Re: Compile GLIBC without AVX support

[Florian Weimer <fweimer@redhat.com>]

* marekzmyslowski:

> I'm trying to compile the glibc 2.28 version without AVX support. However,
> whatever option I used, AVX files are still compiled and used. 
> I need this to use gdb record/replay tool.
> Here is the configure command I used:
>
> ../configure --enable-mathvec=no --prefix=/work/bin libc_cv_asm_avx512=no
> libc_cv_asm_avx512dq=no libc_cv_cc_avx512=no  libc_cv_cc_sse2avx=no
> --disable-avx2  --disable-avx512 --disable-avx-128-fma
>
> After make the result is following:
>
> root@d93e9cc876c3:/work/glibc/build# readelf -Ws libc.so | grep avx
>   1194: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS
> strcasecmp_l-avx.os
>   1198: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS strncase_l-avx.os
>   3257: 0000000000159070   337 FUNC    LOCAL  DEFAULT   13
> __memset_avx512_unaligned_erms
>   3272: 0000000000158a40     9 FUNC    LOCAL  DEFAULT   13
> __mempcpy_chk_avx512_unaligned
>   3276: 000000000014f1d0  1288 FUNC    LOCAL  DEFAULT   13 __strncmp_avx2
> .

GDB needs to communicate to the process that there is no AVX support, by
masking CPU flags.  I don't know if GDB has implemented that.  You may
have to use KVM or boot the system with a suitable clearcpuid argument
on the kernel command line.  If you do that, you won't have to rebuild
glibc because glibc will automatically stop using AVX.

You can reduce AVX usage further by using --disable-multi-arch, but some
uses will remain if the CPU advertises support for AVX because there are
some corner cases where the dynamic linker needs to save and restore
AVX-related registers.

Thanks,
Florian