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Add support for cross block dse.
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This patch enables dead stroe elimination across basicblocks.

Example:
define void @test_02(i32 %N) {
  %1 = alloca i32
  store i32 %N, i32* %1
  store i32 10, i32* @x
  %2 = load i32, i32* %1
  %3 = icmp ne i32 %2, 0
  br i1 %3, label %4, label %5

; <label>:4
  store i32 5, i32* @x
  br label %7

; <label>:5
  %6 = load i32, i32* @x
  store i32 %6, i32* @y
  br label %7

; <label>:7
  store i32 15, i32* @x
  ret void
}
In the above example dead store "store i32 5, i32* @x" is now eliminated.

Differential Revision: http://reviews.llvm.org/D11143

llvm-svn: 245025
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@kvbhat
kvbhat committed Aug 14, 2015
1 parent d541e73 commit ddc2a86
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Showing 3 changed files with 404 additions and 51 deletions.
275 changes: 224 additions & 51 deletions llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
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Expand Up @@ -16,13 +16,16 @@
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
Expand All @@ -42,19 +45,49 @@ using namespace llvm;

STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumCrossBlockStores, "Number of cross block stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");

namespace {
struct DSE : public FunctionPass {
AliasAnalysis *AA;
MemoryDependenceAnalysis *MD;
DominatorTree *DT;
PostDominatorTree *PDT;
const TargetLibraryInfo *TLI;

SmallVector<SmallVector<StoreInst *, 8>, 16> Candidates;
SetVector<StoreInst *> DeadStores;
SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32>
BackEdges;
DenseSet<std::pair<const BasicBlock *, const BasicBlock *>> BackEdgesMap;
static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass(ID), AA(nullptr), MD(nullptr), DT(nullptr) {
DSE()
: FunctionPass(ID), AA(nullptr), MD(nullptr), DT(nullptr),
PDT(nullptr) {
initializeDSEPass(*PassRegistry::getPassRegistry());
}
// Return all stores in a given BasicBlock.
SmallVector<StoreInst *, 8> getStores(BasicBlock *BB) {
SmallVector<StoreInst *, 8> VecStores;
for (auto &BI : *BB) {
if (StoreInst *SI = dyn_cast<StoreInst>(&BI))
VecStores.push_back(SI);
}
return VecStores;
}

// Get dfs in/out on the PDT and populate Candidates store list which
// is used to find potential dead stores for a given block
void populateCandidateStores(Function &F) {
for (auto &I : F) {
DomTreeNode *DTNode = PDT->getNode(&I);
if (!DTNode)
continue;
int DFSIn = DTNode->getDFSNumIn();
SmallVector<StoreInst *, 8> VecStores = getStores(&I);
Candidates[DFSIn] = VecStores;
}
}

bool runOnFunction(Function &F) override {
if (skipOptnoneFunction(F))
Expand All @@ -64,7 +97,21 @@ namespace {
MD = &getAnalysis<MemoryDependenceAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();

PDT = &getAnalysis<PostDominatorTree>();
if (PDT->getRootNode()) {
int Count = PDT->getRootNode()->getDFSNumOut();
SmallVector<StoreInst *, 8> VecStores;
Candidates.resize(Count + 1);
Candidates.assign(Count + 1, VecStores);

// If we have more than 1 block try to populate candidate store.
if (Count > 1) {
populateCandidateStores(F);
FindFunctionBackedges(F, BackEdges);
for (auto I : BackEdges)
BackEdgesMap.insert(I);
}
}
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
// Only check non-dead blocks. Dead blocks may have strange pointer
Expand All @@ -83,16 +130,23 @@ namespace {
void RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
SmallSetVector<Value *, 16> &DeadStackObjects,
const DataLayout &DL);

void handleNonLocalStoreDeletion(StoreInst *SI);
bool isSafeCandidateForDeletion(BasicBlock *SrcBlock, BasicBlock *SinkBlock,
StoreInst *SI);
void DeleteDeadInstruction(Instruction *I, MemoryDependenceAnalysis &MD,
const TargetLibraryInfo &TLI,
SmallSetVector<Value *, 16> *ValueSet = nullptr);
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addRequired<PostDominatorTree>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addPreserved<AliasAnalysis>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<MemoryDependenceAnalysis>();
AU.addPreserved<PostDominatorTree>();
}
};
}
Expand All @@ -102,6 +156,7 @@ INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTree)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)

Expand All @@ -111,50 +166,6 @@ FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
// Helper functions
//===----------------------------------------------------------------------===//

/// DeleteDeadInstruction - 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,
MemoryDependenceAnalysis &MD,
const TargetLibraryInfo &TLI,
SmallSetVector<Value*, 16> *ValueSet = nullptr) {
SmallVector<Instruction*, 32> NowDeadInsts;

NowDeadInsts.push_back(I);
--NumFastOther;

// Before we touch this instruction, remove it from memdep!
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
++NumFastOther;

// 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);
}

DeadInst->eraseFromParent();

if (ValueSet) ValueSet->remove(DeadInst);
} while (!NowDeadInsts.empty());
}


/// hasMemoryWrite - Does this instruction write some memory? This only returns
/// true for things that we can analyze with other helpers below.
static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo &TLI) {
Expand Down Expand Up @@ -527,10 +538,15 @@ bool DSE::runOnBasicBlock(BasicBlock &BB) {

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())
if (!InstDep.isDef() && !InstDep.isClobber() && !InstDep.isNonLocal())
continue;
if (InstDep.isNonLocal()) {
if (!PDT->getRootNode())
continue;
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
handleNonLocalStoreDeletion(SI);
continue;
}

// Figure out what location is being stored to.
MemoryLocation Loc = getLocForWrite(Inst, *AA);
Expand Down Expand Up @@ -704,6 +720,50 @@ static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
}
}

/// DeleteDeadInstruction - 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.
void DSE::DeleteDeadInstruction(Instruction *I, MemoryDependenceAnalysis &MD,
const TargetLibraryInfo &TLI,
SmallSetVector<Value *, 16> *ValueSet) {
SmallVector<Instruction *, 32> NowDeadInsts;

NowDeadInsts.push_back(I);
--NumFastOther;

// Before we touch this instruction, remove it from memdep!
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
++NumFastOther;
if (StoreInst *SI = dyn_cast<StoreInst>(DeadInst))
DeadStores.insert(SI);

// 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);
}

DeadInst->eraseFromParent();

if (ValueSet)
ValueSet->remove(DeadInst);
} while (!NowDeadInsts.empty());
}

/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
Expand Down Expand Up @@ -931,3 +991,116 @@ void DSE::RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
return !AA->isNoAlias(StackLoc, LoadedLoc);
});
}

/// isSafeCandidateForDeletion- Check all paths from the SrcBlock till
/// SinkBlock to see if Store 'SI' is safe to be remove.
/// Returns true if the candidate store SI is safe to delete
/// else returns false.
bool DSE::isSafeCandidateForDeletion(BasicBlock *SrcBlock,
BasicBlock *SinkBlock, StoreInst *SI) {
SmallVector<BasicBlock *, 16> WorkList;
SmallPtrSet<BasicBlock *, 8> Visited;
BasicBlock::iterator BBI(SI);

// Check from the store till end of block and make sure we have no references
// to memory stored by this Store Instruction.
for (auto BI = ++BBI, BE = SrcBlock->end(); BI != BE; ++BI) {
Instruction *I = BI;
StoreInst *CSI = dyn_cast<StoreInst>(I);
if (CSI) {
AliasResult R =
AA->alias(MemoryLocation::get(SI), MemoryLocation::get(CSI));
if (R == MustAlias)
return true;
} else {
ModRefInfo Res = AA->getModRefInfo(I, MemoryLocation::get(SI));
if (Res != MRI_NoModRef)
return false;
}
}

// Add successors of the block to stack and start DFS.
for (succ_iterator I = succ_begin(SrcBlock), E = succ_end(SrcBlock); I != E;
++I) {
if (!Visited.insert(*I).second)
continue;
// A path with backedge may not be safe. Conservatively mark
// this store unsafe.
if (BackEdgesMap.count(std::make_pair(SrcBlock, *I)))
return false;
WorkList.push_back(*I);
}

while (!WorkList.empty()) {
BasicBlock *B = WorkList.pop_back_val();
auto BI = B->begin();
auto BE = B->end();
for (; BI != BE; ++BI) {
Instruction *I = BI;
StoreInst *CSI = dyn_cast<StoreInst>(I);
if (CSI) {
AliasResult R =
AA->alias(MemoryLocation::get(SI), MemoryLocation::get(CSI));
if (R == MustAlias)
break;
} else {
ModRefInfo Res = AA->getModRefInfo(I, MemoryLocation::get(SI));
if (Res != MRI_NoModRef)
return false;
}
}

// If we reached the sink node or we found a block which has a stores that
// overwrites the candidate block we need not look at their successors.
if (B == SinkBlock || BI != BE)
continue;

for (succ_iterator I = succ_begin(B), E = succ_end(B); I != E; ++I) {
if (!Visited.insert(*I).second)
continue;
// A path with backedge may not be safe.Conservatively mark
// this store unsafe.
if (BackEdgesMap.count(std::make_pair(B, *I)))
return false;
WorkList.push_back(*I);
}
}

return true;
}

/// handleNonLocalStoreDeletion - Handle non local dead store elimination.
/// This works by finding candidate stores using PDT and then running DFS
/// from candidate store block checking all paths to make sure the store is
/// safe to delete.
void DSE::handleNonLocalStoreDeletion(StoreInst *SI) {
BasicBlock *BB = SI->getParent();
Value *Pointer = SI->getPointerOperand();
DomTreeNode *DTNode = PDT->getNode(BB);
if (!DTNode)
return;

int DFSNumIn = DTNode->getDFSNumIn();
int DFSNumOut = DTNode->getDFSNumOut();
for (int i = DFSNumIn + 1; i < DFSNumOut; ++i) {
for (auto &I : Candidates[i]) {
StoreInst *CandidateSI = I;
if (DeadStores.count(CandidateSI))
continue;
Value *MemPtr = CandidateSI->getPointerOperand();
if (!MemPtr)
continue;
if (Pointer->getType() != MemPtr->getType())
continue;
AliasResult R =
AA->alias(MemoryLocation::get(SI), MemoryLocation::get(CandidateSI));
if (R != MustAlias)
continue;
if (isSafeCandidateForDeletion(CandidateSI->getParent(), BB,
CandidateSI)) {
DeleteDeadInstruction(CandidateSI, *MD, *TLI);
++NumCrossBlockStores;
}
}
}
}

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