calls in nothrow functions can be marked nothrow even if the callee

is not known to be nothrow.  This allows readnone/readonly functions
to be deleted even if we don't know whether the callee can throw.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71676 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 18 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index c76403e039..527ed43010 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -9578,6 +9578,16 @@ Instruction *InstCombiner::SimplifyMemSet(MemSetInst *MI) {
 /// the heavy lifting.
 ///
 Instruction *InstCombiner::visitCallInst(CallInst &CI) {
+  // If the caller function is nounwind, mark the call as nounwind, even if the
+  // callee isn't.
+  if (CI.getParent()->getParent()->doesNotThrow() &&
+      !CI.doesNotThrow()) {
+    CI.setDoesNotThrow();
+    return &CI;
+  }
+  
+  
+  
   IntrinsicInst *II = dyn_cast<IntrinsicInst>(&CI);
   if (!II) return visitCallSite(&CI);
   
diff --git a/test/Transforms/InstCombine/nothrow.ll b/test/Transforms/InstCombine/nothrow.ll
new file mode 100644
index 0000000000..fbf162a1db
--- /dev/null
+++ b/test/Transforms/InstCombine/nothrow.ll
@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | not grep call
+; rdar://6880732
+declare double @t1(i32) readonly
+
+define void @t2() nounwind {
+  call double @t1(i32 42)  ;; dead call even though callee is not nothrow.
+  ret void
+}