[IPSCCP] Remove noundef when zapping return values

[nikic]

Summary

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

[fhahn]

LGTM, thanks!

llvm/lib/Transforms/IPO/SCCP.cpp

for (ReturnInst *RI : ReturnsToZap) {
     Function *F = RI->getParent()->getParent();
     RI->setOperand(0, UndefValue::get(F->getReturnType()));
     // Record all functions that are zapped.
     FuncZappedReturn.insert(F);
   }
 
   // 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) {
         assert(isa<BlockAddress>(U.getUser()) ||
                (isa<Constant>(U.getUser()) &&
                 all_of(U.getUser()->users(), [](const User *UserUser) {
                   return cast<IntrinsicInst>(UserUser)->isAssumeLikeIntrinsic();
                 })));
         continue;
       }
 
       for (Use &Arg : CB->args())
         CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
+      CB->removeRetAttrs(UBImplyingAttributes);
     }
   }
 
   // If we inferred constant or undef values for globals variables, we can
   // delete the global and any stores that remain to it.
   for (const auto &I : make_early_inc_range(Solver.getTrackedGlobals())) {
     GlobalVariable *GV = I.first;
     if (SCCPSolver::isOverdefined(I.second))

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

 ; RUN: opt -S -passes=ipsccp < %s | FileCheck %s
 @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()
   call dereferenceable(1) ptr @ret_dereferenceable()
   call dereferenceable_or_null(1) ptr @ret_dereferenceable_or_null()
   call nonnull ptr @ret_nonnull()
   call nonnull noundef ptr @ret_nonnull_noundef()
   ret void
 }