glibc ifunc debug (was Re: glibc tunables to disable axv not updating memcpy)

[Vineet Gupta <vineet.gupta@linux.dev>]

On 12/8/22 00:31, Florian Weimer wrote:
> * 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.

Unfortunately gdb doesn't want to put a breakpoint in the ifunc header.
I tried a -g3 -O2 build and excellent tips at [1] - Ubuntu 22.04 - gdb 12.1

| gdb elf/ld-linux-x86-64.so.2
| set exec-wrapper env LD_LIBRARY_PATH=....
| b _start
| ..
| b sysdeps/x86_64/multiarch/ifunc-memmove.h:48
| No source file named sysdeps/x86_64/multiarch/ifunc-memmove.h
| b ../sysdeps/x86_64/multiarch/ifunc-memmove.h:48

It does seem to recognize other headers in x86_64.
Do I need a newer gdb ? Also can't build glibc with -O0 anyways.

Thx,
-Vineet

[1] https://sourceware.org/glibc/wiki/Debugging/Development_Debugging