[IPSCCP] Remove noundef when zapping return values

When replacing return values with undef, we should also drop the
noundef attribute (and other UB implying attributes).

Differential Revision: https://reviews.llvm.org/D144461

Author: nikic

llvm/lib/Transforms/IPO/SCCP.cpp

@@ -338,9 +338,14 @@ static bool runIPSCCP(
 
   // Remove the returned attribute for zapped functions and the
   // corresponding call sites.
+  // Also remove any attributes that convert an undef return value into
+  // immediate undefined behavior
+  AttributeMask UBImplyingAttributes =
+      AttributeFuncs::getUBImplyingAttributes();
   for (Function *F : FuncZappedReturn) {
     for (Argument &A : F->args())
       F->removeParamAttr(A.getArgNo(), Attribute::Returned);
+    F->removeRetAttrs(UBImplyingAttributes);
     for (Use &U : F->uses()) {
       CallBase *CB = dyn_cast<CallBase>(U.getUser());
       if (!CB) {
@@ -354,6 +359,7 @@ static bool runIPSCCP(
 
       for (Use &Arg : CB->args())
         CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
+      CB->removeRetAttrs(UBImplyingAttributes);
     }
   }
 

llvm/test/Transforms/SCCP/ipsccp-noundef.ll

@@ -2,47 +2,48 @@
 @g = external global i8
 
 define internal noundef i32 @ret_noundef() {
-; CHECK-LABEL: define internal noundef i32 @ret_noundef() {
+; CHECK-LABEL: define internal i32 @ret_noundef() {
 ; CHECK-NEXT:    ret i32 undef
 ;
   ret i32 0
 }
 
 define internal dereferenceable(1) ptr @ret_dereferenceable() {
-; CHECK-LABEL: define internal dereferenceable(1) ptr @ret_dereferenceable() {
+; CHECK-LABEL: define internal ptr @ret_dereferenceable() {
 ; CHECK-NEXT:    ret ptr undef
 ;
   ret ptr @g
 }
 
 define internal dereferenceable_or_null(1) ptr @ret_dereferenceable_or_null() {
-; CHECK-LABEL: define internal dereferenceable_or_null(1) ptr @ret_dereferenceable_or_null() {
+; CHECK-LABEL: define internal ptr @ret_dereferenceable_or_null() {
 ; CHECK-NEXT:    ret ptr undef
 ;
   ret ptr @g
 }
 
+; Non-null is fine, because it does not cause immediate UB.
 define internal nonnull ptr @ret_nonnull() {
 ; CHECK-LABEL: define internal nonnull ptr @ret_nonnull() {
 ; CHECK-NEXT:    ret ptr undef
 ;
   ret ptr @g
 }
 
-define internal nonnull noundef ptr @ret_nonnull_noundef() {
-; CHECK-LABEL: define internal noundef nonnull ptr @ret_nonnull_noundef() {
+define internal nonnull ptr @ret_nonnull_noundef() {
+; CHECK-LABEL: define internal nonnull ptr @ret_nonnull_noundef() {
 ; CHECK-NEXT:    ret ptr undef
 ;
   ret ptr @g
 }
 
 define void @test() {
 ; CHECK-LABEL: define void @test() {
-; CHECK-NEXT:    [[TMP1:%.*]] = call noundef i32 @ret_noundef()
-; CHECK-NEXT:    [[TMP2:%.*]] = call dereferenceable(1) ptr @ret_dereferenceable()
-; CHECK-NEXT:    [[TMP3:%.*]] = call dereferenceable_or_null(1) ptr @ret_dereferenceable_or_null()
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @ret_noundef()
+; CHECK-NEXT:    [[TMP2:%.*]] = call ptr @ret_dereferenceable()
+; CHECK-NEXT:    [[TMP3:%.*]] = call ptr @ret_dereferenceable_or_null()
 ; CHECK-NEXT:    [[TMP4:%.*]] = call nonnull ptr @ret_nonnull()
-; CHECK-NEXT:    [[TMP5:%.*]] = call noundef nonnull ptr @ret_nonnull_noundef()
+; CHECK-NEXT:    [[TMP5:%.*]] = call nonnull ptr @ret_nonnull_noundef()
 ; CHECK-NEXT:    ret void
 ;
   call noundef i32 @ret_noundef()