[PATCH] D110422: [AIX] Enable PGO without LTO

[Fangrui Song via Phabricator via cfe-commits]

MaskRay added a comment.

Sorry, I still don't understand this response:

> We have 3 sets weak symbols here: weak_func, profc_weak_foo, profd_weak_func. We can't ensure that binder always choose 3 of them from same object.

So this part of the description isn't clear to me:

> However, on AIX, the current binder can NOT discard the weak symbols if we put all of them into the same csect, as binder can NOT discard only part of a csect.
>
> This creates a unique challenge for using those symbols to calculate some relative offset.

Say a.o has a weak `foo, __profc_foo, __profd_foo`. The `__profd_foo` references the `__profc_foo`

b.o has another set of weak `foo, __profc_foo, __profd_foo`. The `__profd_foo` references the `__profc_foo`.

What's the linker (called binder?) behavior? "We can't ensure that binder always choose 3 of them from same object."?
But is that a problem? For linkonce_odr functions, they should have identical semantics in a.o and b.o.
(There can be ODR issues if you use advanced PGO techniques like value profiling, but I assume that it does not work and will be difficult to make work and more importantly, it is also of lower value anyway.)

So the linker picking the a.o `foo` and the b.o `__profc_foo/__profd_foo` isn't a poblem.

I understand that PrivateLinkage can avoid the issues but you need to justify the complexity in `InstrProfiling.cpp` with the value.
Currently I don't think using weak symbol is a big problem, especially if the AIX linker will be fixed anyway in the future.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D110422/new/

https://reviews.llvm.org/D110422