[SimplifyCFG] Don't split musttail call from ret

Summary:
When inserting an `unreachable` after a noreturn call, we must ensure
that it's not a musttail call to avoid breaking the IR invariants for
musttail calls.

Reviewers: fedor.sergeev, majnemer

Reviewed By: majnemer

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60080

llvm-svn: 357485

2 files changed, 21 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 429ae85596f..7024c663e30 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -2054,7 +2054,7 @@ static bool markAliveBlocks(Function &F,
           Changed = true;
           break;
         }
-        if (CI->doesNotReturn()) {
+        if (CI->doesNotReturn() && !CI->isMustTailCall()) {
           // If we found a call to a no-return function, insert an unreachable
           // instruction after it.  Make sure there isn't *already* one there
           // though.
diff --git a/llvm/test/Transforms/SimplifyCFG/noreturn-call.ll b/llvm/test/Transforms/SimplifyCFG/noreturn-call.ll
index b4547782837..88d4f85713b 100644
--- a/llvm/test/Transforms/SimplifyCFG/noreturn-call.ll
+++ b/llvm/test/Transforms/SimplifyCFG/noreturn-call.ll
@@ -1,11 +1,29 @@
-; RUN: opt < %s -simplifycfg -S | grep unreachable
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
 ; PR1796
 
 declare void @Finisher(i32) noreturn
 
-define void @YYY(i32) {
+; Make sure we optimize a sequence of two calls (second unreachable);
+define void @double_call(i32) {
+; CHECK-LABEL: @double_call(
+; CHECK-NEXT:    tail call void @Finisher(i32 %0) #0
+; CHECK-NEXT:    unreachable
+;
   tail call void @Finisher(i32 %0) noreturn
   tail call void @Finisher(i32 %0) noreturn
   ret void
 }
 
+; Make sure we DON'T try to optimize a musttail call (the IR invariant
+; is that it must be followed by [optional bitcast then] ret).
+define void @must_tail(i32) {
+; CHECK-LABEL: @must_tail(
+; CHECK-NEXT:    musttail call void @Finisher(i32 %0) #0
+; CHECK-NEXT:    ret void
+;
+  musttail call void @Finisher(i32 %0) #0
+  ret void
+}
+
+; CHECK: attributes #0 = { noreturn }
+attributes #0 = { noreturn }