Use "willreturn" in isGuaranteedToTransferExecutionToSuccessor

The `willreturn` function attribute guarantees that a function call will
come back to the call site if the call is also known not to throw.
Therefore, this attribute can be used in
`isGuaranteedToTransferExecutionToSuccessor`.

Patch by Hideto Ueno (@uenoku)

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63372

llvm-svn: 364580

llvm/lib/Analysis/ValueTracking.cpp

@@ -4285,6 +4285,11 @@ bool llvm::isGuaranteedToTransferExecutionToSuccessor(const Instruction *I) {
     if (!CS.doesNotThrow())
       return false;
 
+    // A function which doens't throw and has "willreturn" attribute will
+    // always return.
+    if (CS.hasFnAttr(Attribute::WillReturn))
+      return true;
+
     // Non-throwing call sites can loop infinitely, call exit/pthread_exit
     // etc. and thus not return.  However, LLVM already assumes that
     //

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -464,6 +464,7 @@ TEST(ValueTracking, GuaranteedToTransferExecutionToSuccessor) {
       "declare void @nounwind_argmemonly(i32*) nounwind argmemonly "
       "declare void @throws_but_readonly(i32*) readonly "
       "declare void @throws_but_argmemonly(i32*) argmemonly "
+      "declare void @nounwind_willreturn(i32*) nounwind willreturn"
       " "
       "declare void @unknown(i32*) "
       " "
@@ -476,6 +477,7 @@ TEST(ValueTracking, GuaranteedToTransferExecutionToSuccessor) {
       "  call void @unknown(i32* %p) nounwind argmemonly "
       "  call void @unknown(i32* %p) readonly "
       "  call void @unknown(i32* %p) argmemonly "
+      "  call void @nounwind_willreturn(i32* %p)"
       "  ret void "
       "} ";
 
@@ -497,6 +499,7 @@ TEST(ValueTracking, GuaranteedToTransferExecutionToSuccessor) {
       true,  // call void @unknown(i32* %p) nounwind argmemonly
       false, // call void @unknown(i32* %p) readonly
       false, // call void @unknown(i32* %p) argmemonly
+      true,  // call void @nounwind_willreturn(i32* %p)
       false, // ret void
   };