[PATCH] D154324: [C++20] [Modules] [ODRHash] Use CanonicalType for base classes

Sat Jul 22 02:50:48 PDT 2023

v.g.vassilev added a comment.

In D154324#4524368 <https://reviews.llvm.org/D154324#4524368>, @rsmith wrote:

> In D154324#4524235 <https://reviews.llvm.org/D154324#4524235>, @v.g.vassilev wrote:
>
>> @rsmith, thanks for the suggestions! Could you go over `ODRHash::AddTemplateName` suggest how to fix it to address https://reviews.llvm.org/D153003 and https://reviews.llvm.org/D41416#4496451?
>
> `AddTemplateName` looks fine as-is to me; I think the problem in D153003 <https://reviews.llvm.org/D153003> is that we'd stepped outside of the entity we were odr-hashing and started hashing something else, which (legitimately) was different between translation units.
>
> For D41416 <https://reviews.llvm.org/D41416>, ODR hashing may not be the best mechanism to hash the template arguments, unfortunately. ODR hashing is (or perhaps, should be) about determining whether two things are spelled the same way and have the same meaning (as required by the C++ ODR), whereas I think what you're looking for is whether they have the same meaning regardless of spelling. Maybe we can get away with reusing ODR hashing anyway, on the basis that any canonical, non-dependent template argument should have the same (invented) spelling in every translation unit, but I'm not certain that's true in all cases. There may still be cases where the canonical type includes some aspect of "whatever we saw first", in which case the ODR hash can differ across translation units for non-dependent, canonical template arguments that are spelled differently but have the same meaning, though I can't think of one off-hand.

Thanks for investigating. I am happy to try to get away with (mis)using ODR hashing and see if we (and for how long) could get away with it. @Hahnfeld and I discussed to use the llvm FoldingSet technique if ODR hash falls short. Is that or it was something else what you had in mind as an alternative to ODR hashing?

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