[swift-dev] SILVerifier: Should SuperMethodInst's result type be the same type as the referenced member?

[David Farler]

> On Dec 10, 2015, at 08:16, John McCall <rjmccall at apple.com> wrote:
> 
>> On Dec 9, 2015, at 6:09 PM, David Farler via swift-dev <swift-dev at swift.org> wrote:
>> [Joe, Roman, sorry for resend, I got the e-mail group wrong]
>> 
>> In SuperMethodInst's verifier, we have:
>> 
>> require(CMI->getType() == TC.getConstantType(CMI->getMember()),
>>        "result type of super_method must match type of method");
>> 
>> I think this assumption was valid when we only allowed super_method on foreign classes, without needing to worry about reabstractions. Now that we're allowing super_method with operands of native class type which can be generic, does this check make sense anymore?
>> 
>> Consider the following:
>> 
>> class Parent<A> {
>>  let x: A
>>  required init(x: A) { self.x = x }
>> }
>> 
>> class Child : Parent<String> {
>>  required init(x: String) {
>>    super.init(x: x)
>>  }
>> }
>> 
>> class Grandchild : Child {}
>> 
>> 
>> Here, the vtable thunks for their initializers have respective types:
>> 
>> $@convention(method) <T> (@in T, @owned Base<T>) -> @owned Base<T>
>> $@convention(method) (@in String, @owned Child) -> @owned Child
>> $@convention(method) (@in String, @owned Grandchild) -> @owned Grandchild
>> 
>> 
>> However, the real backing implementations have these respective types:
>> 
>> $@convention(method) <T> (@in T, @owned Base<T>) -> @owned Base<T>
>> $@convention(method) (@owned String, @owned Child) -> @owned Child
>> $@convention(method) (@owned String, @owned Grandchild) -> @owned Grandchild
>> 
>> 
>> So, Child and Grandchild have abstraction differences because their initializers aren't generic. When I make a super_method instruction, the constant appears to always point to the backing implementation, not the thunk, so I needed to get the overridden vtable entry from the constant and I think that's reasonable. That gives me:
>> 
>> super_method %10 : $Child, #Base.init!initializer.1 : <T> Base<T>.Type -> (x: T) -> Base<T> , $@convention(method) <τ_0_0> (@in τ_0_0, @owned Base<τ_0_0>) -> @owned Base<τ_0_0>
>> 
>> and
>> 
>> super_method %6 : $Grandchild, #Child.init!initializer.1 : Child.Type -> (x: String) -> Child , $@convention(method) (@in String, @owned Child) -> @owned Child
>> 
>> 
>> which look good to me.
>> 
>> With my changes today to fix generic substitutions of partial super methods and getting the right type from the vtable, if I disable that verifier check, devirtualization works correctly with super_method instructions.
>> 
>> Is this a problem with SILDeclRef or is this check simply no longer valid in the verifier? If so, I wonder what the suitable replacement check should be. Maybe something like:
> 
> I’d look at the verifier for ClassMethodInst.  It turns out that TypeConverter has a getConstantOverrideType that lowers according to the overridden abstraction pattern.
> 
> John.

Yep, that's what I moved over to using yesterday to fix the abstraction mismatch in the devirtualizer and that's when I started getting the verifier error.  I'll take a look at the class method's verifier and see what it says. 

David
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