[Bug c/58872] RFC: more builtins for bit manipulation

["yann at droneaud dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58872

Yann Droneaud <yann at droneaud dot fr> changed:

           What    |Removed                     |Added
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                 CC|                            |yann at droneaud dot fr

--- Comment #2 from Yann Droneaud <yann at droneaud dot fr> ---
(In reply to Marc Glisse from comment #1)
> What do the arguments of brev mean?
> Related to PR 50481.

The prototype for the reverse bit string builtin was designed to be useful to
reverse bits in a subset of a integer value, starting at "index" with "count"
bits.

The general use case is likely to reverse all bits in a variable, (uint64_t for
example):

    uint64_t v;
    __builtin_brev64(v, 0, sizeof(v) * 8);

But I thought it might be interesting to have a "more" general purpose builtin.

Actually, the builtin arguments for reverse should probably match what's
currently available on modern hardware.