[PATCH] D77491: [Sema] Introduce BuiltinAttr, per-declaration builtin-ness

[Raul Tambre via Phabricator via cfe-commits]

tambre added a comment.

In D77491#2299938 <https://reviews.llvm.org/D77491#2299938>, @rsmith wrote:

> We've hit a fairly subtle miscompile caused by this patch.
>
> glibc's setjmp.h looks like this (irrelevant parts removed):
>
>   struct __jmp_buf_tag { /*...*/ };
>   extern int __sigsetjmp(struct __jmp_buf_tag __env[1], int);
>   typedef struct __jmp_buf_tag sigjmp_buf[1];
>   #define sigsetjmp __sigsetjmp
>
> This worked fine with the old approach. But with the new approach, we decide the declaration of `__sigsetjmp` is not a builtin, because at its point of declaration, we can't compute the "proper" type because `sigjmp_buf` has not been declared yet. As a result, we don't add a `BuiltinAttr` to `__sigsetjmp`, but much more critically, we don't add a `ReturnsTwiceAttr`, which results in miscompiles in calls to this function. (I think `sigsetjmp` is the only affected function with glibc. `jmp_buf` is declared prior to `__setjmp` and friends.)
>
> I suppose we don't actually care what the parameter types for `__sigsetjmp` are, and it would be fine (and much safer) to treat any function with that name as a builtin, like we used to. Perhaps we should have a way of marking builtins as "the given type is what we expect / what we will implicitly declare, but it's OK if it doesn't actually match"?

Marking `__sigsetjmp` as having custom typechecking should suffice (`t` attribute in Builtins.def), no? Though a case in `Sema::CheckBuiltinFunctionCall()` might also then be necessary.


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