[Bug target/104611] memcmp/strcmp/strncmp can be optimized when the result is tested for [in]equality with 0 on aarch64

["redbeard0531 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104611

Mathias Stearn <redbeard0531 at gmail dot com> changed:

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--- Comment #4 from Mathias Stearn <redbeard0531 at gmail dot com> ---
clang has already been using the optimized memcmp code since v16, even at -O1:
https://www.godbolt.org/z/qEd768TKr. Older versions (at least since v9) were
still branch-free, but via a less optimal sequence of instructions.

GCC's code gets even more ridiculous at 32 bytes, because it does a branch
after every 8-byte compare, while the clang code is fully branch-free (not that
branch-free is always better, but it seems clearly so in this case).

Judging by the codegen, there seems to be three deficiencies in GCC: 1) an
inability to take advantage of the load-pair instructions to load 16-bytes at a
time, and 2) an inability to use ccmp to combine comparisons. 3) using
branching rather than cset to fill the output register. Ideally these could all
be done in the general case by the low level instruction optimizer, but even
getting them special cased for memcmp (and friends) would be an improvement.