Enable the tail call optimization when the caller returns undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88737 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 12 insertions, 1 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuild.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuild.cpp
index f2c623cd61..90fd95eb63 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuild.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuild.cpp
@@ -4376,6 +4376,10 @@ isInTailCallPosition(const Instruction *I, Attributes CalleeRetAttr,
   // what the call's return type is.
   if (!Ret || Ret->getNumOperands() == 0) return true;
 
+  // If the return value is undef, it doesn't matter what the call's
+  // return type is.
+  if (isa<UndefValue>(Ret->getOperand(0))) return true;
+
   // Conservatively require the attributes of the call to match those of
   // the return. Ignore noalias because it doesn't affect the call sequence.
   unsigned CallerRetAttr = F->getAttributes().getRetAttributes();
diff --git a/test/CodeGen/X86/tailcall1.ll b/test/CodeGen/X86/tailcall1.ll
index 215d00946d..4923df26b4 100644
--- a/test/CodeGen/X86/tailcall1.ll
+++ b/test/CodeGen/X86/tailcall1.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s -march=x86 -tailcallopt | grep TAILCALL | count 3
+; RUN: llc < %s -march=x86 -tailcallopt | grep TAILCALL | count 4
 define fastcc i32 @tailcallee(i32 %a1, i32 %a2, i32 %a3, i32 %a4) {
 entry:
 	ret i32 %a3
@@ -23,3 +23,10 @@ define fastcc i8* @alias_caller() nounwind {
   %p = tail call fastcc noalias i8* @noalias_callee()
   ret i8* %p
 }
+
+declare fastcc i32 @i32_callee()
+
+define fastcc i32 @ret_undef() nounwind {
+  %p = tail call fastcc i32 @i32_callee()
+  ret i32 undef
+}