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1104 lines
41 KiB
1104 lines
41 KiB
//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a trivial dead store elimination that only considers
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// basic-block local redundant stores.
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//
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// FIXME: This should eventually be extended to be a post-dominator tree
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// traversal. Doing so would be pretty trivial.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/CaptureTracking.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/MemoryBuiltins.h"
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#include "llvm/Analysis/MemoryDependenceAnalysis.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/GlobalVariable.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include <map>
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using namespace llvm;
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#define DEBUG_TYPE "dse"
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STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
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STATISTIC(NumFastStores, "Number of stores deleted");
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STATISTIC(NumFastOther , "Number of other instrs removed");
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STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
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static cl::opt<bool>
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EnablePartialOverwriteTracking("enable-dse-partial-overwrite-tracking",
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cl::init(true), cl::Hidden,
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cl::desc("Enable partial-overwrite tracking in DSE"));
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//===----------------------------------------------------------------------===//
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// Helper functions
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//===----------------------------------------------------------------------===//
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/// Delete this instruction. Before we do, go through and zero out all the
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/// operands of this instruction. If any of them become dead, delete them and
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/// the computation tree that feeds them.
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/// If ValueSet is non-null, remove any deleted instructions from it as well.
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static void
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deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
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MemoryDependenceResults &MD, const TargetLibraryInfo &TLI,
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SmallSetVector<Value *, 16> *ValueSet = nullptr) {
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SmallVector<Instruction*, 32> NowDeadInsts;
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NowDeadInsts.push_back(I);
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--NumFastOther;
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// Keeping the iterator straight is a pain, so we let this routine tell the
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// caller what the next instruction is after we're done mucking about.
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BasicBlock::iterator NewIter = *BBI;
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// Before we touch this instruction, remove it from memdep!
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do {
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Instruction *DeadInst = NowDeadInsts.pop_back_val();
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++NumFastOther;
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// This instruction is dead, zap it, in stages. Start by removing it from
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// MemDep, which needs to know the operands and needs it to be in the
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// function.
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MD.removeInstruction(DeadInst);
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for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
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Value *Op = DeadInst->getOperand(op);
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DeadInst->setOperand(op, nullptr);
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// If this operand just became dead, add it to the NowDeadInsts list.
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if (!Op->use_empty()) continue;
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if (Instruction *OpI = dyn_cast<Instruction>(Op))
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if (isInstructionTriviallyDead(OpI, &TLI))
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NowDeadInsts.push_back(OpI);
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}
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if (NewIter == DeadInst->getIterator())
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NewIter = DeadInst->eraseFromParent();
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else
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DeadInst->eraseFromParent();
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if (ValueSet) ValueSet->remove(DeadInst);
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} while (!NowDeadInsts.empty());
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*BBI = NewIter;
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}
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/// Does this instruction write some memory? This only returns true for things
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/// that we can analyze with other helpers below.
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static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo &TLI) {
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if (isa<StoreInst>(I))
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return true;
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default:
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return false;
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case Intrinsic::memset:
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case Intrinsic::memmove:
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case Intrinsic::memcpy:
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case Intrinsic::init_trampoline:
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case Intrinsic::lifetime_end:
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return true;
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}
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}
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if (auto CS = CallSite(I)) {
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if (Function *F = CS.getCalledFunction()) {
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StringRef FnName = F->getName();
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if (TLI.has(LibFunc::strcpy) && FnName == TLI.getName(LibFunc::strcpy))
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return true;
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if (TLI.has(LibFunc::strncpy) && FnName == TLI.getName(LibFunc::strncpy))
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return true;
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if (TLI.has(LibFunc::strcat) && FnName == TLI.getName(LibFunc::strcat))
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return true;
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if (TLI.has(LibFunc::strncat) && FnName == TLI.getName(LibFunc::strncat))
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return true;
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}
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}
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return false;
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}
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/// Return a Location stored to by the specified instruction. If isRemovable
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/// returns true, this function and getLocForRead completely describe the memory
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/// operations for this instruction.
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static MemoryLocation getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
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if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
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return MemoryLocation::get(SI);
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if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
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// memcpy/memmove/memset.
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MemoryLocation Loc = MemoryLocation::getForDest(MI);
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return Loc;
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}
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IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
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if (!II)
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return MemoryLocation();
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switch (II->getIntrinsicID()) {
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default:
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return MemoryLocation(); // Unhandled intrinsic.
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case Intrinsic::init_trampoline:
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// FIXME: We don't know the size of the trampoline, so we can't really
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// handle it here.
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return MemoryLocation(II->getArgOperand(0));
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case Intrinsic::lifetime_end: {
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uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
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return MemoryLocation(II->getArgOperand(1), Len);
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}
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}
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}
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/// Return the location read by the specified "hasMemoryWrite" instruction if
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/// any.
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static MemoryLocation getLocForRead(Instruction *Inst,
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const TargetLibraryInfo &TLI) {
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assert(hasMemoryWrite(Inst, TLI) && "Unknown instruction case");
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// The only instructions that both read and write are the mem transfer
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// instructions (memcpy/memmove).
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if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
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return MemoryLocation::getForSource(MTI);
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return MemoryLocation();
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}
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/// If the value of this instruction and the memory it writes to is unused, may
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/// we delete this instruction?
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static bool isRemovable(Instruction *I) {
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// Don't remove volatile/atomic stores.
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if (StoreInst *SI = dyn_cast<StoreInst>(I))
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return SI->isUnordered();
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
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case Intrinsic::lifetime_end:
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// Never remove dead lifetime_end's, e.g. because it is followed by a
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// free.
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return false;
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case Intrinsic::init_trampoline:
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// Always safe to remove init_trampoline.
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return true;
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case Intrinsic::memset:
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case Intrinsic::memmove:
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case Intrinsic::memcpy:
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// Don't remove volatile memory intrinsics.
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return !cast<MemIntrinsic>(II)->isVolatile();
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}
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}
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if (auto CS = CallSite(I))
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return CS.getInstruction()->use_empty();
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return false;
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}
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/// Returns true if the end of this instruction can be safely shortened in
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/// length.
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static bool isShortenableAtTheEnd(Instruction *I) {
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// Don't shorten stores for now
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if (isa<StoreInst>(I))
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return false;
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: return false;
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case Intrinsic::memset:
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case Intrinsic::memcpy:
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// Do shorten memory intrinsics.
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// FIXME: Add memmove if it's also safe to transform.
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return true;
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}
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}
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// Don't shorten libcalls calls for now.
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return false;
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}
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/// Returns true if the beginning of this instruction can be safely shortened
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/// in length.
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static bool isShortenableAtTheBeginning(Instruction *I) {
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// FIXME: Handle only memset for now. Supporting memcpy/memmove should be
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// easily done by offsetting the source address.
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IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
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return II && II->getIntrinsicID() == Intrinsic::memset;
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}
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/// Return the pointer that is being written to.
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static Value *getStoredPointerOperand(Instruction *I) {
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if (StoreInst *SI = dyn_cast<StoreInst>(I))
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return SI->getPointerOperand();
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if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
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return MI->getDest();
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
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switch (II->getIntrinsicID()) {
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default: llvm_unreachable("Unexpected intrinsic!");
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case Intrinsic::init_trampoline:
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return II->getArgOperand(0);
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}
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}
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CallSite CS(I);
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// All the supported functions so far happen to have dest as their first
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// argument.
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return CS.getArgument(0);
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}
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static uint64_t getPointerSize(const Value *V, const DataLayout &DL,
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const TargetLibraryInfo &TLI) {
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uint64_t Size;
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if (getObjectSize(V, Size, DL, &TLI))
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return Size;
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return MemoryLocation::UnknownSize;
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}
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namespace {
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enum OverwriteResult {
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OverwriteBegin,
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OverwriteComplete,
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OverwriteEnd,
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OverwriteUnknown
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};
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}
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typedef DenseMap<Instruction *,
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std::map<int64_t, int64_t>> InstOverlapIntervalsTy;
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/// Return 'OverwriteComplete' if a store to the 'Later' location completely
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/// overwrites a store to the 'Earlier' location, 'OverwriteEnd' if the end of
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/// the 'Earlier' location is completely overwritten by 'Later',
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/// 'OverwriteBegin' if the beginning of the 'Earlier' location is overwritten
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/// by 'Later', or 'OverwriteUnknown' if nothing can be determined.
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static OverwriteResult isOverwrite(const MemoryLocation &Later,
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const MemoryLocation &Earlier,
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const DataLayout &DL,
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const TargetLibraryInfo &TLI,
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int64_t &EarlierOff, int64_t &LaterOff,
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Instruction *DepWrite,
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InstOverlapIntervalsTy &IOL) {
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// If we don't know the sizes of either access, then we can't do a comparison.
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if (Later.Size == MemoryLocation::UnknownSize ||
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Earlier.Size == MemoryLocation::UnknownSize)
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return OverwriteUnknown;
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const Value *P1 = Earlier.Ptr->stripPointerCasts();
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const Value *P2 = Later.Ptr->stripPointerCasts();
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// If the start pointers are the same, we just have to compare sizes to see if
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// the later store was larger than the earlier store.
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if (P1 == P2) {
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// Make sure that the Later size is >= the Earlier size.
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if (Later.Size >= Earlier.Size)
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return OverwriteComplete;
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}
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// Check to see if the later store is to the entire object (either a global,
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// an alloca, or a byval/inalloca argument). If so, then it clearly
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// overwrites any other store to the same object.
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const Value *UO1 = GetUnderlyingObject(P1, DL),
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*UO2 = GetUnderlyingObject(P2, DL);
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// If we can't resolve the same pointers to the same object, then we can't
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// analyze them at all.
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if (UO1 != UO2)
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return OverwriteUnknown;
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// If the "Later" store is to a recognizable object, get its size.
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uint64_t ObjectSize = getPointerSize(UO2, DL, TLI);
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if (ObjectSize != MemoryLocation::UnknownSize)
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if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
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return OverwriteComplete;
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// Okay, we have stores to two completely different pointers. Try to
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// decompose the pointer into a "base + constant_offset" form. If the base
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// pointers are equal, then we can reason about the two stores.
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EarlierOff = 0;
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LaterOff = 0;
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const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
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const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
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// If the base pointers still differ, we have two completely different stores.
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if (BP1 != BP2)
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return OverwriteUnknown;
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// The later store completely overlaps the earlier store if:
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//
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// 1. Both start at the same offset and the later one's size is greater than
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// or equal to the earlier one's, or
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//
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// |--earlier--|
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// |-- later --|
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//
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// 2. The earlier store has an offset greater than the later offset, but which
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// still lies completely within the later store.
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//
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// |--earlier--|
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// |----- later ------|
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//
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// We have to be careful here as *Off is signed while *.Size is unsigned.
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if (EarlierOff >= LaterOff &&
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Later.Size >= Earlier.Size &&
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uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
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return OverwriteComplete;
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// We may now overlap, although the overlap is not complete. There might also
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// be other incomplete overlaps, and together, they might cover the complete
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// earlier write.
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// Note: The correctness of this logic depends on the fact that this function
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// is not even called providing DepWrite when there are any intervening reads.
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if (EnablePartialOverwriteTracking &&
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LaterOff < int64_t(EarlierOff + Earlier.Size) &&
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int64_t(LaterOff + Later.Size) >= EarlierOff) {
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// Insert our part of the overlap into the map.
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auto &IM = IOL[DepWrite];
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DEBUG(dbgs() << "DSE: Partial overwrite: Earlier [" << EarlierOff << ", " <<
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int64_t(EarlierOff + Earlier.Size) << ") Later [" <<
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LaterOff << ", " << int64_t(LaterOff + Later.Size) << ")\n");
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// Make sure that we only insert non-overlapping intervals and combine
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// adjacent intervals. The intervals are stored in the map with the ending
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// offset as the key (in the half-open sense) and the starting offset as
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// the value.
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int64_t LaterIntStart = LaterOff, LaterIntEnd = LaterOff + Later.Size;
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// Find any intervals ending at, or after, LaterIntStart which start
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// before LaterIntEnd.
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auto ILI = IM.lower_bound(LaterIntStart);
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if (ILI != IM.end() && ILI->second <= LaterIntEnd) {
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// This existing interval is overlapped with the current store somewhere
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// in [LaterIntStart, LaterIntEnd]. Merge them by erasing the existing
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// intervals and adjusting our start and end.
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LaterIntStart = std::min(LaterIntStart, ILI->second);
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LaterIntEnd = std::max(LaterIntEnd, ILI->first);
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ILI = IM.erase(ILI);
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// Continue erasing and adjusting our end in case other previous
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// intervals are also overlapped with the current store.
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//
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// |--- ealier 1 ---| |--- ealier 2 ---|
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// |------- later---------|
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//
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while (ILI != IM.end() && ILI->second <= LaterIntEnd) {
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assert(ILI->second > LaterIntStart && "Unexpected interval");
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LaterIntEnd = std::max(LaterIntEnd, ILI->first);
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ILI = IM.erase(ILI);
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}
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}
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IM[LaterIntEnd] = LaterIntStart;
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ILI = IM.begin();
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if (ILI->second <= EarlierOff &&
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ILI->first >= int64_t(EarlierOff + Earlier.Size)) {
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DEBUG(dbgs() << "DSE: Full overwrite from partials: Earlier [" <<
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EarlierOff << ", " <<
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int64_t(EarlierOff + Earlier.Size) <<
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") Composite Later [" <<
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ILI->second << ", " << ILI->first << ")\n");
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++NumCompletePartials;
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return OverwriteComplete;
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}
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}
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// Another interesting case is if the later store overwrites the end of the
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// earlier store.
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//
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// |--earlier--|
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// |-- later --|
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//
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// In this case we may want to trim the size of earlier to avoid generating
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// writes to addresses which will definitely be overwritten later
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if (LaterOff > EarlierOff &&
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LaterOff < int64_t(EarlierOff + Earlier.Size) &&
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int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
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return OverwriteEnd;
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// Finally, we also need to check if the later store overwrites the beginning
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// of the earlier store.
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//
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// |--earlier--|
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// |-- later --|
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//
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// In this case we may want to move the destination address and trim the size
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// of earlier to avoid generating writes to addresses which will definitely
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// be overwritten later.
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if (LaterOff <= EarlierOff && int64_t(LaterOff + Later.Size) > EarlierOff) {
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assert (int64_t(LaterOff + Later.Size) < int64_t(EarlierOff + Earlier.Size)
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&& "Expect to be handled as OverwriteComplete" );
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return OverwriteBegin;
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}
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// Otherwise, they don't completely overlap.
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return OverwriteUnknown;
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}
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|
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/// If 'Inst' might be a self read (i.e. a noop copy of a
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/// memory region into an identical pointer) then it doesn't actually make its
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/// input dead in the traditional sense. Consider this case:
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///
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/// memcpy(A <- B)
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/// memcpy(A <- A)
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///
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/// In this case, the second store to A does not make the first store to A dead.
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/// The usual situation isn't an explicit A<-A store like this (which can be
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/// trivially removed) but a case where two pointers may alias.
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///
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/// This function detects when it is unsafe to remove a dependent instruction
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/// because the DSE inducing instruction may be a self-read.
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static bool isPossibleSelfRead(Instruction *Inst,
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const MemoryLocation &InstStoreLoc,
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Instruction *DepWrite,
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const TargetLibraryInfo &TLI,
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AliasAnalysis &AA) {
|
|
// Self reads can only happen for instructions that read memory. Get the
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// 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;
|
|
|
|
// Okay, 'Inst' may copy over itself. However, we can still remove a the
|
|
// DepWrite instruction if we can prove that it reads from the same location
|
|
// as Inst. This handles useful cases like:
|
|
// memcpy(A <- B)
|
|
// memcpy(A <- B)
|
|
// Here we don't know if A/B may alias, but we do know that B/B are must
|
|
// aliases, so removing the first memcpy is safe (assuming it writes <= #
|
|
// bytes as the second one.
|
|
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) {
|
|
SmallVector<BasicBlock *, 16> WorkList;
|
|
SmallPtrSet<BasicBlock *, 8> Visited;
|
|
BasicBlock::iterator FirstBBI(FirstI);
|
|
++FirstBBI;
|
|
BasicBlock::iterator SecondBBI(SecondI);
|
|
BasicBlock *FirstBB = FirstI->getParent();
|
|
BasicBlock *SecondBB = SecondI->getParent();
|
|
MemoryLocation MemLoc = MemoryLocation::get(SecondI);
|
|
|
|
// Start checking the store-block.
|
|
WorkList.push_back(SecondBB);
|
|
bool isFirstBlock = true;
|
|
|
|
// Check all blocks going backward until we reach the load-block.
|
|
while (!WorkList.empty()) {
|
|
BasicBlock *B = WorkList.pop_back_val();
|
|
|
|
// Ignore instructions before LI if this is the FirstBB.
|
|
BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
|
|
|
|
BasicBlock::iterator EI;
|
|
if (isFirstBlock) {
|
|
// Ignore instructions after SI 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 SI.
|
|
EI = B->end();
|
|
}
|
|
for (; BI != EI; ++BI) {
|
|
Instruction *I = &*BI;
|
|
if (I->mayWriteToMemory() && I != SecondI) {
|
|
auto Res = AA->getModRefInfo(I, MemLoc);
|
|
if (Res != MRI_NoModRef)
|
|
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) {
|
|
if (!Visited.insert(*PredI).second)
|
|
continue;
|
|
WorkList.push_back(*PredI);
|
|
}
|
|
}
|
|
}
|
|
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;
|
|
TerminatorInst *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) {
|
|
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 (!hasMemoryWrite(Dependency, *TLI) || !isRemovable(Dependency))
|
|
break;
|
|
|
|
Value *DepPointer =
|
|
GetUnderlyingObject(getStoredPointerOperand(Dependency), DL);
|
|
|
|
// Check for aliasing.
|
|
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
|
|
break;
|
|
|
|
// DCE instructions only used to calculate that store.
|
|
BasicBlock::iterator BBI(Dependency);
|
|
deleteDeadInstruction(Dependency, &BBI, *MD, *TLI);
|
|
++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<Value *, 16> &DeadStackObjects,
|
|
const DataLayout &DL, AliasAnalysis *AA,
|
|
const TargetLibraryInfo *TLI) {
|
|
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(const_cast<Value*>(UnderlyingPointer));
|
|
return;
|
|
}
|
|
|
|
// Remove objects that could alias LoadedLoc.
|
|
DeadStackObjects.remove_if([&](Value *I) {
|
|
// See if the loaded location could alias the stack location.
|
|
MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI));
|
|
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) {
|
|
bool MadeChange = false;
|
|
|
|
// Keep track of all of the stack objects that are dead at the end of the
|
|
// function.
|
|
SmallSetVector<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.hasByValOrInAllocaAttr())
|
|
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 (hasMemoryWrite(&*BBI, *TLI) && isRemovable(&*BBI)) {
|
|
// See through pointer-to-pointer bitcasts
|
|
SmallVector<Value *, 4> Pointers;
|
|
GetUnderlyingObjects(getStoredPointerOperand(&*BBI), Pointers, DL);
|
|
|
|
// Stores to stack values are valid candidates for removal.
|
|
bool AllDead = true;
|
|
for (Value *Pointer : Pointers)
|
|
if (!DeadStackObjects.count(Pointer)) {
|
|
AllDead = false;
|
|
break;
|
|
}
|
|
|
|
if (AllDead) {
|
|
Instruction *Dead = &*BBI;
|
|
|
|
DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
|
|
<< *Dead << "\n Objects: ";
|
|
for (SmallVectorImpl<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, &DeadStackObjects);
|
|
++NumFastStores;
|
|
MadeChange = true;
|
|
continue;
|
|
}
|
|
}
|
|
|
|
// Remove any dead non-memory-mutating instructions.
|
|
if (isInstructionTriviallyDead(&*BBI, TLI)) {
|
|
deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, &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 CS = CallSite(&*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(CS))
|
|
continue;
|
|
|
|
// If the call might load from any of our allocas, then any store above
|
|
// the call is live.
|
|
DeadStackObjects.remove_if([&](Value *I) {
|
|
// See if the call site touches the value.
|
|
ModRefInfo A = AA->getModRefInfo(CS, I, getPointerSize(I, DL, *TLI));
|
|
|
|
return A == MRI_ModRef || A == MRI_Ref;
|
|
});
|
|
|
|
// 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 (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
|
|
LoadedLoc = MemoryLocation::getForSource(MTI);
|
|
} 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);
|
|
|
|
// 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 eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
|
|
AliasAnalysis *AA, MemoryDependenceResults *MD,
|
|
const DataLayout &DL,
|
|
const TargetLibraryInfo *TLI) {
|
|
// 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)) {
|
|
|
|
DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n LOAD: "
|
|
<< *DepLoad << "\n STORE: " << *SI << '\n');
|
|
|
|
deleteDeadInstruction(SI, &BBI, *MD, *TLI);
|
|
++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)) {
|
|
DEBUG(
|
|
dbgs() << "DSE: Remove null store to the calloc'ed object:\n DEAD: "
|
|
<< *Inst << "\n OBJECT: " << *UnderlyingPointer << '\n');
|
|
|
|
deleteDeadInstruction(SI, &BBI, *MD, *TLI);
|
|
++NumRedundantStores;
|
|
return true;
|
|
}
|
|
}
|
|
return false;
|
|
}
|
|
|
|
static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
|
|
MemoryDependenceResults *MD, DominatorTree *DT,
|
|
const TargetLibraryInfo *TLI) {
|
|
const DataLayout &DL = BB.getModule()->getDataLayout();
|
|
bool MadeChange = false;
|
|
|
|
// 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);
|
|
// Increment BBI after handleFree has potentially deleted instructions.
|
|
// This ensures we maintain a valid iterator.
|
|
++BBI;
|
|
continue;
|
|
}
|
|
|
|
Instruction *Inst = &*BBI++;
|
|
|
|
// Check to see if Inst writes to memory. If not, continue.
|
|
if (!hasMemoryWrite(Inst, *TLI))
|
|
continue;
|
|
|
|
// eliminateNoopStore will update in iterator, if necessary.
|
|
if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI)) {
|
|
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, *AA);
|
|
|
|
// If we didn't get a useful location, fail.
|
|
if (!Loc.Ptr)
|
|
continue;
|
|
|
|
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();
|
|
MemoryLocation DepLoc = getLocForWrite(DepWrite, *AA);
|
|
// If we didn't get a useful location, or if it isn't a size, bail out.
|
|
if (!DepLoc.Ptr)
|
|
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.
|
|
if (isRemovable(DepWrite) &&
|
|
!isPossibleSelfRead(Inst, Loc, DepWrite, *TLI, *AA)) {
|
|
int64_t InstWriteOffset, DepWriteOffset;
|
|
OverwriteResult OR =
|
|
isOverwrite(Loc, DepLoc, DL, *TLI, DepWriteOffset, InstWriteOffset,
|
|
DepWrite, IOL);
|
|
if (OR == OverwriteComplete) {
|
|
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);
|
|
++NumFastStores;
|
|
MadeChange = true;
|
|
|
|
// We erased DepWrite; start over.
|
|
InstDep = MD->getDependency(Inst);
|
|
continue;
|
|
} else if ((OR == OverwriteEnd && isShortenableAtTheEnd(DepWrite)) ||
|
|
((OR == OverwriteBegin &&
|
|
isShortenableAtTheBeginning(DepWrite)))) {
|
|
// 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
|
|
MemIntrinsic *DepIntrinsic = cast<MemIntrinsic>(DepWrite);
|
|
unsigned DepWriteAlign = DepIntrinsic->getAlignment();
|
|
bool IsOverwriteEnd = (OR == OverwriteEnd);
|
|
if (!IsOverwriteEnd)
|
|
InstWriteOffset = int64_t(InstWriteOffset + Loc.Size);
|
|
|
|
if ((llvm::isPowerOf2_64(InstWriteOffset) &&
|
|
DepWriteAlign <= InstWriteOffset) ||
|
|
((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
|
|
|
|
DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
|
|
<< (IsOverwriteEnd ? "END" : "BEGIN") << ": "
|
|
<< *DepWrite << "\n KILLER (offset "
|
|
<< InstWriteOffset << ", " << DepLoc.Size << ")"
|
|
<< *Inst << '\n');
|
|
|
|
int64_t NewLength =
|
|
IsOverwriteEnd
|
|
? InstWriteOffset - DepWriteOffset
|
|
: DepLoc.Size - (InstWriteOffset - DepWriteOffset);
|
|
|
|
Value *DepWriteLength = DepIntrinsic->getLength();
|
|
Value *TrimmedLength =
|
|
ConstantInt::get(DepWriteLength->getType(), NewLength);
|
|
DepIntrinsic->setLength(TrimmedLength);
|
|
|
|
if (!IsOverwriteEnd) {
|
|
int64_t OffsetMoved = (InstWriteOffset - DepWriteOffset);
|
|
Value *Indices[1] = {
|
|
ConstantInt::get(DepWriteLength->getType(), OffsetMoved)};
|
|
GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds(
|
|
DepIntrinsic->getRawDest(), Indices, "", DepWrite);
|
|
DepIntrinsic->setDest(NewDestGEP);
|
|
}
|
|
MadeChange = true;
|
|
}
|
|
}
|
|
}
|
|
|
|
// 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 (AA->getModRefInfo(DepWrite, Loc) & MRI_Ref)
|
|
break;
|
|
|
|
InstDep = MD->getPointerDependencyFrom(Loc, false,
|
|
DepWrite->getIterator(), &BB);
|
|
}
|
|
}
|
|
|
|
// 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);
|
|
|
|
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;
|
|
}
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
// DSE Pass
|
|
//===----------------------------------------------------------------------===//
|
|
PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
|
|
AliasAnalysis *AA = &AM.getResult<AAManager>(F);
|
|
DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
|
|
MemoryDependenceResults *MD = &AM.getResult<MemoryDependenceAnalysis>(F);
|
|
const TargetLibraryInfo *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
|
|
|
|
if (!eliminateDeadStores(F, AA, MD, DT, TLI))
|
|
return PreservedAnalyses::all();
|
|
PreservedAnalyses PA;
|
|
PA.preserve<DominatorTreeAnalysis>();
|
|
PA.preserve<GlobalsAA>();
|
|
PA.preserve<MemoryDependenceAnalysis>();
|
|
return PA;
|
|
}
|
|
|
|
namespace {
|
|
/// A legacy pass for the legacy pass manager that wraps \c DSEPass.
|
|
class DSELegacyPass : public FunctionPass {
|
|
public:
|
|
DSELegacyPass() : FunctionPass(ID) {
|
|
initializeDSELegacyPassPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
bool runOnFunction(Function &F) override {
|
|
if (skipFunction(F))
|
|
return false;
|
|
|
|
DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
|
|
AliasAnalysis *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
|
|
MemoryDependenceResults *MD =
|
|
&getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
|
|
const TargetLibraryInfo *TLI =
|
|
&getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
|
|
|
|
return eliminateDeadStores(F, AA, MD, DT, TLI);
|
|
}
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override {
|
|
AU.setPreservesCFG();
|
|
AU.addRequired<DominatorTreeWrapperPass>();
|
|
AU.addRequired<AAResultsWrapperPass>();
|
|
AU.addRequired<MemoryDependenceWrapperPass>();
|
|
AU.addRequired<TargetLibraryInfoWrapperPass>();
|
|
AU.addPreserved<DominatorTreeWrapperPass>();
|
|
AU.addPreserved<GlobalsAAWrapperPass>();
|
|
AU.addPreserved<MemoryDependenceWrapperPass>();
|
|
}
|
|
|
|
static char ID; // Pass identification, replacement for typeid
|
|
};
|
|
} // end anonymous namespace
|
|
|
|
char DSELegacyPass::ID = 0;
|
|
INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false,
|
|
false)
|
|
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
|
|
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
|
|
INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
|
|
false)
|
|
|
|
FunctionPass *llvm::createDeadStoreEliminationPass() {
|
|
return new DSELegacyPass();
|
|
}
|