Bug#123468: [Bug c/6906] warn about asserts with side effects

["felix.von.s at posteo dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=6906

felix <felix.von.s at posteo dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |felix.von.s at posteo dot de

--- Comment #9 from felix <felix.von.s at posteo dot de> ---
I have just realised that __builtin_pure_p is already expressible:

#define __builtin_pure_p(expr) \
    (__builtin_object_size(((void)(expr), ""), 2))

If (expr) is free of side effects, this is the same as
__builtin_object_size("", 2), which is 1. Otherwise, the expression returns 0.
This is documented behaviour, by the way. The documentation for
__builtin_object_size at
<https://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html> states:

> __builtin_object_size never evaluates its arguments for side effects.
> If there are any side effects in them, it returns (size_t) -1 for
> type 0 or 1 and (size_t) 0 for type 2 or 3.

Experimentation reveals __attribute__((pure)) is enough for
__builtin_object_size to consider a function free of side effects.

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