[Bug other/79469] Feature request: provide `__builtin_assume` builtin function to allow more aggressive optimizations and to match clang

[felix.von.s at posteo dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79469

--- Comment #2 from felix <felix.von.s at posteo dot de> ---
I realised recently that this is already expressible:

#define __builtin_assume(expr) \
    (__builtin_pure_p(expr) \
        ? ((expr) \
            ? (void) 0 \
            : __builtin_unreachable()) \
        : (void) 0 /* XXX: warn about side effects */)

where __builtin_pure_p has the same definition that I gave under PR 6906:

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

As for the corner case I gave earlier, GCC manages to optimise the is_pow2 loop
away at -O2 and -O3 (though not entirely at -O1), and it apparently considers
statement-expressions that contain more than one statement, or any kind of
loop, to have side effects. One can always check __OPTIMIZE__ to prevent
spurious code from being generated at -O0. So this solution apparently performs
no worse :)