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

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

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"?


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