Teach the optimizer to delete zero sized alloca's (but not mallocs!)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12507 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 4 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 93b2487..105ef5b 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -2367,11 +2367,13 @@ Instruction *InstCombiner::visitAllocationInst(AllocationInst &AI) {
 
       // Create and insert the replacement instruction...
       if (isa<MallocInst>(AI))
-        New = new MallocInst(NewTy, 0, AI.getName(), &AI);
+        New = new MallocInst(NewTy, 0, AI.getName());
       else {
         assert(isa<AllocaInst>(AI) && "Unknown type of allocation inst!");
-        New = new AllocaInst(NewTy, 0, AI.getName(), &AI);
+        New = new AllocaInst(NewTy, 0, AI.getName());
       }
+
+      InsertNewInstBefore(New, AI);
       
       // Scan to the end of the allocation instructions, to skip over a block of
       // allocas if possible...
@@ -2387,9 +2389,15 @@ Instruction *InstCombiner::visitAllocationInst(AllocationInst &AI) {
 
       // Now make everything use the getelementptr instead of the original
       // allocation.
-      ReplaceInstUsesWith(AI, V);
-      return &AI;
+      return ReplaceInstUsesWith(AI, V);
     }
+
+  // If alloca'ing a zero byte object, replace the alloca with a null pointer.
+  // Note that we only do this for alloca's, because malloc should allocate and
+  // return a unique pointer, even for a zero byte allocation.
+  if (isa<AllocaInst>(AI) && TD->getTypeSize(AI.getAllocatedType()) == 0)
+    return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
+
   return 0;
 }