[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

@@ -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;

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