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* [Bug string/31675] New: glibc.cpu.hwcaps doesn't work as documented
@ 2024-04-23 18:11 mikulas at artax dot karlin.mff.cuni.cz
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From: mikulas at artax dot karlin.mff.cuni.cz @ 2024-04-23 18:11 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=31675
Bug ID: 31675
Summary: glibc.cpu.hwcaps doesn't work as documented
Product: glibc
Version: 2.37
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: string
Assignee: unassigned at sourceware dot org
Reporter: mikulas at artax dot karlin.mff.cuni.cz
Target Milestone: ---
If I disable AVX512 and AVX2 with
"export GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX512F,-AVX2,-AVX512VL",
glibc uses __memcpy_avx_unaligned_erms version of memcpy that uses AVX2
instructions.
We need to add "AVX_Fast_Unaligned_Load" to suppress AVX2. If I set:
"export
GLIBC_TUNABLES=glibc.cpu.hwcaps=-AVX512F,-AVX2,-AVX512VL,-AVX_Fast_Unaligned_Load",
then glibc will use the SSE3 version of memcpy.
The need to set "AVX_Fast_Unaligned_Load" should be either documented or fixed.
The documentation says "The glibc.cpu.hwcaps=-xxx,yyy,-zzz... tunable allows
the user to enable CPU/ARCH feature yyy, disable CPU/ARCH feature xxx and zzz
where the feature name is case-sensitive and has to match the ones in
sysdeps/x86/include/cpu-features.h.", but the file "cpu-features.h" doesn't
contain "AVX_Fast_Unaligned_Load".
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2024-04-23 18:11 [Bug string/31675] New: glibc.cpu.hwcaps doesn't work as documented mikulas at artax dot karlin.mff.cuni.cz
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