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

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//===- DeadStoreElimination.cpp - Fast 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
//
//===----------------------------------------------------------------------===//
//
// This file implements a trivial dead store elimination that only considers
// basic-block local redundant stores.
//
// FIXME: This should eventually be extended to be a post-dominator tree
// traversal. Doing so would be pretty trivial.
//
//===----------------------------------------------------------------------===//
#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/MemoryDependenceAnalysis.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(NumNoopStores, "Number of noop stores deleted");
STATISTIC(NumCFGChecks, "Number of stores modified");
STATISTIC(NumCFGTries, "Number of stores modified");
STATISTIC(NumCFGSuccess, "Number of stores modified");
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<bool>
EnableMemorySSA("enable-dse-memoryssa", cl::init(false), cl::Hidden,
cl::desc("Use the new MemorySSA-backed DSE."));
static cl::opt<unsigned>
MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(100), cl::Hidden,
cl::desc("The number of memory instructions to scan for "
"dead store elimination (default = 100)"));
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> 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>;
/// Delete this instruction. Before we do, go through and zero out all the
/// operands of this instruction. If any of them become dead, delete them and
/// the computation tree that feeds them.
/// If ValueSet is non-null, remove any deleted instructions from it as well.
static void
deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
MemoryDependenceResults &MD, const TargetLibraryInfo &TLI,
InstOverlapIntervalsTy &IOL,
MapVector<Instruction *, bool> &ThrowableInst,
SmallSetVector<const Value *, 16> *ValueSet = nullptr) {
SmallVector<Instruction*, 32> NowDeadInsts;
NowDeadInsts.push_back(I);
--NumFastOther;
// Keeping the iterator straight is a pain, so we let this routine tell the
// caller what the next instruction is after we're done mucking about.
BasicBlock::iterator NewIter = *BBI;
// Before we touch this instruction, remove it from memdep!
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
// Mark the DeadInst as dead in the list of throwable instructions.
auto It = ThrowableInst.find(DeadInst);
if (It != ThrowableInst.end())
ThrowableInst[It->first] = false;
++NumFastOther;
// Try to preserve debug information attached to the dead instruction.
salvageDebugInfo(*DeadInst);
salvageKnowledge(DeadInst);
// This instruction is dead, zap it, in stages. Start by removing it from
// MemDep, which needs to know the operands and needs it to be in the
// function.
MD.removeInstruction(DeadInst);
for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
Value *Op = DeadInst->getOperand(op);
DeadInst->setOperand(op, nullptr);
// If this operand just became dead, add it to the NowDeadInsts list.
if (!Op->use_empty()) continue;
if (Instruction *OpI = dyn_cast<Instruction>(Op))
if (isInstructionTriviallyDead(OpI, &TLI))
NowDeadInsts.push_back(OpI);
}
if (ValueSet) ValueSet->remove(DeadInst);
IOL.erase(DeadInst);
if (NewIter == DeadInst->getIterator())
NewIter = DeadInst->eraseFromParent();
else
DeadInst->eraseFromParent();
} while (!NowDeadInsts.empty());
*BBI = NewIter;
// Pop dead entries from back of ThrowableInst till we find an alive entry.
while (!ThrowableInst.empty() && !ThrowableInst.back().second)
ThrowableInst.pop_back();
}
/// 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_element_unordered_atomic:
case Intrinsic::memmove_element_unordered_atomic:
case Intrinsic::memset_element_unordered_atomic:
case Intrinsic::init_trampoline:
case Intrinsic::lifetime_end:
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) {
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
return MemoryLocation::get(SI);
if (auto *MI = dyn_cast<AnyMemIntrinsic>(Inst)) {
// memcpy/memmove/memset.
MemoryLocation Loc = MemoryLocation::getForDest(MI);
return Loc;
}
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
switch (II->getIntrinsicID()) {
default:
return MemoryLocation(); // Unhandled intrinsic.
case Intrinsic::init_trampoline:
return MemoryLocation(II->getArgOperand(0));
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(CB->getArgOperand(0));
return MemoryLocation();
}
/// Return the location read by the specified "hasAnalyzableMemoryWrite"
/// instruction if any.
static MemoryLocation getLocForRead(Instruction *Inst,
const TargetLibraryInfo &TLI) {
assert(hasAnalyzableMemoryWrite(Inst, TLI) && "Unknown instruction case");
// The only instructions that both read and write are the mem transfer
// instructions (memcpy/memmove).
if (auto *MTI = dyn_cast<AnyMemTransferInst>(Inst))
return MemoryLocation::getForSource(MTI);
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:
// 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:
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);
}
/// Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
//TODO: factor this to reuse getLocForWrite
MemoryLocation Loc = getLocForWrite(I);
assert(Loc.Ptr &&
"unable to find pointer written for analyzable instruction?");
// TODO: most APIs don't expect const Value *
return const_cast<Value*>(Loc.Ptr);
}
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_Unknown
};
} // end anonymous namespace
/// 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.
static OverwriteResult isOverwrite(const MemoryLocation &Later,
const MemoryLocation &Earlier,
const DataLayout &DL,
const TargetLibraryInfo &TLI,
int64_t &EarlierOff, int64_t &LaterOff,
Instruction *DepWrite,
InstOverlapIntervalsTy &IOL,
AliasAnalysis &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())
return OW_Unknown;
const uint64_t LaterSize = Later.Size.getValue();
const uint64_t EarlierSize = Earlier.Size.getValue();
const Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
// 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 (P1 == P2 || AA.isMustAlias(P1, P2)) {
// Make sure that the Later size is >= the Earlier size.
if (LaterSize >= EarlierSize)
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 *UO1 = GetUnderlyingObject(P1, DL),
*UO2 = GetUnderlyingObject(P2, DL);
// 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 store completely overlaps the earlier store if:
//
// 1. Both start at the same offset and the later one's size is greater than
// or equal to the earlier one's, or
//
// |--earlier--|
// |-- later --|
//
// 2. The earlier store has an offset greater than the later offset, but which
// still lies completely within the later store.
//
// |--earlier--|
// |----- later ------|
//
// We have to be careful here as *Off is signed while *.Size is unsigned.
if (EarlierOff >= LaterOff &&
LaterSize >= EarlierSize &&
uint64_t(EarlierOff - LaterOff) + EarlierSize <= LaterSize)
return OW_Complete;
// 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;
}
/// If 'Inst' might be a self read (i.e. a noop copy of a
/// memory region into an identical pointer) then it doesn't actually make its
/// input dead in the traditional sense. Consider this case:
///
/// memmove(A <- B)
/// memmove(A <- A)
///
/// In this case, the second store to A does not make the first store to A dead.
/// The usual situation isn't an explicit A<-A store like this (which can be
/// trivially removed) but a case where two pointers may alias.
///
/// This function detects when it is unsafe to remove a dependent instruction
/// because the DSE inducing instruction may be a self-read.
static bool isPossibleSelfRead(Instruction *Inst,
const MemoryLocation &InstStoreLoc,
Instruction *DepWrite,
const TargetLibraryInfo &TLI,
AliasAnalysis &AA) {
// Self reads can only happen for instructions that read memory. Get the
// location read.
MemoryLocation InstReadLoc = getLocForRead(Inst, TLI);
if (!InstReadLoc.Ptr)
return false; // Not a reading instruction.
// If the read and written loc obviously don't alias, it isn't a read.
if (AA.isNoAlias(InstReadLoc, InstStoreLoc))
return false;
if (isa<AnyMemCpyInst>(Inst)) {
// LLVM's memcpy overlap semantics are not fully fleshed out (see PR11763)
// but in practice memcpy(A <- B) either means that A and B are disjoint or
// are equal (i.e. there are not partial overlaps). Given that, if we have:
//
// memcpy/memmove(A <- B) // DepWrite
// memcpy(A <- B) // Inst
//
// with Inst reading/writing a >= size than DepWrite, we can reason as
// follows:
//
// - If A == B then both the copies are no-ops, so the DepWrite can be
// removed.
// - If A != B then A and B are disjoint locations in Inst. Since
// Inst.size >= DepWrite.size A and B are disjoint in DepWrite too.
// Therefore DepWrite can be removed.
MemoryLocation DepReadLoc = getLocForRead(DepWrite, TLI);
if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
return false;
}
// If DepWrite doesn't read memory or if we can't prove it is a must alias,
// then it can't be considered dead.
return true;
}
/// 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,
AliasAnalysis *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 (auto PredI = pred_begin(B), PE = pred_end(B); PredI != PE; ++PredI) {
PHITransAddr PredAddr = Addr;
if (PredAddr.NeedsPHITranslationFromBlock(B)) {
if (!PredAddr.IsPotentiallyPHITranslatable())
return false;
if (PredAddr.PHITranslateValue(B, *PredI, DT, false))
return false;
}
Value *TranslatedPtr = PredAddr.getAddr();
auto Inserted = Visited.insert(std::make_pair(*PredI, 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(*PredI, PredAddr));
}
}
}
return true;
}
/// Find all blocks that will unconditionally lead to the block BB and append
/// them to F.
static void findUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
BasicBlock *BB, DominatorTree *DT) {
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
BasicBlock *Pred = *I;
if (Pred == BB) continue;
Instruction *PredTI = Pred->getTerminator();
if (PredTI->getNumSuccessors() != 1)
continue;
if (DT->isReachableFromEntry(Pred))
Blocks.push_back(Pred);
}
}
/// Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
static bool handleFree(CallInst *F, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL,
MapVector<Instruction *, bool> &ThrowableInst) {
bool MadeChange = false;
MemoryLocation Loc = MemoryLocation(F->getOperand(0));
SmallVector<BasicBlock *, 16> Blocks;
Blocks.push_back(F->getParent());
const DataLayout &DL = F->getModule()->getDataLayout();
while (!Blocks.empty()) {
BasicBlock *BB = Blocks.pop_back_val();
Instruction *InstPt = BB->getTerminator();
if (BB == F->getParent()) InstPt = F;
MemDepResult Dep =
MD->getPointerDependencyFrom(Loc, false, InstPt->getIterator(), BB);
while (Dep.isDef() || Dep.isClobber()) {
Instruction *Dependency = Dep.getInst();
if (!hasAnalyzableMemoryWrite(Dependency, *TLI) ||
!isRemovable(Dependency))
break;
Value *DepPointer =
GetUnderlyingObject(getStoredPointerOperand(Dependency), DL);
// Check for aliasing.
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
break;
LLVM_DEBUG(
dbgs() << "DSE: Dead Store to soon to be freed memory:\n DEAD: "
<< *Dependency << '\n');
// DCE instructions only used to calculate that store.
BasicBlock::iterator BBI(Dependency);
deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL,
ThrowableInst);
++NumFastStores;
MadeChange = true;
// Inst's old Dependency is now deleted. Compute the next dependency,
// which may also be dead, as in
// s[0] = 0;
// s[1] = 0; // This has just been deleted.
// free(s);
Dep = MD->getPointerDependencyFrom(Loc, false, BBI, BB);
}
if (Dep.isNonLocal())
findUnconditionalPreds(Blocks, BB, DT);
}
return MadeChange;
}
/// Check to see if the specified location may alias any of the stack objects in
/// the DeadStackObjects set. If so, they become live because the location is
/// being loaded.
static void removeAccessedObjects(const MemoryLocation &LoadedLoc,
SmallSetVector<const Value *, 16> &DeadStackObjects,
const DataLayout &DL, AliasAnalysis *AA,
const TargetLibraryInfo *TLI,
const Function *F) {
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr, DL);
// A constant can't be in the dead pointer set.
if (isa<Constant>(UnderlyingPointer))
return;
// If the kill pointer can be easily reduced to an alloca, don't bother doing
// extraneous AA queries.
if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
DeadStackObjects.remove(UnderlyingPointer);
return;
}
// Remove objects that could alias LoadedLoc.
DeadStackObjects.remove_if([&](const Value *I) {
// See if the loaded location could alias the stack location.
MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI, F));
return !AA->isNoAlias(StackLoc, LoadedLoc);
});
}
/// Remove dead stores to stack-allocated locations in the function end block.
/// Ex:
/// %A = alloca i32
/// ...
/// store i32 1, i32* %A
/// ret void
static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
MemoryDependenceResults *MD,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL,
MapVector<Instruction *, bool> &ThrowableInst) {
bool MadeChange = false;
// Keep track of all of the stack objects that are dead at the end of the
// function.
SmallSetVector<const Value*, 16> DeadStackObjects;
// Find all of the alloca'd pointers in the entry block.
BasicBlock &Entry = BB.getParent()->front();
for (Instruction &I : Entry) {
if (isa<AllocaInst>(&I))
DeadStackObjects.insert(&I);
// Okay, so these are dead heap objects, but if the pointer never escapes
// then it's leaked by this function anyways.
else if (isAllocLikeFn(&I, TLI) && !PointerMayBeCaptured(&I, true, true))
DeadStackObjects.insert(&I);
}
// Treat byval or inalloca arguments the same, stores to them are dead at the
// end of the function.
for (Argument &AI : BB.getParent()->args())
if (AI.hasPassPointeeByValueAttr())
DeadStackObjects.insert(&AI);
const DataLayout &DL = BB.getModule()->getDataLayout();
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
--BBI;
// If we find a store, check to see if it points into a dead stack value.
if (hasAnalyzableMemoryWrite(&*BBI, *TLI) && isRemovable(&*BBI)) {
// See through pointer-to-pointer bitcasts
SmallVector<const Value *, 4> Pointers;
GetUnderlyingObjects(getStoredPointerOperand(&*BBI), Pointers, DL);
// Stores to stack values are valid candidates for removal.
bool AllDead = true;
for (const Value *Pointer : Pointers)
if (!DeadStackObjects.count(Pointer)) {
AllDead = false;
break;
}
if (AllDead) {
Instruction *Dead = &*BBI;
LLVM_DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
<< *Dead << "\n Objects: ";
for (SmallVectorImpl<const Value *>::iterator I =
Pointers.begin(),
E = Pointers.end();
I != E; ++I) {
dbgs() << **I;
if (std::next(I) != E)
dbgs() << ", ";
} dbgs()
<< '\n');
// DCE instructions only used to calculate that store.
deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, ThrowableInst,
&DeadStackObjects);
++NumFastStores;
MadeChange = true;
continue;
}
}
// Remove any dead non-memory-mutating instructions.
if (isInstructionTriviallyDead(&*BBI, TLI)) {
LLVM_DEBUG(dbgs() << "DSE: Removing trivially dead instruction:\n DEAD: "
<< *&*BBI << '\n');
deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, ThrowableInst,
&DeadStackObjects);
++NumFastOther;
MadeChange = true;
continue;
}
if (isa<AllocaInst>(BBI)) {
// Remove allocas from the list of dead stack objects; there can't be
// any references before the definition.
DeadStackObjects.remove(&*BBI);
continue;
}
if (auto *Call = dyn_cast<CallBase>(&*BBI)) {
// Remove allocation function calls from the list of dead stack objects;
// there can't be any references before the definition.
if (isAllocLikeFn(&*BBI, TLI))
DeadStackObjects.remove(&*BBI);
// If this call does not access memory, it can't be loading any of our
// pointers.
if (AA->doesNotAccessMemory(Call))
continue;
// If the call might load from any of our allocas, then any store above
// the call is live.
DeadStackObjects.remove_if([&](const Value *I) {
// See if the call site touches the value.
return isRefSet(AA->getModRefInfo(
Call, I, getPointerSize(I, DL, *TLI, BB.getParent())));
});
// If all of the allocas were clobbered by the call then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
break;
continue;
}
// 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>(*BBI))
continue;
MemoryLocation LoadedLoc;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
if (!L->isUnordered()) // Be conservative with atomic/volatile load
break;
LoadedLoc = MemoryLocation::get(L);
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
LoadedLoc = MemoryLocation::get(V);
} else if (!BBI->mayReadFromMemory()) {
// Instruction doesn't read memory. Note that stores that weren't removed
// above will hit this case.
continue;
} else {
// Unknown inst; assume it clobbers everything.
break;
}
// Remove any allocas from the DeadPointer set that are loaded, as this
// makes any stores above the access live.
removeAccessedObjects(LoadedLoc, DeadStackObjects, DL, AA, TLI, BB.getParent());
// If all of the allocas were clobbered by the access then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
break;
}
return MadeChange;
}
static bool tryToShorten(Instruction *EarlierWrite, int64_t &EarlierOffset,
int64_t &EarlierSize, int64_t LaterOffset,
int64_t LaterSize, bool IsOverwriteEnd) {
// TODO: base this on the target vector size so that if the earlier
// store was too small to get vector writes anyway then its likely
// a good idea to shorten it
// Power of 2 vector writes are probably always a bad idea to optimize
// as any store/memset/memcpy is likely using vector instructions so
// shortening it to not vector size is likely to be slower
auto *EarlierIntrinsic = cast<AnyMemIntrinsic>(EarlierWrite);
unsigned EarlierWriteAlign = EarlierIntrinsic->getDestAlignment();
if (!IsOverwriteEnd)
LaterOffset = int64_t(LaterOffset + LaterSize);
if (!(isPowerOf2_64(LaterOffset) && EarlierWriteAlign <= LaterOffset) &&
!((EarlierWriteAlign != 0) && LaterOffset % EarlierWriteAlign == 0))
return false;
int64_t NewLength = IsOverwriteEnd
? LaterOffset - EarlierOffset
: EarlierSize - (LaterOffset - EarlierOffset);
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 != NewLength % ElementSize)
return false;
}
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
<< (IsOverwriteEnd ? "END" : "BEGIN") << ": "
<< *EarlierWrite << "\n KILLER (offset " << LaterOffset
<< ", " << EarlierSize << ")\n");
Value *EarlierWriteLength = EarlierIntrinsic->getLength();
Value *TrimmedLength =
ConstantInt::get(EarlierWriteLength->getType(), NewLength);
EarlierIntrinsic->setLength(TrimmedLength);
EarlierSize = NewLength;
if (!IsOverwriteEnd) {
int64_t OffsetMoved = (LaterOffset - EarlierOffset);
Value *Indices[1] = {
ConstantInt::get(EarlierWriteLength->getType(), OffsetMoved)};
GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds(
EarlierIntrinsic->getRawDest()->getType()->getPointerElementType(),
EarlierIntrinsic->getRawDest(), Indices, "", EarlierWrite);
NewDestGEP->setDebugLoc(EarlierIntrinsic->getDebugLoc());
EarlierIntrinsic->setDest(NewDestGEP);
EarlierOffset = EarlierOffset + OffsetMoved;
}
return true;
}
static bool tryToShortenEnd(Instruction *EarlierWrite,
OverlapIntervalsTy &IntervalMap,
int64_t &EarlierStart, int64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheEnd(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = --IntervalMap.end();
int64_t LaterStart = OII->second;
int64_t LaterSize = OII->first - LaterStart;
if (LaterStart > EarlierStart && LaterStart < EarlierStart + EarlierSize &&
LaterStart + LaterSize >= EarlierStart + EarlierSize) {
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, int64_t &EarlierSize) {
if (IntervalMap.empty() || !isShortenableAtTheBeginning(EarlierWrite))
return false;
OverlapIntervalsTy::iterator OII = IntervalMap.begin();
int64_t LaterStart = OII->second;
int64_t LaterSize = OII->first - LaterStart;
if (LaterStart <= EarlierStart && LaterStart + LaterSize > EarlierStart) {
assert(LaterStart + LaterSize < EarlierStart + 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(AliasAnalysis *AA,
const DataLayout &DL,
InstOverlapIntervalsTy &IOL) {
bool Changed = false;
for (auto OI : IOL) {
Instruction *EarlierWrite = OI.first;
MemoryLocation Loc = getLocForWrite(EarlierWrite);
assert(isRemovable(EarlierWrite) && "Expect only removable instruction");
const Value *Ptr = Loc.Ptr->stripPointerCasts();
int64_t EarlierStart = 0;
int64_t EarlierSize = int64_t(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 bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
AliasAnalysis *AA, MemoryDependenceResults *MD,
const DataLayout &DL,
const TargetLibraryInfo *TLI,
InstOverlapIntervalsTy &IOL,
MapVector<Instruction *, bool> &ThrowableInst,
DominatorTree *DT) {
// Must be a store instruction.
StoreInst *SI = dyn_cast<StoreInst>(Inst);
if (!SI)
return false;
// If we're storing the same value back to a pointer that we just loaded from,
// then the store can be removed.
if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
isRemovable(SI) &&
memoryIsNotModifiedBetween(DepLoad, SI, AA, DL, DT)) {
LLVM_DEBUG(
dbgs() << "DSE: Remove Store Of Load from same pointer:\n LOAD: "
<< *DepLoad << "\n STORE: " << *SI << '\n');
deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst);
++NumRedundantStores;
return true;
}
}
// Remove null stores into the calloc'ed objects
Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand());
if (StoredConstant && StoredConstant->isNullValue() && isRemovable(SI)) {
Instruction *UnderlyingPointer =
dyn_cast<Instruction>(GetUnderlyingObject(SI->getPointerOperand(), DL));
if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) &&
memoryIsNotModifiedBetween(UnderlyingPointer, SI, AA, DL, DT)) {
LLVM_DEBUG(
dbgs() << "DSE: Remove null store to the calloc'ed object:\n DEAD: "
<< *Inst << "\n OBJECT: " << *UnderlyingPointer << '\n');
deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst);
++NumRedundantStores;
return true;
}
}
return false;
}
static Constant *
tryToMergePartialOverlappingStores(StoreInst *Earlier, StoreInst *Later,
int64_t InstWriteOffset,
int64_t DepWriteOffset, const DataLayout &DL,
AliasAnalysis *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;
}
static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI) {
const DataLayout &DL = BB.getModule()->getDataLayout();
bool MadeChange = false;
MapVector<Instruction *, bool> ThrowableInst;
// A map of interval maps representing partially-overwritten value parts.
InstOverlapIntervalsTy IOL;
// Do a top-down walk on the BB.
for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
// Handle 'free' calls specially.
if (CallInst *F = isFreeCall(&*BBI, TLI)) {
MadeChange |= handleFree(F, AA, MD, DT, TLI, IOL, ThrowableInst);
// Increment BBI after handleFree has potentially deleted instructions.
// This ensures we maintain a valid iterator.
++BBI;
continue;
}
Instruction *Inst = &*BBI++;
if (Inst->mayThrow()) {
ThrowableInst[Inst] = true;
continue;
}
// Check to see if Inst writes to memory. If not, continue.
if (!hasAnalyzableMemoryWrite(Inst, *TLI))
continue;
// eliminateNoopStore will update in iterator, if necessary.
if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI, IOL,
ThrowableInst, DT)) {
MadeChange = true;
continue;
}
// If we find something that writes memory, get its memory dependence.
MemDepResult InstDep = MD->getDependency(Inst);
// Ignore any store where we can't find a local dependence.
// FIXME: cross-block DSE would be fun. :)
if (!InstDep.isDef() && !InstDep.isClobber())
continue;
// Figure out what location is being stored to.
MemoryLocation Loc = getLocForWrite(Inst);
// If we didn't get a useful location, fail.
if (!Loc.Ptr)
continue;
// Loop until we find a store we can eliminate or a load that
// invalidates the analysis. Without an upper bound on the number of
// instructions examined, this analysis can become very time-consuming.
// However, the potential gain diminishes as we process more instructions
// without eliminating any of them. Therefore, we limit the number of
// instructions we look at.
auto Limit = MD->getDefaultBlockScanLimit();
while (InstDep.isDef() || InstDep.isClobber()) {
// Get the memory clobbered by the instruction we depend on. MemDep will
// skip any instructions that 'Loc' clearly doesn't interact with. If we
// end up depending on a may- or must-aliased load, then we can't optimize
// away the store and we bail out. However, if we depend on something
// that overwrites the memory location we *can* potentially optimize it.
//
// Find out what memory location the dependent instruction stores.
Instruction *DepWrite = InstDep.getInst();
if (!hasAnalyzableMemoryWrite(DepWrite, *TLI))
break;
MemoryLocation DepLoc = getLocForWrite(DepWrite);
// If we didn't get a useful location, or if it isn't a size, bail out.
if (!DepLoc.Ptr)
break;
// Find the last throwable instruction not removed by call to
// deleteDeadInstruction.
Instruction *LastThrowing = nullptr;
if (!ThrowableInst.empty())
LastThrowing = ThrowableInst.back().first;
// Make sure we don't look past a call which might throw. This is an
// issue because MemoryDependenceAnalysis works in the wrong direction:
// it finds instructions which dominate the current instruction, rather than
// instructions which are post-dominated by the current instruction.
//
// If the underlying object is a non-escaping memory allocation, any store
// to it is dead along the unwind edge. Otherwise, we need to preserve
// the store.
if (LastThrowing && DepWrite->comesBefore(LastThrowing)) {
const Value* Underlying = GetUnderlyingObject(DepLoc.Ptr, DL);
bool IsStoreDeadOnUnwind = isa<AllocaInst>(Underlying);
if (!IsStoreDeadOnUnwind) {
// We're looking for a call to an allocation function
// where the allocation doesn't escape before the last
// throwing instruction; PointerMayBeCaptured
// reasonably fast approximation.
IsStoreDeadOnUnwind = isAllocLikeFn(Underlying, TLI) &&
!PointerMayBeCaptured(Underlying, false, true);
}
if (!IsStoreDeadOnUnwind)
break;
}
// If we find a write that is a) removable (i.e., non-volatile), b) is
// completely obliterated by the store to 'Loc', and c) which we know that
// 'Inst' doesn't load from, then we can remove it.
// Also try to merge two stores if a later one only touches memory written
// to by the earlier one.
if (isRemovable(DepWrite) &&
!isPossibleSelfRead(Inst, Loc, DepWrite, *TLI, *AA)) {
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR = isOverwrite(Loc, DepLoc, DL, *TLI, DepWriteOffset,
InstWriteOffset, DepWrite, IOL, *AA,
BB.getParent());
if (OR == OW_Complete) {
LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *DepWrite
<< "\n KILLER: " << *Inst << '\n');
// Delete the store and now-dead instructions that feed it.
deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL,
ThrowableInst);
++NumFastStores;
MadeChange = true;
// We erased DepWrite; start over.
InstDep = MD->getDependency(Inst);
continue;
} else if ((OR == OW_End && isShortenableAtTheEnd(DepWrite)) ||
((OR == OW_Begin &&
isShortenableAtTheBeginning(DepWrite)))) {
assert(!EnablePartialOverwriteTracking && "Do not expect to perform "
"when partial-overwrite "
"tracking is enabled");
// The overwrite result is known, so these must be known, too.
int64_t EarlierSize = DepLoc.Size.getValue();
int64_t LaterSize = Loc.Size.getValue();
bool IsOverwriteEnd = (OR == OW_End);
MadeChange |= tryToShorten(DepWrite, DepWriteOffset, EarlierSize,
InstWriteOffset, LaterSize, IsOverwriteEnd);
} else if (EnablePartialStoreMerging &&
OR == OW_PartialEarlierWithFullLater) {
auto *Earlier = dyn_cast<StoreInst>(DepWrite);
auto *Later = dyn_cast<StoreInst>(Inst);
if (Constant *C = tryToMergePartialOverlappingStores(
Earlier, Later, InstWriteOffset, DepWriteOffset, DL, AA,
DT)) {
auto *SI = new StoreInst(
C, Earlier->getPointerOperand(), false, Earlier->getAlign(),
Earlier->getOrdering(), Earlier->getSyncScopeID(), DepWrite);
unsigned MDToKeep[] = {LLVMContext::MD_dbg, LLVMContext::MD_tbaa,
LLVMContext::MD_alias_scope,
LLVMContext::MD_noalias,
LLVMContext::MD_nontemporal};
SI->copyMetadata(*DepWrite, MDToKeep);
++NumModifiedStores;
// Delete the old stores and now-dead instructions that feed them.
deleteDeadInstruction(Inst, &BBI, *MD, *TLI, IOL,
ThrowableInst);
deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL,
ThrowableInst);
MadeChange = true;
// We erased DepWrite and Inst (Loc); start over.
break;
}
}
}
// If this is a may-aliased store that is clobbering the store value, we
// can keep searching past it for another must-aliased pointer that stores
// to the same location. For example, in:
// store -> P
// store -> Q
// store -> P
// we can remove the first store to P even though we don't know if P and Q
// alias.
if (DepWrite == &BB.front()) break;
// Can't look past this instruction if it might read 'Loc'.
if (isRefSet(AA->getModRefInfo(DepWrite, Loc)))
break;
InstDep = MD->getPointerDependencyFrom(Loc, /*isLoad=*/ false,
DepWrite->getIterator(), &BB,
/*QueryInst=*/ nullptr, &Limit);
}
}
if (EnablePartialOverwriteTracking)
MadeChange |= removePartiallyOverlappedStores(AA, DL, IOL);
// If this block ends in a return, unwind, or unreachable, all allocas are
// dead at its end, which means stores to them are also dead.
if (BB.getTerminator()->getNumSuccessors() == 0)
MadeChange |= handleEndBlock(BB, AA, MD, TLI, IOL, ThrowableInst);
return MadeChange;
}
static bool eliminateDeadStores(Function &F, AliasAnalysis *AA,
MemoryDependenceResults *MD, DominatorTree *DT,
const TargetLibraryInfo *TLI) {
bool MadeChange = false;
for (BasicBlock &BB : F)
// Only check non-dead blocks. Dead blocks may have strange pointer
// cycles that will confuse alias analysis.
if (DT->isReachableFromEntry(&BB))
MadeChange |= eliminateDeadStores(BB, AA, MD, DT, TLI);
return MadeChange;
}
namespace {
//=============================================================================
// MemorySSA backed dead store elimination.
//
// 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 (DomAccess) by walking
// upwards.
// 2. Check that there are no reads between DomAccess and the StartDef by
// checking all uses starting at DomAccess and walking until we see StartDef.
// 3. For each found DomDef, check that:
// 1. There are no barrier instructions between DomDef and StartDef (like
// throws or stores with ordering constraints).
// 2. StartDef is executed whenever DomDef is executed.
// 3. StartDef completely overwrites DomDef.
// 4. Erase DomDef from the function and MemorySSA.
// Returns true if \p M is an intrisnic that does not read or write memory.
bool isNoopIntrinsic(MemoryUseOrDef *M) {
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(M->getMemoryInst())) {
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))
return true;
return false;
}
struct DSEState {
Function &F;
AliasAnalysis &AA;
MemorySSA &MSSA;
DominatorTree &DT;
PostDominatorTree &PDT;
const TargetLibraryInfo &TLI;
// 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;
// Keep track of all of the objects that are invisible to the caller after
// the function returns.
SmallPtrSet<const Value *, 16> 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), MSSA(MSSA), DT(DT), PDT(PDT), TLI(TLI) {}
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);
// Track whether alloca and alloca-like objects are visible in the
// caller before and after the function returns. Alloca objects are
// invalid in the caller, so they are neither visible before or after
// the function returns.
if (isa<AllocaInst>(&I)) {
State.InvisibleToCallerBeforeRet.insert(&I);
State.InvisibleToCallerAfterRet.insert(&I);
}
// For alloca-like objects we need to check if they are captured before
// the function returns and if the return might capture the object.
if (isAllocLikeFn(&I, &TLI)) {
bool CapturesBeforeRet = PointerMayBeCaptured(&I, false, true);
if (!CapturesBeforeRet) {
State.InvisibleToCallerBeforeRet.insert(&I);
if (!PointerMayBeCaptured(&I, true, false))
State.InvisibleToCallerAfterRet.insert(&I);
}
}
}
}
// 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.hasPassPointeeByValueAttr()) {
// For byval, the caller doesn't know the address of the allocation.
if (AI.hasByValAttr())
State.InvisibleToCallerBeforeRet.insert(&AI);
State.InvisibleToCallerAfterRet.insert(&AI);
}
return State;
}
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)) {
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(CB->getArgOperand(0))};
default:
break;
}
}
return None;
}
return MemoryLocation::getOrNone(I);
}
/// Returns true if \p Use completely overwrites \p DefLoc.
bool isCompleteOverwrite(MemoryLocation DefLoc, Instruction *UseInst) const {
// 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;
auto CC = getLocForWriteEx(UseInst);
InstOverlapIntervalsTy IOL;
const DataLayout &DL = F.getParent()->getDataLayout();
return CC &&
isOverwrite(*CC, DefLoc, DL, TLI, DepWriteOffset, InstWriteOffset,
UseInst, IOL, AA, &F) == OW_Complete;
}
/// 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];
if (isa<MemoryPhi>(UseAccess)) {
PushMemUses(UseAccess);
continue;
}
// 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(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 the same
/// underlying object as \p DefLoc.
bool isMemTerminator(MemoryLocation DefLoc, Instruction *MaybeTerm) const {
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 (MaybeTermLoc->second) {
DataLayout DL = MaybeTerm->getParent()->getModule()->getDataLayout();
DefLoc = MemoryLocation(GetUnderlyingObject(DefLoc.Ptr, DL));
}
return AA.isMustAlias(MaybeTermLoc->first, DefLoc);
}
// Returns true if \p Use may read from \p DefLoc.
bool isReadClobber(MemoryLocation DefLoc, Instruction *UseInst) const {
if (!UseInst->mayReadFromMemory())
return false;
if (auto *CB = dyn_cast<CallBase>(UseInst))
if (CB->onlyAccessesInaccessibleMemory())
return false;
ModRefInfo MR = AA.getModRefInfo(UseInst, DefLoc);
// If necessary, perform additional analysis.
if (isRefSet(MR))
MR = AA.callCapturesBefore(UseInst, DefLoc, &DT);
return isRefSet(MR);
}
// Find a MemoryDef writing to \p DefLoc and dominating \p Current, 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 *Current,
MemoryLocation DefLoc, bool DefVisibleToCallerBeforeRet,
bool DefVisibleToCallerAfterRet, int &ScanLimit) const {
MemoryAccess *DomAccess;
bool StepAgain;
LLVM_DEBUG(dbgs() << " trying to get dominating access for " << *Current
<< "\n");
// Find the next clobbering Mod access for DefLoc, starting at Current.
do {
StepAgain = false;
// Reached TOP.
if (MSSA.isLiveOnEntryDef(Current))
return None;
if (isa<MemoryPhi>(Current)) {
DomAccess = Current;
break;
}
MemoryUseOrDef *CurrentUD = cast<MemoryUseOrDef>(Current);
// Look for access that clobber DefLoc.
DomAccess = MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(CurrentUD,
DefLoc);
if (MSSA.isLiveOnEntryDef(DomAccess))
return None;
if (isa<MemoryPhi>(DomAccess))
break;
// Check if we can skip DomDef for DSE.
MemoryDef *DomDef = dyn_cast<MemoryDef>(DomAccess);
if (DomDef && canSkipDef(DomDef, DefVisibleToCallerBeforeRet)) {
StepAgain = true;
Current = DomDef->getDefiningAccess();
}
} 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 DomAccess to any function exit.
SmallPtrSet<BasicBlock *, 16> KillingBlocks = {KillingDef->getBlock()};
LLVM_DEBUG({
dbgs() << " Checking for reads of " << *DomAccess;
if (isa<MemoryDef>(DomAccess))
dbgs() << " (" << *cast<MemoryDef>(DomAccess)->getMemoryInst() << ")\n";
else
dbgs() << ")\n";
});
SmallSetVector<MemoryAccess *, 32> WorkList;
auto PushMemUses = [&WorkList](MemoryAccess *Acc) {
for (Use &U : Acc->uses())
WorkList.insert(cast<MemoryAccess>(U.getUser()));
};
PushMemUses(DomAccess);
// Check if DomDef may be read.
for (unsigned I = 0; I < WorkList.size(); I++) {
MemoryAccess *UseAccess = WorkList[I];
LLVM_DEBUG(dbgs() << " " << *UseAccess);
if (--ScanLimit == 0) {
LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
return None;
}
if (isa<MemoryPhi>(UseAccess)) {
LLVM_DEBUG(dbgs() << "\n ... adding PHI uses\n");
PushMemUses(UseAccess);
continue;
}
Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");
if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess))) {
LLVM_DEBUG(dbgs() << " ... adding uses of intrinsic\n");
PushMemUses(UseAccess);
continue;
}
// A memory terminator kills all preceeding MemoryDefs and all succeeding
// MemoryAccesses. We do not have to check it's users.
if (isMemTerminator(DefLoc, UseInst))
continue;
// Uses which may read the original MemoryDef mean we cannot eliminate the
// original MD. Stop walk.
if (isReadClobber(DefLoc, UseInst)) {
LLVM_DEBUG(dbgs() << " ... found read clobber\n");
return None;
}
// For the KillingDef and DomAccess 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 || DomAccess == 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) ; <----- DomDef 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(DefLoc, UseInst)) {
if (DefVisibleToCallerAfterRet && UseAccess != DomAccess) {
BasicBlock *MaybeKillingBlock = UseInst->getParent();
if (PostOrderNumbers.find(MaybeKillingBlock)->second <
PostOrderNumbers.find(DomAccess->getBlock())->second) {
LLVM_DEBUG(dbgs() << " ... found killing block "
<< MaybeKillingBlock->getName() << "\n");
KillingBlocks.insert(MaybeKillingBlock);
}
}
} else
PushMemUses(UseDef);
}
}
// For accesses to locations visible after the function returns, make sure
// that the location is killed (=overwritten) along all paths from DomAccess
// to the exit.
if (DefVisibleToCallerAfterRet) {
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 DomAccess.
if (KillingBlocks.count(CommonPred)) {
if (PDT.dominates(CommonPred, DomAccess->getBlock()))
return {DomAccess};
return None;
}
// If the common post-dominator does not post-dominate DomAccess, there
// is a path from DomAccess to an exit not going through a killing block.
if (PDT.dominates(CommonPred, DomAccess->getBlock())) {
SetVector<BasicBlock *> WorkList;
// DomAccess's post-order number provides an upper bound of the blocks
// on a path starting at DomAccess.
unsigned UpperBound =
PostOrderNumbers.find(DomAccess->getBlock())->second;
// 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 DomAccess.
for (unsigned I = 0; I < WorkList.size(); I++) {
NumCFGChecks++;
BasicBlock *Current = WorkList[I];
if (KillingBlocks.count(Current))
continue;
if (Current == DomAccess->getBlock())
return None;
// DomAccess is reachable from the entry, so we don't have to explore
// unreachable blocks further.
if (!DT.isReachableFromEntry(Current))
continue;
unsigned CPO = PostOrderNumbers.find(Current)->second;
// Current block is not on a path starting at DomAccess.
if (CPO > UpperBound)
continue;
for (BasicBlock *Pred : predecessors(Current))
WorkList.insert(Pred);
if (WorkList.size() >= MemorySSAPathCheckLimit)
return None;
}
NumCFGSuccess++;
return {DomAccess};
}
return None;
}
// No aliasing MemoryUses of DomAccess found, DomAccess is potentially dead.
return {DomAccess};
}
// 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) const {
// 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 && InvisibleToCallerBeforeRet.count(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) const {
// 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() && !InvisibleToCallerBeforeRet.count(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());
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() {
const DataLayout &DL = F.getParent()->getDataLayout();
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.find(Def) != SkipStores.end() ||
!isRemovable(Def->getMemoryInst()))
continue;
// TODO: Consider doing the underlying object check first, if it is
// beneficial compile-time wise.
if (isWriteAtEndOfFunction(Def)) {
Instruction *DefI = Def->getMemoryInst();
// See through pointer-to-pointer bitcasts
SmallVector<const Value *, 4> Pointers;
GetUnderlyingObjects(getLocForWriteEx(DefI)->Ptr, Pointers, DL);
LLVM_DEBUG(dbgs() << " ... MemoryDef is not accessed until the end "
"of the function\n");
bool CanKill = true;
for (const Value *Pointer : Pointers) {
if (!InvisibleToCallerAfterRet.count(Pointer)) {
CanKill = false;
break;
}
}
if (CanKill) {
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, 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)) {
auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
// If both accesses share the same defining access, no instructions
// between them can modify the memory location.
return LoadAccess == Def->getDefiningAccess();
}
}
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 eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA,
MemorySSA &MSSA, DominatorTree &DT,
PostDominatorTree &PDT,
const TargetLibraryInfo &TLI) {
const DataLayout &DL = F.getParent()->getDataLayout();
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();
auto MaybeSILoc = State.getLocForWriteEx(SI);
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, DL);
// Check if the store is a no-op.
if (isRemovable(SI) && State.storeIsNoop(KillingDef, SILoc, SILocUnd)) {
LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *SI << '\n');
State.deleteDeadInstruction(SI);
NumNoopStores++;
MadeChange = true;
continue;
}
Instruction *DefObj =
const_cast<Instruction *>(dyn_cast<Instruction>(SILocUnd));
bool DefVisibleToCallerBeforeRet =
!State.InvisibleToCallerBeforeRet.count(SILocUnd);
bool DefVisibleToCallerAfterRet =
!State.InvisibleToCallerAfterRet.count(SILocUnd);
if (DefObj && isAllocLikeFn(DefObj, &TLI)) {
if (DefVisibleToCallerBeforeRet)
DefVisibleToCallerBeforeRet =
PointerMayBeCapturedBefore(DefObj, false, true, SI, &DT);
}
MemoryAccess *Current = KillingDef;
LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
<< *KillingDef << " (" << *SI << ")\n");
int ScanLimit = MemorySSAScanLimit;
// Worklist of MemoryAccesses that may be killed by KillingDef.
SetVector<MemoryAccess *> ToCheck;
ToCheck.insert(KillingDef->getDefiningAccess());
// 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, DefVisibleToCallerBeforeRet,
DefVisibleToCallerAfterRet, ScanLimit);
if (!Next) {
LLVM_DEBUG(dbgs() << " finished walk\n");
continue;
}
MemoryAccess *DomAccess = *Next;
LLVM_DEBUG(dbgs() << " Checking if we can kill " << *DomAccess);
if (isa<MemoryPhi>(DomAccess)) {
LLVM_DEBUG(dbgs() << "\n ... adding incoming values to worklist\n");
for (Value *V : cast<MemoryPhi>(DomAccess)->incoming_values()) {
MemoryAccess *IncomingAccess = cast<MemoryAccess>(V);
BasicBlock *IncomingBlock = IncomingAccess->getBlock();
BasicBlock *PhiBlock = DomAccess->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;
}
MemoryDef *NextDef = dyn_cast<MemoryDef>(DomAccess);
Instruction *NI = NextDef->getMemoryInst();
LLVM_DEBUG(dbgs() << " (" << *NI << ")\n");
// Before we try to remove anything, check for any extra throwing
// instructions that block us from DSEing
if (State.mayThrowBetween(SI, NI, SILocUnd)) {
LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
break;
}
// Check for anything that looks like it will be a barrier to further
// removal
if (State.isDSEBarrier(SILocUnd, NI)) {
LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
continue;
}
ToCheck.insert(NextDef->getDefiningAccess());
if (!hasAnalyzableMemoryWrite(NI, TLI)) {
LLVM_DEBUG(dbgs() << " ... skip, cannot analyze def\n");
continue;
}
if (!isRemovable(NI)) {
LLVM_DEBUG(dbgs() << " ... skip, cannot remove def\n");
continue;
}
if (!DebugCounter::shouldExecute(MemorySSACounter))
continue;
MemoryLocation NILoc = *State.getLocForWriteEx(NI);
if (State.isMemTerminatorInst(SI)) {
const Value *NIUnd = GetUnderlyingObject(NILoc.Ptr, DL);
if (!SILocUnd || 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;
auto Iter = State.IOLs.insert(
std::make_pair<BasicBlock *, InstOverlapIntervalsTy>(
NI->getParent(), InstOverlapIntervalsTy()));
auto &IOL = Iter.first->second;
OverwriteResult OR = isOverwrite(SILoc, NILoc, DL, TLI, DepWriteOffset,
InstWriteOffset, NI, IOL, AA, &F);
if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
auto *Earlier = dyn_cast<StoreInst>(NI);
auto *Later = dyn_cast<StoreInst>(SI);
if (Constant *Merged = tryToMergePartialOverlappingStores(
Earlier, Later, InstWriteOffset, DepWriteOffset, DL, &AA,
&DT)) {
// Update stored value of earlier store to merged constant.
Earlier->setOperand(0, Merged);
++NumModifiedStores;
MadeChange = 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;
}
}
}
}
if (EnablePartialOverwriteTracking)
for (auto &KV : State.IOLs)
MadeChange |= removePartiallyOverlappedStores(&AA, DL, KV.second);
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);
bool Changed = false;
if (EnableMemorySSA) {
MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
} else {
MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F);
Changed = eliminateDeadStores(F, &AA, &MD, &DT, &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>();
if (EnableMemorySSA)
PA.preserve<MemorySSAAnalysis>();
else
PA.preserve<MemoryDependenceAnalysis>();
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);
bool Changed = false;
if (EnableMemorySSA) {
MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
PostDominatorTree &PDT =
getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
} else {
MemoryDependenceResults &MD =
getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
Changed = eliminateDeadStores(F, &AA, &MD, &DT, &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>();
if (EnableMemorySSA) {
AU.addRequired<PostDominatorTreeWrapperPass>();
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<PostDominatorTreeWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
} else {
AU.addRequired<MemoryDependenceWrapperPass>();
AU.addPreserved<MemoryDependenceWrapperPass>();
}
}
};
} // 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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