Inliner: don't mark call sites as 'nounwind' if that would be redundant

When F calls G calls H, G is nounwind, and G is inlined into F, then the
inlined call-site to H should be effectively nounwind so as not to lose
information during inlining.

If H itself is nounwind (which often happens when H is an intrinsic), we
no longer mark the callsite explicitly as nounwind. Previously, there
were cases where the inlined call-site of H differs from a pre-existing
call-site of H in F *only* in the explicitly added nounwind attribute,
thus preventing common subexpression elimination.

v2:
- just check CI->doesNotThrow

Differential Revision: https://reviews.llvm.org/D129860

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/InlineFunction.cpp b/llvm/lib/Transforms/Utils/InlineFunction.cpp
index 2fb00f9..7e835bd 100644
--- a/llvm/lib/Transforms/Utils/InlineFunction.cpp
+++ b/llvm/lib/Transforms/Utils/InlineFunction.cpp
@@ -2194,9 +2194,11 @@ llvm::InlineResult llvm::InlineFunction(CallBase &CB, InlineFunctionInfo &IFI,
         CI->setTailCallKind(ChildTCK);
         InlinedMustTailCalls |= CI->isMustTailCall();
 
-        // Calls inlined through a 'nounwind' call site should be marked
-        // 'nounwind'.
-        if (MarkNoUnwind)
+        // Call sites inlined through a 'nounwind' call site should be
+        // 'nounwind' as well. However, avoid marking call sites explicitly
+        // where possible. This helps expose more opportunities for CSE after
+        // inlining, commonly when the callee is an intrinsic.
+        if (MarkNoUnwind && !CI->doesNotThrow())
           CI->setDoesNotThrow();
       }
     }