From 03544ec2a43fab162d25cf44627d1d08430bcccd Mon Sep 17 00:00:00 2001
From: Nadav Rotem <nrotem@apple.com>
Date: Tue, 19 Feb 2013 20:02:09 +0000
Subject: [PATCH] Fix a bug in mayHaveSideEffects. Functions that do not return
 are now considered as instructions with side effects.

rdar://13227456



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175553 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/llvm/IR/Instruction.h             |  8 +++++++-
 lib/IR/Instruction.cpp                    |  8 ++++++--
 test/Transforms/FunctionAttrs/noreturn.ll | 18 ++++++++++++++++++
 3 files changed, 31 insertions(+), 3 deletions(-)
 create mode 100644 test/Transforms/FunctionAttrs/noreturn.ll

diff --git a/include/llvm/IR/Instruction.h b/include/llvm/IR/Instruction.h
index e5e5f96d561..5721d8f2f3f 100644
--- a/include/llvm/IR/Instruction.h
+++ b/include/llvm/IR/Instruction.h
@@ -309,6 +309,12 @@ public:
   ///
   bool mayThrow() const;
 
+  /// mayReturn - Return true if this is a function that may return.
+  /// this is true for all normal instructions. The only exception
+  /// is functions that are marked with the 'noreturn' attribute.
+  ///
+  bool mayReturn() const;
+
   /// mayHaveSideEffects - Return true if the instruction may have side effects.
   ///
   /// Note that this does not consider malloc and alloca to have side
@@ -316,7 +322,7 @@ public:
   /// instructions which don't used the returned value.  For cases where this
   /// matters, isSafeToSpeculativelyExecute may be more appropriate.
   bool mayHaveSideEffects() const {
-    return mayWriteToMemory() || mayThrow();
+    return mayWriteToMemory() || mayThrow() || !mayReturn();
   }
 
   /// clone() - Create a copy of 'this' instruction that is identical in all
diff --git a/lib/IR/Instruction.cpp b/lib/IR/Instruction.cpp
index 42df5d71182..2b5a0b39c31 100644
--- a/lib/IR/Instruction.cpp
+++ b/lib/IR/Instruction.cpp
@@ -455,14 +455,18 @@ bool Instruction::mayWriteToMemory() const {
   }
 }
 
-/// mayThrow - Return true if this instruction may throw an exception.
-///
 bool Instruction::mayThrow() const {
   if (const CallInst *CI = dyn_cast<CallInst>(this))
     return !CI->doesNotThrow();
   return isa<ResumeInst>(this);
 }
 
+bool Instruction::mayReturn() const {
+  if (const CallInst *CI = dyn_cast<CallInst>(this))
+    return !CI->doesNotReturn();
+  return true;
+}
+
 /// isAssociative - Return true if the instruction is associative:
 ///
 ///   Associative operators satisfy:  x op (y op z) === (x op y) op z
diff --git a/test/Transforms/FunctionAttrs/noreturn.ll b/test/Transforms/FunctionAttrs/noreturn.ll
new file mode 100644
index 00000000000..470ebcb1d3c
--- /dev/null
+++ b/test/Transforms/FunctionAttrs/noreturn.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -functionattrs -instcombine -S | FileCheck %s
+
+define void @endless_loop() noreturn nounwind readnone ssp uwtable {
+entry:
+  br label %while.body
+
+while.body:
+  br label %while.body
+}
+;CHECK: @main
+;CHECK: endless_loop
+;CHECK: ret
+define i32 @main() noreturn nounwind ssp uwtable {
+entry:
+  tail call void @endless_loop()
+  unreachable
+}
+