Hi Pádraig,
Pádraig Brady <[email protected]> writes:
> The attached 2 patches, add a new cpu-supports module,
> and updates the crc-x86_64 module to use it.
> This ensures that any users of the crc-x86_64 module
> will honor the GLIBC_TUNABLES environment variable.
> Specifically it will allow disabling of the hardware acceleration
> used by crc-x86_64 using:
> export GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX
>
> Note this is a general mechanism provided by glibc
> to tune hardware acceleration within glibc and also
> determines what libs are loaded with its dynamic dispatch mechanism:
> On my "x86-64-v3" system for example:
>
> $ ld.so --help | grep -B1 x86-64-v
> Subdirectories of glibc-hwcaps directories, in priority order:
> x86-64-v4
> x86-64-v3 (supported, searched)
> x86-64-v2 (supported, searched)
> $ ld.so --help | GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX \
> grep -B1 x86-64-v
> Subdirectories of glibc-hwcaps directories, in priority order:
> x86-64-v4
> x86-64-v3 (supported, searched)
> x86-64-v2 (supported, searched)
>
> This module will also be used by coreutils to ensure
> its various uses of __builtin_cpu_supports() are similarly gated.
Ah, I should have known that we needed this in Gnulib for crc32. Thanks
for working on it.
I'll give a more through review later, but one thing that that popped
out to me during an initial skim:
> + if (0);
> +# if defined __x86_64__
> + else if (STREQ (feature, "avx")) hwcap = "-AVX";
> + else if (STREQ (feature, "avx2")) hwcap = "-AVX2";
> + else if (STREQ (feature, "avx512bw")) hwcap = "-AVX512BW";
> + else if (STREQ (feature, "avx512f")) hwcap = "-AVX512F";
> + else if (STREQ (feature, "pclmul")) hwcap = "-PCLMULQDQ";
> + else if (STREQ (feature, "vpclmulqdq")) hwcap = "-VPCLMULQDQ";
> +# elif defined __aarch64__
> + else if (STREQ (feature, "pmull")) hwcap "-PMULL";
> +# endif
> +
> + return hwcap;
> +}
I like the attempt to minimize lines of code, but I recall clang warning
about this by default:
$ cat main.c
int
main (void)
{
if (0);
return 0;
}
$ clang main.c
main.c:5:9: warning: if statement has empty body [-Wempty-body]
5 | if (0);
| ^
main.c:5:9: note: put the semicolon on a separate line to silence this
warning
1 warning generated.
$ clang -v 2>&1 | head -n 1
clang version 20.1.8 (Fedora 20.1.8-4.fc42)
Also, lib/cpu-supports.c needs a copyright header.
Collin
