From 62cdcd6c5aee89e2cbdc498e5a6193d19bd498fb Mon Sep 17 00:00:00 2001
From: sstefan1 <sstipanovic@s-energize.com>
Date: Wed, 21 Apr 2021 21:02:28 +0200
Subject: [PATCH] [FuncAttrs] Don't infer willreturn for nonexact definitions

Discovered during attributor testing comparing stats with
and without the attributor. Willreturn should not be inferred
for nonexact definitions.

Differential Revision: https://reviews.llvm.org/D100988
---
 llvm/lib/Transforms/IPO/FunctionAttrs.cpp        | 6 ++++++
 llvm/test/Transforms/FunctionAttrs/willreturn.ll | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 8f80a61ae08e..ca8660a98ded 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -1432,6 +1432,12 @@ static bool addNoReturnAttrs(const SCCNodeSet &SCCNodes) {
 }
 
 static bool functionWillReturn(const Function &F) {
+  // We can infer and propagate function attributes only when we know that the
+  // definition we'll get at link time is *exactly* the definition we see now.
+  // For more details, see GlobalValue::mayBeDerefined.
+  if (!F.hasExactDefinition())
+    return false;
+
   // Must-progress function without side-effects must return.
   if (F.mustProgress() && F.onlyReadsMemory())
     return true;
diff --git a/llvm/test/Transforms/FunctionAttrs/willreturn.ll b/llvm/test/Transforms/FunctionAttrs/willreturn.ll
index 1f69c3d80d8b..35f48e7a344c 100644
--- a/llvm/test/Transforms/FunctionAttrs/willreturn.ll
+++ b/llvm/test/Transforms/FunctionAttrs/willreturn.ll
@@ -153,5 +153,12 @@ bb2:
   br label %bb1
 }
 
+define linkonce i32 @square(i32) {
+; CHECK-NOT: Function Attrs: {{.*}}willreturn
+; CHECK: define linkonce i32 @square(
+    %2 = mul nsw i32 %0, %0
+    ret i32 %2
+}
+
 declare i64 @fn_noread() readnone
 declare void @fn_willreturn() willreturn