From: Vineet Gupta <vineet.gupta@linux.dev>
To: libc-help@sourceware.org
Cc: siddhesh@gotplt.org, goldstein.w.n@gmail.com, hjl.tools@gmail.com
Subject: glibc tunables to disable axv not updating memcpy
Date: Wed, 7 Dec 2022 19:47:43 -0800 [thread overview]
Message-ID: <6d615a11-b863-1d76-e0ce-1f064cda06f1@linux.dev> (raw)
Hi,
I've been playing with x86_64 tunables to see how they are plumbed into
ifunc machinery, but running into a something which I don't understand
(this is glibc 2.35 so pretty recent).
I'm disabling AVX and friends as:
export
GLIBC_TUNABLES=glibc.cpu.hwcaps=--SSSE3,-SSE4_2,AVX,-AVX2,-AVX512F,-AVX512VL
A simple program which calls memcpy, seems to be calling
__memmove_evex_unaligned_erms() despite my tunable disabling AVX512VL.
|
| main (argc=2, argv=0x7fffffffdce8) at test-mcpu.c:10
| 10 memcpy(dest, src, atoi(argv[1]));
| (gdb) s
| __memmove_evex_unaligned_erms () at
../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:307
| 307 ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S
glibc dependency on AVX512VL for the said routine/
| IFUNC_IMPL_ADD (array, i, memmove,
| CPU_FEATURE_USABLE (AVX512VL),
| __memmove_evex_unaligned_erms)
|
There used to be AVX*_Usable for tunables but it seems that was changed
in 107e6a3c2212ba7a3a (" x86: Support usable check for all CPU features")
I also don't understand how the memcpy is being redirected to memmove,
since assembler in test has a PLT call to memcpy.
LD_DEBUG=all doesn't print any ifunc related info.
I've tried hacking x86 elf_machine_lazy_rel() add dl_debug_printf()
around R_X86_64_IRELATIVE - which seems to work but there's too less
info to help answer the exact ifunc plumbing.
Any ideas where to look ?
Thx,
-Vineet
next reply other threads:[~2022-12-08 3:47 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-08 3:47 Vineet Gupta [this message]
2022-12-08 8:31 ` Florian Weimer
2023-01-05 0:04 ` glibc ifunc debug (was Re: glibc tunables to disable axv not updating memcpy) Vineet Gupta
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