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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