[DSE] Eliminate zero memset after calloc

Solves PR11896

As noted, this can be improved futher (calloc -> malloc) in some cases. But for know, this is the first step.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D101391

Author: davidbolvansky

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1770,6 +1770,25 @@ struct DSEState {
   bool storeIsNoop(MemoryDef *Def, const MemoryLocation &DefLoc,
                    const Value *DefUO) {
     StoreInst *Store = dyn_cast<StoreInst>(Def->getMemoryInst());
+    MemSetInst *MemSet = dyn_cast<MemSetInst>(Def->getMemoryInst());
+    Constant *StoredConstant = nullptr;
+    if (Store)
+      StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
+    if (MemSet)
+      StoredConstant = dyn_cast<Constant>(MemSet->getValue());
+
+    if (StoredConstant && StoredConstant->isNullValue()) {
+      auto *DefUOInst = dyn_cast<Instruction>(DefUO);
+      if (DefUOInst && isCallocLikeFn(DefUOInst, &TLI)) {
+        auto *UnderlyingDef = cast<MemoryDef>(MSSA.getMemoryAccess(DefUOInst));
+        // If UnderlyingDef is the clobbering access of Def, no instructions
+        // between them can modify the memory location.
+        auto *ClobberDef =
+            MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def);
+        return UnderlyingDef == ClobberDef;
+      }
+    }
+
     if (!Store)
       return false;
 
@@ -1817,18 +1836,6 @@ struct DSEState {
       }
     }
 
-    Constant *StoredConstant = dyn_cast<Constant>(Store->getOperand(0));
-    if (StoredConstant && StoredConstant->isNullValue()) {
-      auto *DefUOInst = dyn_cast<Instruction>(DefUO);
-      if (DefUOInst && isCallocLikeFn(DefUOInst, &TLI)) {
-        auto *UnderlyingDef = cast<MemoryDef>(MSSA.getMemoryAccess(DefUOInst));
-        // If UnderlyingDef is the clobbering access of Def, no instructions
-        // between them can modify the memory location.
-        auto *ClobberDef =
-            MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def);
-        return UnderlyingDef == ClobberDef;
-      }
-    }
     return false;
   }
 };

llvm/test/Transforms/DeadStoreElimination/noop-stores.ll

@@ -324,18 +324,27 @@ define i8* @store_zero_after_calloc_inaccessiblememonly() {
 define i8* @zero_memset_after_calloc()  {
 ; CHECK-LABEL: @zero_memset_after_calloc(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 40000, i1 false)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;
   %call = tail call i8* @calloc(i64 10000, i64 4)
   call void @llvm.memset.p0i8.i64(i8* %call, i8 0, i64 40000, i1 false)
   ret i8* %call
 }
 
+define i8* @volatile_zero_memset_after_calloc()  {
+; CHECK-LABEL: @volatile_zero_memset_after_calloc(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 40000, i1 true)
+; CHECK-NEXT:    ret i8* [[CALL]]
+;
+  %call = tail call i8* @calloc(i64 10000, i64 4)
+  call void @llvm.memset.p0i8.i64(i8* %call, i8 0, i64 40000, i1 true)
+  ret i8* %call
+}
+
 define i8* @zero_memset_and_store_after_calloc(i8 %v)  {
 ; CHECK-LABEL: @zero_memset_and_store_after_calloc(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 40000, i1 false)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;
   %call = tail call i8* @calloc(i64 10000, i64 4)
@@ -347,7 +356,6 @@ define i8* @zero_memset_and_store_after_calloc(i8 %v)  {
 define i8* @partial_zero_memset_after_calloc() {
 ; CHECK-LABEL: @partial_zero_memset_after_calloc(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 20, i1 false)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;
   %call = tail call i8* @calloc(i64 10000, i64 4)
@@ -360,7 +368,6 @@ define i8* @partial_zero_memset_and_store_after_calloc(i8 %v)  {
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, i8* [[CALL]], i64 30
 ; CHECK-NEXT:    store i8 [[V:%.*]], i8* [[GEP]], align 1
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 20, i1 false)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;
   %call = tail call i8* @calloc(i64 10000, i64 4)
@@ -373,7 +380,6 @@ define i8* @partial_zero_memset_and_store_after_calloc(i8 %v)  {
 define i8* @zero_memset_and_store_with_dyn_index_after_calloc(i8 %v, i64 %idx)  {
 ; CHECK-LABEL: @zero_memset_and_store_with_dyn_index_after_calloc(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 0, i64 40000, i1 false)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;
   %call = tail call i8* @calloc(i64 10000, i64 4)
@@ -409,6 +415,16 @@ define i8* @zero_memset_after_calloc_inaccessiblememonly()  {
   ret i8* %call
 }
 
+define i8* @cst_nonzero_memset_after_calloc() {
+; CHECK-LABEL: @cst_nonzero_memset_after_calloc(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[CALL]], i8 1, i64 40000, i1 false)
+; CHECK-NEXT:    ret i8* [[CALL]]
+;
+  %call = tail call i8* @calloc(i64 10000, i64 4)
+  call void @llvm.memset.p0i8.i64(i8* %call, i8 1, i64 40000, i1 false)
+  ret i8* %call
+}
 
 define i8* @nonzero_memset_after_calloc(i8 %v) {
 ; CHECK-LABEL: @nonzero_memset_after_calloc(
@@ -427,7 +443,6 @@ define i8* @nonzero_memset_after_calloc(i8 %v) {
 define i8* @memset_pattern16_after_calloc(i8* %pat) {
 ; CHECK-LABEL: @memset_pattern16_after_calloc(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @calloc(i64 10000, i64 4)
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 4 [[CALL]], i8 0, i64 40000, i1 false)
 ; CHECK-NEXT:    call void @memset_pattern16(i8* [[CALL]], i8* [[PAT:%.*]], i64 40000)
 ; CHECK-NEXT:    ret i8* [[CALL]]
 ;