dead calls to llvm.stacksave can be deleted, even though they

have potential side-effects.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45392 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/Local.cpp

@@ -17,6 +17,7 @@
 #include "llvm/DerivedTypes.h"
 #include "llvm/Instructions.h"
 #include "llvm/Intrinsics.h"
+#include "llvm/IntrinsicInst.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Support/GetElementPtrTypeIterator.h"
@@ -173,8 +174,16 @@ bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
 bool llvm::isInstructionTriviallyDead(Instruction *I) {
   if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
 
-  if (!I->mayWriteToMemory()) return true;
+  if (!I->mayWriteToMemory())
+    return true;
 
+  // Special case intrinsics that "may write to memory" but can be deleted when
+  // dead.
+  if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+    // Safe to delete llvm.stacksave if dead.
+    if (II->getIntrinsicID() == Intrinsic::stacksave)
+      return true;
+  
   return false;
 }
 

test/Transforms/InstCombine/deadcode.ll

@@ -1,4 +1,5 @@
 ; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {ret i32 %A}
+; RUN: llvm-as < %s | opt -die | llvm-dis | not grep call.*llvm.stacksave
 
 define i32 @test(i32 %A) {
 	%X = or i1 false, false		
@@ -12,3 +13,12 @@ C:		; preds = %T, %0
 	%C.upgrd.1 = phi i32 [ %B, %T ], [ %A, %0 ]
 	ret i32 %C.upgrd.1
 }
+
+define i32* @test2(i32 %width) {
+	%tmp = call i8* @llvm.stacksave( )
+        %tmp14 = alloca i32, i32 %width
+	ret i32* %tmp14
+} 
+
+declare i8* @llvm.stacksave()
+