[llvm-dev] Are unknown types now intentionally casual?

[Nikita Popov via llvm-dev]

On Sun, Jan 9, 2022 at 6:29 AM Russell Wallace via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> ... no, when I feed the second output as input into clang again, it
> objects to the undeclared type:
>
> (c1) C:\t>clang -mllvm -opaque-pointers 2.ll
> 2.ll:6:38: error: use of undefined type named 'struct.Unknown'
> @Unknown = external dso_local global %struct.Unknown, align 1
>                                      ^
> 1 error generated.
>
> So this seems to be just a bug in this particular version of clang,
> probably because the opaque pointers conversion is still a work in progress?
>

Yes, this is a bug in the opaque pointers implementation. The TypeFinder
does not find the struct type here, because it is neither reachable from
the global type, nor the (absent) global initializer, but only from the
global value type. We'll have to port some changes from the ValueEnumerator
to the TypeFinder.

Regards,
Nikita


> On Sun, Jan 9, 2022 at 4:20 AM Russell Wallace <russell.wallace at gmail.com>
> wrote:
>
>> Given the following code:
>>
>> struct Unknown;
>> extern struct Unknown Unknown;
>>
>> struct Unknown *foo(void) {
>>   return &Unknown;
>> }
>>
>> And compiling it with a recent (trunk of a few days ago) clang with
>> default options gives (abridged):
>>
>> %struct.Unknown = type opaque
>>
>> @Unknown = external dso_local global %struct.Unknown, align 1
>>
>> ; Function Attrs: noinline nounwind optnone uwtable
>> define dso_local %struct.Unknown* @foo() #0 {
>> entry:
>>   ret %struct.Unknown* @Unknown
>> }
>>
>> Okay, so I think I understand what's going on there. Now, trying the same
>> input with the same compiler and adding the option -mllvm -opaque-pointers
>> (because opaque pointers are good; I think they are a very worthwhile
>> simplification):
>>
>> @Unknown = external dso_local global %struct.Unknown, align 1
>>
>> ; Function Attrs: noinline nounwind optnone uwtable
>> define dso_local ptr @foo() #0 {
>> entry:
>>   ret ptr @Unknown
>> }
>>
>> So that's mostly clear except one thing I'm not quite clear
>> about: %struct.Unknown is referred to, but not declared anywhere. Is that
>> intentional? Is the rule now intended to be that whenever a %type.name
>> is used that was not declared anywhere, it should be taken to be an opaque
>> type? (I'm fine with that if it is, just figure I better make sure.)
>>
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