[DSE] Don't remove stores made live by a call which unwinds.

Issue exposed by noalias or more aggressive alias analysis.

Fixes http://llvm.org/PR25422.

Differential revision: https://reviews.llvm.org/D21007

llvm-svn: 278451

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -70,6 +70,7 @@ static void
 deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
                       MemoryDependenceResults &MD, const TargetLibraryInfo &TLI,
                       InstOverlapIntervalsTy &IOL,
+                      DenseMap<Instruction*, size_t> *InstrOrdering,
                       SmallSetVector<Value *, 16> *ValueSet = nullptr) {
   SmallVector<Instruction*, 32> NowDeadInsts;
 
@@ -102,15 +103,14 @@ deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI,
           NowDeadInsts.push_back(OpI);
     }
 
+    if (ValueSet) ValueSet->remove(DeadInst);
+    InstrOrdering->erase(DeadInst);
+    IOL.erase(DeadInst);
 
     if (NewIter == DeadInst->getIterator())
       NewIter = DeadInst->eraseFromParent();
     else
       DeadInst->eraseFromParent();
-
-    IOL.erase(DeadInst);
-
-    if (ValueSet) ValueSet->remove(DeadInst);
   } while (!NowDeadInsts.empty());
   *BBI = NewIter;
 }
@@ -590,7 +590,8 @@ static void findUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
 static bool handleFree(CallInst *F, AliasAnalysis *AA,
                        MemoryDependenceResults *MD, DominatorTree *DT,
                        const TargetLibraryInfo *TLI,
-                       InstOverlapIntervalsTy &IOL) {
+                       InstOverlapIntervalsTy &IOL,
+                       DenseMap<Instruction*, size_t> *InstrOrdering) {
   bool MadeChange = false;
 
   MemoryLocation Loc = MemoryLocation(F->getOperand(0));
@@ -622,7 +623,7 @@ static bool handleFree(CallInst *F, AliasAnalysis *AA,
 
       // DCE instructions only used to calculate that store.
       BasicBlock::iterator BBI(Dependency);
-      deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL);
+      deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL, InstrOrdering);
       ++NumFastStores;
       MadeChange = true;
 
@@ -678,7 +679,8 @@ static void removeAccessedObjects(const MemoryLocation &LoadedLoc,
 static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
                              MemoryDependenceResults *MD,
                              const TargetLibraryInfo *TLI,
-                             InstOverlapIntervalsTy &IOL) {
+                             InstOverlapIntervalsTy &IOL,
+                             DenseMap<Instruction*, size_t> *InstrOrdering) {
   bool MadeChange = false;
 
   // Keep track of all of the stack objects that are dead at the end of the
@@ -737,7 +739,7 @@ static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
               dbgs() << '\n');
 
         // DCE instructions only used to calculate that store.
-        deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, &DeadStackObjects);
+        deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, InstrOrdering, &DeadStackObjects);
         ++NumFastStores;
         MadeChange = true;
         continue;
@@ -748,7 +750,7 @@ static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA,
     if (isInstructionTriviallyDead(&*BBI, TLI)) {
       DEBUG(dbgs() << "DSE: Removing trivially dead instruction:\n  DEAD: "
                    << *&*BBI << '\n');
-      deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, &DeadStackObjects);
+      deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, InstrOrdering, &DeadStackObjects);
       ++NumFastOther;
       MadeChange = true;
       continue;
@@ -947,7 +949,8 @@ static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
                                AliasAnalysis *AA, MemoryDependenceResults *MD,
                                const DataLayout &DL,
                                const TargetLibraryInfo *TLI,
-                               InstOverlapIntervalsTy &IOL) {
+                               InstOverlapIntervalsTy &IOL,
+                               DenseMap<Instruction*, size_t> *InstrOrdering) {
   // Must be a store instruction.
   StoreInst *SI = dyn_cast<StoreInst>(Inst);
   if (!SI)
@@ -962,7 +965,7 @@ static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
       DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  LOAD: "
                    << *DepLoad << "\n  STORE: " << *SI << '\n');
 
-      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL);
+      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, InstrOrdering);
       ++NumRedundantStores;
       return true;
     }
@@ -980,7 +983,7 @@ static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI,
           dbgs() << "DSE: Remove null store to the calloc'ed object:\n  DEAD: "
                  << *Inst << "\n  OBJECT: " << *UnderlyingPointer << '\n');
 
-      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL);
+      deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, InstrOrdering);
       ++NumRedundantStores;
       return true;
     }
@@ -994,6 +997,12 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
   const DataLayout &DL = BB.getModule()->getDataLayout();
   bool MadeChange = false;
 
+  // FIXME: Maybe change this to use some abstraction like OrderedBasicBlock?
+  // The current OrderedBasicBlock can't deal with mutation at the moment.
+  size_t LastThrowingInstIndex = 0;
+  DenseMap<Instruction*, size_t> InstrOrdering;
+  size_t InstrIndex = 1;
+
   // A map of interval maps representing partially-overwritten value parts.
   InstOverlapIntervalsTy IOL;
 
@@ -1001,7 +1010,7 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
   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);
+      MadeChange |= handleFree(F, AA, MD, DT, TLI, IOL, &InstrOrdering);
       // Increment BBI after handleFree has potentially deleted instructions.
       // This ensures we maintain a valid iterator.
       ++BBI;
@@ -1010,12 +1019,19 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
 
     Instruction *Inst = &*BBI++;
 
+    size_t CurInstNumber = InstrIndex++;
+    InstrOrdering.insert(std::make_pair(Inst, CurInstNumber));
+    if (Inst->mayThrow()) {
+      LastThrowingInstIndex = CurInstNumber;
+      continue;
+    }
+
     // 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, IOL)) {
+    if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI, IOL, &InstrOrdering)) {
       MadeChange = true;
       continue;
     }
@@ -1049,6 +1065,31 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
       if (!DepLoc.Ptr)
         break;
 
+      // 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.
+      size_t DepIndex = InstrOrdering.lookup(DepWrite);
+      assert(DepIndex && "Unexpected instruction");
+      if (DepIndex <= LastThrowingInstIndex) {
+        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.
@@ -1063,7 +1104,7 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
                 << *DepWrite << "\n  KILLER: " << *Inst << '\n');
 
           // Delete the store and now-dead instructions that feed it.
-          deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL);
+          deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, &InstrOrdering);
           ++NumFastStores;
           MadeChange = true;
 
@@ -1109,7 +1150,7 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
   // 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);
+    MadeChange |= handleEndBlock(BB, AA, MD, TLI, IOL, &InstrOrdering);
 
   return MadeChange;
 }
@@ -1123,6 +1164,7 @@ static bool eliminateDeadStores(Function &F, AliasAnalysis *AA,
     // cycles that will confuse alias analysis.
     if (DT->isReachableFromEntry(&BB))
       MadeChange |= eliminateDeadStores(BB, AA, MD, DT, TLI);
+
   return MadeChange;
 }
 

llvm/test/Analysis/GlobalsModRef/func-memattributes.ll

@@ -2,30 +2,30 @@
 
 @X = internal global i32 4
 
-define void @test0() {
+define i32 @test0() {
 ; CHECK-LABEL: @test0
 ; CHECK: store i32 0, i32* @X
-; CHECK-NEXT: call void @func_readonly() #0
+; CHECK-NEXT: call i32 @func_readonly() #0
 ; CHECK-NEXT: store i32 1, i32* @X
   store i32 0, i32* @X
-  call void @func_readonly() #0
+  %x = call i32 @func_readonly() #0
   store i32 1, i32* @X
-  ret void
+  ret i32 %x
 }
 
-define void @test1() {
+define i32 @test1() {
 ; CHECK-LABEL: @test1
 ; CHECK-NOT: store
-; CHECK: call void @func_read_argmem_only() #1
+; CHECK: call i32 @func_read_argmem_only() #1
 ; CHECK-NEXT: store i32 3, i32* @X
   store i32 2, i32* @X
-  call void @func_read_argmem_only() #1
+  %x = call i32 @func_read_argmem_only() #1
   store i32 3, i32* @X
-  ret void
+  ret i32 %x
 }
 
-declare void @func_readonly() #0
+declare i32 @func_readonly() #0
-declare void @func_read_argmem_only() #1
+declare i32 @func_read_argmem_only() #1
 
-attributes #0 = { readonly }
+attributes #0 = { readonly nounwind }
-attributes #1 = { readonly argmemonly }
+attributes #1 = { readonly argmemonly nounwind }

llvm/test/Transforms/DeadStoreElimination/free.ll

@@ -13,7 +13,7 @@ define void @test(i32* %Q, i32* %P) {
         %DEAD = load i32, i32* %Q            ; <i32> [#uses=1]
         store i32 %DEAD, i32* %P
         %1 = bitcast i32* %P to i8*
-        tail call void @free(i8* %1)
+        tail call void @free(i8* %1) nounwind
         ret void
 }
 
@@ -25,7 +25,7 @@ define void @test2({i32, i32}* %P) {
 	%Q = getelementptr {i32, i32}, {i32, i32} *%P, i32 0, i32 1
 	store i32 4, i32* %Q
         %1 = bitcast {i32, i32}* %P to i8*
-        tail call void @free(i8* %1)
+        tail call void @free(i8* %1) nounwind
 	ret void
 }
 
@@ -37,7 +37,7 @@ define void @test3() {
   store i8 0, i8* %m
   %m1 = getelementptr i8, i8* %m, i64 1
   store i8 1, i8* %m1
-  call void @free(i8* %m)
+  call void @free(i8* %m) nounwind
   ret void
 }
 

llvm/test/Transforms/DeadStoreElimination/simple.ll

@@ -498,3 +498,26 @@ bb3:
   ret i32 0
 }
 
+; Don't remove redundant store: unknown_func could unwind
+; CHECK-LABEL: @test34(
+; CHECK: store i32 1
+; CHECK: store i32 0
+; CHECK: ret
+define void @test34(i32* noalias %p) {
+  store i32 1, i32* %p
+  call void @unknown_func()
+  store i32 0, i32* %p
+  ret void
+}
+
+; Remove redundant store even with an unwinding function in the same block
+; CHECK-LABEL: @test35(
+; CHECK: call void @unknown_func
+; CHECK-NEXT: store i32 0
+; CHECK-NEXT: ret void
+define void @test35(i32* noalias %p) {
+  call void @unknown_func()
+  store i32 1, i32* %p
+  store i32 0, i32* %p
+  ret void
+}