[DeadStoreElimination] Remove dead zero store to calloc initialized memory

[igor-laevsky]

Summary

This change allows dead store elimination to remove zero and null stores into memory freshly allocated with calloc-like function.

Philip, this is variation of your old change (http://reviews.llvm.org/D3942). Hopefully all issues with msan should be fixed.

[reames]

Mostly looks pretty good. Minor, but required, comments.

lib/Transforms/Scalar/DeadStoreElimination.cpp

535

This should be outside the surrounding loop.

537

The code here looks correct, but I'm a bit concerned about compile time impact. GetUnderlyingObject might be fairly slow (since it has to chase through GEPs and bitcasts). Possibly you should check to see if you have an appropriate constant before executing this query?

663

The rename looks correct, but I haven't reviewed the correctness of the existing code.

test/Transforms/DeadStoreElimination/calloc-store.ll

9

Please add a couple of negative tests, specifically:

• base object is not a calloc function
• memory is clobbered
• store is volatile (i.e. not removable)
• store of wrong constant
• store of non-constant

[reames]

LGTM w/one minor comment.

test/Transforms/DeadStoreElimination/calloc-store.ll

47

Stale comment.

lib/Transforms/Scalar/DeadStoreElimination.cpp

bool runOnFunction(Function &F) override {
         if (DT->isReachableFromEntry(I))
           Changed |= runOnBasicBlock(*I);
 
       AA = nullptr; MD = nullptr; DT = nullptr;
       return Changed;
     }
 
     bool runOnBasicBlock(BasicBlock &BB);
-    bool MemoryIsNotModifiedBetween(LoadInst *LI, StoreInst *SI);
+    bool MemoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI);
     bool HandleFree(CallInst *F);
     bool handleEndBlock(BasicBlock &BB);
     void RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
                                SmallSetVector<Value *, 16> &DeadStackObjects,
                                const DataLayout &DL);
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.setPreservesCFG();
for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
 
     // If we find something that writes memory, get its memory dependence.
     if (!hasMemoryWrite(Inst, *TLI))
       continue;
 
     // If we're storing the same value back to a pointer that we just
     // loaded from, then the store can be removed.
     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
-      if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
-        if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
-            isRemovable(SI) &&
-            MemoryIsNotModifiedBetween(DepLoad, SI)) {
-
-          DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  "
-                       << "LOAD: " << *DepLoad << "\n  STORE: " << *SI << '\n');
 
-          // DeleteDeadInstruction can delete the current instruction.  Save BBI
-          // in case we need it.
-          WeakVH NextInst(BBI);
+      auto RemoveDeadInstAndUpdateBBI = [&](Instruction *DeadInst) {
 
-          DeleteDeadInstruction(SI, *MD, *TLI);
+        DeleteDeadInstruction(DeadInst, *MD, *TLI);
 
-          if (!NextInst)  // Next instruction deleted.
-            BBI = BB.begin();
-          else if (BBI != BB.begin())  // Revisit this instruction if possible.
-            --BBI;
-          ++NumRedundantStores;
-          MadeChange = true;
+        if (!NextInst) // Next instruction deleted.
+        else if (BBI != BB.begin()) // Revisit this instruction if possible.
+      };
+
+      if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
+        if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
+            isRemovable(SI) &&
+            MemoryIsNotModifiedBetween(DepLoad, SI)) {
+
+          DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  "
+                       << "LOAD: " << *DepLoad << "\n  STORE: " << *SI << '\n');
+
+          RemoveDeadInstAndUpdateBBI(SI);
           continue;
         }
       }
+
+      // Remove null stores into the calloc'ed objects
+      const DataLayout &DL = BB.getModule()->getDataLayout();
+      Instruction *UnderlyingPointer = dyn_cast<Instruction>(
+          GetUnderlyingObject(SI->getPointerOperand(), DL));
+      Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand());
+
+      if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) &&
+          StoredConstant && StoredConstant->isNullValue() &&
+          isRemovable(SI) &&
+          MemoryIsNotModifiedBetween(UnderlyingPointer, SI)) {
+        DEBUG(dbgs()
+              << "DSE: Remove null store to the calloc'ed object:\n  DEAD: "
+              << *Inst << "\n  OBJECT: " << *UnderlyingPointer << '\n');
+
+        RemoveDeadInstAndUpdateBBI(SI);
+        continue;
+      }
     }
 
     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;
bool DSE::runOnBasicBlock(BasicBlock &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);
 
   return MadeChange;
 }
 
-/// Returns true if the memory which is accessed by the store instruction is not
-/// modified between the load and the store instruction.
-/// Precondition: The store instruction must be dominated by the load
+/// 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.
-bool DSE::MemoryIsNotModifiedBetween(LoadInst *LI, StoreInst *SI) {
+bool DSE::MemoryIsNotModifiedBetween(Instruction *FirstI,
+                                     Instruction *SecondI) {
   SmallVector<BasicBlock *, 16> WorkList;
   SmallPtrSet<BasicBlock *, 8> Visited;
-  BasicBlock::iterator LoadBBI(LI);
-  ++LoadBBI;
-  BasicBlock::iterator StoreBBI(SI);
-  BasicBlock *LoadBB = LI->getParent();
-  BasicBlock *StoreBB = SI->getParent();
-  MemoryLocation StoreLoc = MemoryLocation::get(SI);
+  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(StoreBB);
+  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 LoadBB.
-    BasicBlock::iterator BI = (B == LoadBB ? LoadBBI : B->begin());
+    // 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 StoreBB.
-      assert(B == StoreBB && "first block is not the store block");
-      EI = StoreBBI;
+      // 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 StoreBB or (in case of a loop) the second visit of StoreBB.
+      // 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 != SI) {
-        auto Res = AA->getModRefInfo(I, StoreLoc);
+      if (I->mayWriteToMemory() && I != SecondI) {
+        auto Res = AA->getModRefInfo(I, MemLoc);
         if (Res != MRI_NoModRef)
           return false;
       }
     }
-    if (B != LoadBB) {
-      assert(B != &LoadBB->getParent()->getEntryBlock() &&
+    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);
       }
     }
   }

test/Transforms/DeadStoreElimination/calloc-store.ll

+; RUN: opt < %s -basicaa -dse -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+; Function Attrs: nounwind
+declare noalias i8* @calloc(i64, i64)
+
+; Function Attrs: nounwind uwtable
+define noalias i32* @test_store() {
+; CHECK-LABEL: test_store
+  %1 = tail call noalias i8* @calloc(i64 1, i64 4)
+  %2 = bitcast i8* %1 to i32*
+  ; This store is dead and should be removed
+  store i32 0, i32* %2, align 4
+; CHECK-NOT: store i32 0, i32* %2, align 4
+  ret i32* %2
+}
+