[DeadArgElim][FIX] Drop more attributes when the return type changes

[jdoerfert]

Summary

We should not have align or noundef on void return types.

[jdoerfert]

It seems align is allowed on non-pointers, will investigate.

[fhahn]

This looks fine, but might be better to split up into 2 patches (1 for undef, 1 for align)? Small nit for the title: this patch fixes AttributeFuncs::typeIncompatible, which as a consequence fixes DeadArgElim, but potentially other places too. Might be good to reflect that in the title.

[jdoerfert]

In D86157#2258063, @fhahn wrote:

This looks fine, but might be better to split up into 2 patches (1 for undef, 1 for align)? Small nit for the title: this patch fixes AttributeFuncs::typeIncompatible, which as a consequence fixes DeadArgElim, but potentially other places too. Might be good to reflect that in the title.

I will revisit this soon. split it and fix the tests.

[jdoerfert]

Split into D87304 and D87306

llvm/lib/IR/Attributes.cpp

Incompatible.addAttribute(Attribute::SExt)
       .addAttribute(Attribute::ZExt);
 
   if (!Ty->isPointerTy())
     // Attribute that only apply to pointers.
     Incompatible.addAttribute(Attribute::Nest)
         .addAttribute(Attribute::NoAlias)
         .addAttribute(Attribute::NoCapture)
         .addAttribute(Attribute::NonNull)
+        .addAlignmentAttr(1)             // the int here is ignored
         .addDereferenceableAttr(1)       // the int here is ignored
         .addDereferenceableOrNullAttr(1) // the int here is ignored
         .addAttribute(Attribute::ReadNone)
         .addAttribute(Attribute::ReadOnly)
         .addAttribute(Attribute::StructRet)
         .addAttribute(Attribute::InAlloca)
         .addPreallocatedAttr(Ty)
         .addByValAttr(Ty)
         .addByRefAttr(Ty);
 
+  // Some attributes can apply to all "values" but there are no `void` values.
+  if (Ty->isVoidTy())
+    Incompatible.addAttribute(Attribute::NoUndef);
+
   return Incompatible;
 }
 
 template<typename AttrClass>
 static bool isEqual(const Function &Caller, const Function &Callee) {
   return Caller.getFnAttribute(AttrClass::getKind()) ==
          Callee.getFnAttribute(AttrClass::getKind());
 }

llvm/test/Transforms/DeadArgElim/returned.ll

 ; don't do this if 'returned' is on the call site...
 ; CHECK-LABEL: define internal void @test5(%Ty* %this)
 
 define internal %Ty* @test5(%Ty* %this) {
   store volatile %Ty* %this, %Ty** @dummy
   ret %Ty* %this
 }
 
+; Drop all these attributes
+; CHECK-LABEL: define internal void @test6
+define internal align 8 dereferenceable_or_null(2) noundef noalias i8* @test6() {
+  ret i8* null
+}
+
 define %Ty* @caller(%Ty* %this) {
   %1 = call %Ty* @test1(%Ty* %this)
   %2 = call %Ty* @test2(%Ty* %this)
   %3 = call %Ty* @test3(%Ty* %this)
   %4 = call %Ty* @test4(%Ty* %this)
 ; ...instead, drop 'returned' form the call site
 ; CHECK: call void @test5(%Ty* %this)
   %5 = call %Ty* @test5(%Ty* returned %this)
+  %6 = call i8* @test6()
   ret %Ty* %this
 }