheckLimit)
return None;
}
NumCFGSuccess++;
return {EarlierAccess};
}
return None;
}
// No aliasing MemoryUses of EarlierAccess found, EarlierAccess is
// potentially dead.
return {EarlierAccess};
}
// Delete dead memory defs
void deleteDeadInstruction(Instruction *SI) {
MemorySSAUpdater Updater(&MSSA);
SmallVector<Instruction *, 32> NowDeadInsts;
NowDeadInsts.push_back(SI);
--NumFastOther;
while (!NowDeadInsts.empty()) {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
++NumFastOther;
// Try to preserve debug information attached to the dead instruction.
salvageDebugInfo(*DeadInst);
salvageKnowledge(DeadInst);
// Remove the Instruction from MSSA.
if (MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst)) {
if (MemoryDef *MD = dyn_cast<MemoryDef>(MA)) {
SkipStores.insert(MD);
}
Updater.removeMemoryAccess(MA);
}
auto I = IOLs.find(DeadInst->getParent());
if (I != IOLs.end())
I->second.erase(DeadInst);
// Remove its operands
for (Use &O : DeadInst->operands())
if (Instruction *OpI = dyn_cast<Instruction>(O)) {
O = nullptr;
if (isInstructionTriviallyDead(OpI, &TLI))
NowDeadInsts.push_back(OpI);
}
DeadInst->eraseFromParent();
}
}
// Check for any extra throws between SI and NI that block DSE. This only
// checks extra maythrows (those that aren't MemoryDef's). MemoryDef that may
// throw are handled during the walk from one def to the next.
bool mayThrowBetween(Instruction *SI, Instruction *NI,
const Value *SILocUnd) {
// First see if we can ignore it by using the fact that SI is an
// alloca/alloca like object that is not visible to the caller during
// execution of the function.
if (SILocUnd && isInvisibleToCallerBeforeRet(SILocUnd))
return false;
if (SI->getParent() == NI->getParent())
return ThrowingBlocks.count(SI->getParent());
return !ThrowingBlocks.empty();
}
// Check if \p NI acts as a DSE barrier for \p SI. The following instructions
// act as barriers:
// * A memory instruction that may throw and \p SI accesses a non-stack
// object.
// * Atomic stores stronger that monotonic.
bool isDSEBarrier(const Value *SILocUnd, Instruction *NI) {
// If NI may throw it acts as a barrier, unless we are to an alloca/alloca
// like object that does not escape.
if (NI->mayThrow() && !isInvisibleToCallerBeforeRet(SILocUnd))
return true;
// If NI is an atomic load/store stronger than monotonic, do not try to
// eliminate/reorder it.
if (NI->isAtomic()) {
if (auto *LI = dyn_cast<LoadInst>(NI))
return isStrongerThanMonotonic(LI->getOrdering());
if (auto *SI = dyn_cast<StoreInst>(NI))
return isStrongerThanMonotonic(SI->getOrdering());
if (auto *ARMW = dyn_cast<AtomicRMWInst>(NI))
return isStrongerThanMonotonic(ARMW->getOrdering());
if (auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(NI))
return isStrongerThanMonotonic(CmpXchg->getSuccessOrdering()) ||
isStrongerThanMonotonic(CmpXchg->getFailureOrdering());
llvm_unreachable("other instructions should be skipped in MemorySSA");
}
return false;
}
/// Eliminate writes to objects that are not visible in the caller and are not
/// accessed before returning from the function.
bool eliminateDeadWritesAtEndOfFunction() {
bool MadeChange = false;
LLVM_DEBUG(
dbgs()
<< "Trying to eliminate MemoryDefs at the end of the function\n");
for (int I = MemDefs.size() - 1; I >= 0; I--) {
MemoryDef *Def = MemDefs[I];
if (SkipStores.contains(Def) || !isRemovable(Def->getMemoryInst()))
continue;
Instruction *DefI = Def->getMemoryInst();
SmallVector<const Value *, 4> Pointers;
auto DefLoc = getLocForWriteEx(DefI);
if (!DefLoc)
continue;
// NOTE: Currently eliminating writes at the end of a function is limited
// to MemoryDefs with a single underlying object, to save compile-time. In
// practice it appears the case with multiple underlying objects is very
// uncommon. If it turns out to be important, we can use
// getUnderlyingObjects here instead.
const Value *UO = getUnderlyingObject(DefLoc->Ptr);
if (!UO || !isInvisibleToCallerAfterRet(UO))
continue;
if (isWriteAtEndOfFunction(Def)) {
// See through pointer-to-pointer bitcasts
LLVM_DEBUG(dbgs() << " ... MemoryDef is not accessed until the end "
"of the function\n");
deleteDeadInstruction(DefI);
++NumFastStores;
MadeChange = true;
}
}
return MadeChange;
}
/// \returns true if \p Def is a no-op store, either because it
/// directly stores back a loaded value or stores zero to a calloced object.
bool storeIsNoop(MemoryDef *Def, const MemoryLocation &DefLoc,
const Value *DefUO) {
StoreInst *Store = dyn_cast<StoreInst>(Def->getMemoryInst());
if (!Store)
return false;
if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
if (LoadI->getPointerOperand() == Store->getOperand(1)) {
// Get the defining access for the load.
auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
// Fast path: the defining accesses are the same.
if (LoadAccess == Def->getDefiningAccess())
return true;
// Look through phi accesses. Recursively scan all phi accesses by
// adding them to a worklist. Bail when we run into a memory def that
// does not match LoadAccess.
SetVector<MemoryAccess *> ToCheck;
MemoryAccess *Current =
MSSA.getWalker()->getClobberingMemoryAccess(Def);
// We don't want to bail when we run into the store memory def. But,
// the phi access may point to it. So, pretend like we've already
// checked it.
ToCheck.insert(Def);
ToCheck.insert(Current);
// Start at current (1) to simulate already having checked Def.
for (unsigned I = 1; I < ToCheck.size(); ++I) {
Current = ToCheck[I];
if (auto PhiAccess = dyn_cast<MemoryPhi>(Current)) {
// Check all the operands.
for (auto &Use : PhiAccess->incoming_values())
ToCheck.insert(cast<MemoryAccess>(&Use));
continue;
}
// If we found a memory def, bail. This happens when we have an
// unrelated write in between an otherwise noop store.
assert(isa<MemoryDef>(Current) &&
"Only MemoryDefs should reach here.");
// TODO: Skip no alias MemoryDefs that have no aliasing reads.
// We are searching for the definition of the store's destination.
// So, if that is the same definition as the load, then this is a
// noop. Otherwise, fail.
if (LoadAccess != Current)
return false;
}
return true;
}
}
Constant *StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
if (StoredConstant && StoredConstant->isNullValue()) {
auto *DefUOInst = dyn_cast<Instruction>(DefUO);
if (DefUOInst && isCallocLikeFn(DefUOInst, &TLI)) {
auto *UnderlyingDef = cast<MemoryDef>(MSSA.getMemoryAccess(DefUOInst));
// If UnderlyingDef is the clobbering access of Def, no instructions
// between them can modify the memory location.
auto *ClobberDef =
MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def);
return UnderlyingDef == ClobberDef;
}
}
return false;
}
};
bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
DominatorTree &DT, PostDominatorTree &PDT,
const TargetLibraryInfo &TLI) {
bool MadeChange = false;
DSEState State = DSEState::get(F, AA, MSSA, DT, PDT, TLI);
// For each store:
for (unsigned I = 0; I < State.MemDefs.size(); I++) {
MemoryDef *KillingDef = State.MemDefs[I];
if (State.SkipStores.count(KillingDef))
continue;
Instruction *SI = KillingDef->getMemoryInst();
Optional<MemoryLocation> MaybeSILoc;
if (State.isMemTerminatorInst(SI))
MaybeSILoc = State.getLocForTerminator(SI).map(
[](const std::pair<MemoryLocation, bool> &P) { return P.first; });
else
MaybeSILoc = State.getLocForWriteEx(SI);
if (!MaybeSILoc) {
LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
<< *SI << "\n");
continue;
}
MemoryLocation SILoc = *MaybeSILoc;
assert(SILoc.Ptr && "SILoc should not be null");
const Value *SILocUnd = getUnderlyingObject(SILoc.Ptr);
MemoryAccess *Current = KillingDef;
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
<< *Current << " (" << *SI << ")\n");
unsigned ScanLimit = MemorySSAScanLimit;
unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit;
unsigned PartialLimit = MemorySSAPartialStoreLimit;
// Worklist of MemoryAccesses that may be killed by KillingDef.
SetVector<MemoryAccess *> ToCheck;
if (SILocUnd)
ToCheck.insert(KillingDef->getDefiningAccess());
bool Shortend = false;
bool IsMemTerm = State.isMemTerminatorInst(SI);
// Check if MemoryAccesses in the worklist are killed by KillingDef.
for (unsigned I = 0; I < ToCheck.size(); I++) {
Current = ToCheck[I];
if (State.SkipStores.count(Current))
continue;
Optional<MemoryAccess *> Next = State.getDomMemoryDef(
KillingDef, Current, SILoc, SILocUnd, ScanLimit, WalkerStepLimit,
IsMemTerm, PartialLimit);
if (!Next) {
LLVM_DEBUG(dbgs() << " finished walk\n");
continue;
}
MemoryAccess *EarlierAccess = *Next;
LLVM_DEBUG(dbgs() << " Checking if we can kill " << *EarlierAccess);
if (isa<MemoryPhi>(EarlierAccess)) {
LLVM_DEBUG(dbgs() << "\n ... adding incoming values to worklist\n");
for (Value *V : cast<MemoryPhi>(EarlierAccess)->incoming_values()) {
MemoryAccess *IncomingAccess = cast<MemoryAccess>(V);
BasicBlock *IncomingBlock = IncomingAccess->getBlock();
BasicBlock *PhiBlock = EarlierAccess->getBlock();
// We only consider incoming MemoryAccesses that come before the
// MemoryPhi. Otherwise we could discover candidates that do not
// strictly dominate our starting def.
if (State.PostOrderNumbers[IncomingBlock] >
State.PostOrderNumbers[PhiBlock])
ToCheck.insert(IncomingAccess);
}
continue;
}
auto *NextDef = cast<MemoryDef>(EarlierAccess);
Instruction *NI = NextDef->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *NI << ")\n");
ToCheck.insert(NextDef->getDefiningAccess());
NumGetDomMemoryDefPassed++;
if (!DebugCounter::shouldExecute(MemorySSACounter))
continue;
MemoryLocation NILoc = *State.getLocForWriteEx(NI);
if (IsMemTerm) {
const Value *NIUnd = getUnderlyingObject(NILoc.Ptr);
if (SILocUnd != NIUnd)
continue;
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *NI
<< "\n KILLER: " << *SI << '\n');
State.deleteDeadInstruction(NI);
++NumFastStores;
MadeChange = true;
} else {
// Check if NI overwrites SI.
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR =
isOverwrite(SI, NI, SILoc, NILoc, State.DL, TLI, DepWriteOffset,
InstWriteOffset, State.BatchAA, &F);
if (OR == OW_MaybePartial) {
auto Iter = State.IOLs.insert(
std::make_pair<BasicBlock *, InstOverlapIntervalsTy>(
NI->getParent(), InstOverlapIntervalsTy()));
auto &IOL = Iter.first->second;
OR = isPartialOverwrite(SILoc, NILoc, DepWriteOffset, InstWriteOffset,
NI, IOL);
}
if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
auto *Earlier = dyn_cast<StoreInst>(NI);
auto *Later = dyn_cast<StoreInst>(SI);
// We are re-using tryToMergePartialOverlappingStores, which requires
// Earlier to domiante Later.
// TODO: implement tryToMergeParialOverlappingStores using MemorySSA.
if (Earlier && Later && DT.dominates(Earlier, Later)) {
if (Constant *Merged = tryToMergePartialOverlappingStores(
Earlier, Later, InstWriteOffset, DepWriteOffset, State.DL,
State.BatchAA, &DT)) {
// Update stored value of earlier store to merged constant.
Earlier->setOperand(0, Merged);
++NumModifiedStores;
MadeChange = true;
Shortend = true;
// Remove later store and remove any outstanding overlap intervals
// for the updated store.
State.deleteDeadInstruction(Later);
auto I = State.IOLs.find(Earlier->getParent());
if (I != State.IOLs.end())
I->second.erase(Earlier);
break;
}
}
}
if (OR == OW_Complete) {
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *NI
<< "\n KILLER: " << *SI << '\n');
State.deleteDeadInstruction(NI);
++NumFastStores;
MadeChange = true;
}
}
}
// Check if the store is a no-op.
if (!Shortend && isRemovable(SI) &&
State.storeIsNoop(KillingDef, SILoc, SILocUnd)) {
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *SI << '\n');
State.deleteDeadInstruction(SI);
NumRedundantStores++;
MadeChange = true;
continue;
}
}
if (EnablePartialOverwriteTracking)
for (auto &KV : State.IOLs)
MadeChange |= removePartiallyOverlappedStores(State.DL, KV.second, TLI);
MadeChange |= State.eliminateDeadWritesAtEndOfFunction();
return MadeChange;
}
} // end anonymous namespace
//===----------------------------------------------------------------------===//
// DSE Pass
//===----------------------------------------------------------------------===//
PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
AliasAnalysis &AA = AM.getResult<AAManager>(F);
const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F);
DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI);
#ifdef LLVM_ENABLE_STATS
if (AreStatisticsEnabled())
for (auto &I : instructions(F))
NumRemainingStores += isa<StoreInst>(&I);
#endif
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
PA.preserve<MemorySSAAnalysis>();
return PA;
}
namespace {
/// A legacy pass for the legacy pass manager that wraps \c DSEPass.
class DSELegacyPass : public FunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
DSELegacyPass() : FunctionPass(ID) {
initializeDSELegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
const TargetLibraryInfo &TLI =
getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
PostDominatorTree &PDT =
getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI);
#ifdef LLVM_ENABLE_STATS
if (AreStatisticsEnabled())
for (auto &I : instructions(F))
NumRemainingStores += isa<StoreInst>(&I);
#endif
return Changed;
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<PostDominatorTreeWrapperPass>();
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<PostDominatorTreeWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
}
};
} // end anonymous namespace
char DSELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false,
false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
false)
FunctionPass *llvm::createDeadStoreEliminationPass() {
return new DSELegacyPass();
}
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