Re: HEADS UP: plan to switch many ports over to GCC 12 soon

On Mon, 1 Jul 2024, Rhialto wrote:

> Summary: it seems that libdeflate explicitly uses some gcc-intrinsic
> functions on gcc 11+, and it is expected that the assembler can handle
> the result.  So this doesn't depend on a compiler option and it looks
> like we cannot disable it either.
>
> Long version:
>
> This is from .work.log for adler32.c, the offending source file.
>
> [*] cc -DLIBDEFLATE_DLL -Dlibdeflate_shared_EXPORTS -I/tmp/pkgsrc/devel/libdeflate/default/libdeflate-1.20 -O2 -I/usr/pkg/include -I/usr/include -O2 -DNDEBUG -std=gnu99 -fPIC -fvisibility=hidden -Wall -Wdeclaration-after-statement -Wimplicit-fallthrough -Wmissing-field-initializers -Wmissing-prototypes -Wpedantic -Wshadow -Wstrict-prototypes -Wundef -Wvla -MD -MT CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o -MF CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o.d -o CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o -c /tmp/pkgsrc/devel/libdeflate/default/libdeflate-1.20/lib/adler32.c
> <.> /tmp/pkgsrc/devel/libdeflate/default/.gcc/bin/gcc -fcommon -fstack-protector-strong -D_FORTIFY_SOURCE=2 -Wl,-zrelro -fPIC -DLIBDEFLATE_DLL -Dlibdeflate_shared_EXPORTS -I/tmp/pkgsrc/devel/libdeflate/default/libdeflate-1.20 -O2 -I/tmp/pkgsrc/devel/libdeflate/default/.buildlink/include -O2 -DNDEBUG -std=gnu99 -fPIC -fvisibility=hidden -Wall -Wdeclaration-after-statement -Wimplicit-fallthrough -Wmissing-field-initializers -Wmissing-prototypes -Wpedantic -Wshadow -Wstrict-prototypes -Wundef -Wvla -MD -MT CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o -MF CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o.d -o CMakeFiles/libdeflate_shared.dir/lib/adler32.c.o -c /tmp/pkgsrc/devel/libdeflate/default/libdeflate-1.20/lib/adler32.c
>
> pkg_comp:default.conf# ls -l /tmp/pkgsrc/devel/libdeflate/default/.gcc/bin/gcc
> lrwxr-xr-x  1 root  wheel  22 Jul  1 08:30 /tmp/pkgsrc/devel/libdeflate/default/.gcc/bin/gcc -> /usr/pkg/gcc12/bin/gcc
>
> The
>
> /tmp//cceAo651.s:1377: Error: unsupported instruction `vpdpbusd'
>
> seems to be the result of code, included from libdeflate-1.20/lib/adler32_impl.h:
>
> /*
> * AVX-VNNI implementation.  This is used on CPUs that have AVX2 and AVX-VNNI
> * but don't have AVX-512, for example Intel Alder Lake.
> */
> #if GCC_PREREQ(11, 1) || CLANG_PREREQ(12, 0, 13000000) || MSVC_PREREQ(1930)
> #  define adler32_x86_avx2_vnni adler32_x86_avx2_vnni
> #  define SUFFIX                           _avx2_vnni
> #  define ATTRIBUTES            _target_attribute("avx2,avxvnni")
> #  define VL                    32
> #  define USE_VNNI              1
> #  define USE_AVX512            0
> #  include "adler32_template.h"
> #endif
>
> libdeflate-1.20/lib/x86/adler32_template.h has
>
> #if USE_VNNI
>        /*
>         * This is Adler-32 using the vpdpbusd instruction from AVX512VNNI or
>         * AVX-VNNI.  vpdpbusd multiplies the unsigned bytes of one vector by
>         * the signed bytes of another vector and adds the sums in groups of 4
>         * to the 32-bit elements of a third vector.  We use it in two ways:
>         * multiplying the data bytes by a sequence like 64,63,62,...,1 for
>         * calculating part of s2, and multiplying the data bytes by an all-ones
>         * sequence 1,1,1,...,1 for calculating s1 and part of s2.  The all-ones
>         * trick seems to be faster than the alternative of vpsadbw + vpaddd.
>         */
>
> ...
>                                v_s2   = VDPBUSD(v_s2,   data_a, mults);
> and several more such calls
> (VDPBUSD isn't exactly the same word as vpdpbusd but it's the closest one)
>
> and this value for VDPBUSD, chosen because VL == 32 and USE_AVX512 == 0.
> #    define VDPBUSD(a, b, c)    _mm256_dpbusd_avx_epi32((a), (b), (c))
>
>
> So it seems that gcc12 is known to have this intrinsic function, some people
> are using it, and the assembler is expected to cope somehow.
> It doesn't seem like that gcc generates this instruction spontaneously.
>
> /usr/pkg/gcc12/lib/gcc/x86_64--netbsd/12.3.0/include/avxvnniintrin.h has
>
> extern __inline __m256i
> __attribute__((__gnu_inline__, __always_inline__, __artificial__))
> _mm256_dpbusd_avx_epi32(__m256i __A, __m256i __B, __m256i __C)
> {
>  return   (__m256i) __builtin_ia32_vpdpbusd_v8si ((__v8si) __A,
>                                                   (__v8si) __B,
>                                                   (__v8si) __C);
> }
>

In some cases, gcc 12 generates a `{vex}` pseudo-prefix for AVX-NNI intrinsics:

```
$ gcc --version
gcc (nb3 20231008) 10.5.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ /usr/pkg/gcc12/bin/gcc --version
gcc (GCC) 12.3.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ pwd
/tmp/libdeflate-1.20

$ gcc -O2 -S -o /tmp/a1.s lib/adler32.c; as -o /dev/null /tmp/a1.s

$ /usr/pkg/gcc12/bin/gcc -O2 -S -o /tmp/a2.s lib/adler32.c; as -o /dev/null /tmp/a2.s

/tmp/a2.s: Assembler messages:
/tmp/a2.s:761: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:763: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:765: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:766: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:767: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:785: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:786: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:812: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:814: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:907: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:908: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:909: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:910: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:915: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:917: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:918: Error: unsupported instruction `vpdpbusd'
/tmp/a2.s:919: Error: unsupported instruction `vpdpbusd'

$ sed -n 761p /tmp/a2.s
         {vex} vpdpbusd  %ymm7, %ymm2, %ymm0

$
```

You'll need a newer assembler for gcc-12's output -- binutils-2.36, at least.
(The as(1) from binutils-2.39 in -HEAD works fine.)

-RVP