Re: glibc tunables to disable axv not updating memcpy

[Florian Weimer <fweimer@redhat.com>]

* Vineet Gupta via Libc-help:

> 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)
> |

This is for the test suite and the benchmarks.  The production selector
is in sysdeps/x86_64/multiarch/ifunc-memmove.h.  You should be able to
set a line-based breakpoint and single-step through it.

> LD_DEBUG=all doesn't print any ifunc related info.

I want to add logging, which is why I asked on the binutils list:

Subject: Named local symbols in the ELF dynamic symbol table
To: binutils@sourceware.org
Date: Tue, 06 Dec 2022 18:11:08 +0100 (1 day, 15 hours, 17 minutes ago)
Message-ID: <87y1rke9ub.fsf@oldenburg.str.redhat.com>

If we have the symbol names in the dynamic symbol tables, we can use
dladdr or a custom implementation of it to get symbol names.

If I don't get a reply, I'll settle for printing relative addresses.
People can then use an external tool to resolve them to symbols.

Thanks,
Florian