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Debugging setting these flags very early is not very easy.
What is a good approach, for example, attempting to disable AVX, with
several approaches in the pattern:
$ GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX,-AVX2,-AVX_Usable /lib64/ld-2.28.so --list-diagnostics|grep -i AVX
env[0x0]="GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX,-AVX2,-AVX_Usable"
x86.cpu_features.preferred.AVX_Fast_Unaligned_Load=0x1
x86.cpu_features.preferred.Prefer_No_AVX512=0x1
x86.cpu_features.preferred.MathVec_Prefer_No_AVX512=0x0
The desired trest is, for example, to get x86.cpu_features.preferred.AVX_Fast_Unaligned_Load set to zero.
This request is to help an user to experiment with forcing glibc to choose __memcpy_ssse3_back (as was done in rhel7) instead of __memcpy_avx_unaligned_erms
Tried several combinations from the switch in sysdeps/x86/cpu-tunables.c
and could not find a good pattern to experiment. Note that info libc
shows an incorrect path to check for possible values.
The main request is to be able to experiment with different ifunc patterns
for benchmark purposes.
This appears to cause it to use the alternate memcpy.
GLIBC_TUNABLES="glibc.cpu.hwcaps=Fast_Copy_Backward,SSSE3,-Fast_Unaligned_Copy,-Fast_Unaligned_Load,-ERMS,-Prefer_ERMS,-AVX,-AVX_Usable,-AVX_Fast_Unaligned_Load"
I am going to move this bug over to Jira. A link to issues.redhat.com should appear in due course. If you cannot access this ticket, feel free to ask for assistance here.