Re: [PATCH 1/5] x86_64: Add support for bcmp using sse2, sse4_1, avx2, and evex

[Joseph Myers <joseph@codesourcery.com>]

bcmp is an obsolescent function that no modern programs should be using, 
and it's not in the implementation namespace either so compilers shouldn't 
translate memcmp calls to bcmp.

If you want to define memcmp ABI variants optimized for particular usages, 
I suggest the following:

1. Add reserved-namespace names for such variants to the x86_64 psABI 
document (working with the ABI mailing list).  The 32-bit Arm RTABI 
<https://github.com/ARM-software/abi-aa/blob/main/rtabi32/rtabi32.rst> 
provides a precedent for defining such function variants in a psABI (it 
includes various __aeabi_mem*, though no memcmp variants).

2. Add those names to glibc, as well as teaching compilers to generate 
calls to them (with appropriate conditionals for whether the functions are 
known to be available in the target libc; in GCC, that would be based on 
GCC_GLIBC_VERSION_GTE_IFELSE configure tests for targets using glibc).


As a variant, you could define such names as architecture-independent GNU 
extensions rather than in a psABI, especially if there's nothing 
architecture-specific about the variants you think are useful (e.g. no use 
for having changes to calling conventions / call-clobbered registers for 
the variants).  But what should not be done in any case is tying an 
optimization to an obsolescent non-reserved name - any such optimized 
variants should use only implementation-namespace names.

-- 
Joseph S. Myers
joseph@codesourcery.com