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The current IFUNC selection is always using the most recent
features which are available via AT_HWCAP. But in
some scenarios it is useful to adjust this selection.
The environment variable:
GLIBC_TUNABLES=glibc.cpu.hwcaps=-xxx,yyy,zzz,....
can be used to enable HWCAP feature yyy, disable HWCAP feature xxx,
where the feature name is case-sensitive and has to match the ones
used in sysdeps/loongarch/cpu-tunables.c.
Signed-off-by: caiyinyu <caiyinyu@loongson.cn>
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Key Points:
1. On lasx & lsx platforms, We must use _dl_runtime_{profile, resolve}_{lsx, lasx}
to save vector registers.
2. Via "tunables", users can choose str/mem_{lasx,lsx,unaligned} functions with
`export GLIBC_TUNABLES=glibc.cpu.hwcaps=LASX,...`.
Note: glibc.cpu.hwcaps doesn't affect _dl_runtime_{profile, resolve}_{lsx, lasx}
selection.
Usage Notes:
1. Only valid inputs: LASX, LSX, UAL. Case-sensitive, comma-separated, no spaces.
2. Example: `export GLIBC_TUNABLES=glibc.cpu.hwcaps=LASX,UAL` turns on LASX & UAL.
Unmentioned features turn off. With default ifunc: lasx > lsx > unaligned >
aligned > generic, effect is: lasx > unaligned > aligned > generic; lsx off.
3. Incorrect GLIBC_TUNABLES settings will show error messages.
For example: On lsx platforms, you cannot enable lasx features. If you do
that, you will get error messages.
4. Valid input examples:
- GLIBC_TUNABLES=glibc.cpu.hwcaps=LASX: lasx > aligned > generic.
- GLIBC_TUNABLES=glibc.cpu.hwcaps=LSX,UAL: lsx > unaligned > aligned > generic.
- GLIBC_TUNABLES=glibc.cpu.hwcaps=LASX,UAL,LASX,UAL,LSX,LASX,UAL: Repetitions
allowed but not recommended. Results in: lasx > lsx > unaligned > aligned >
generic.
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