Re: [dpdk-dev] [PATCH v2] build: check AVX512 rather than compiler version

[Liang Ma <liangma@liangbit.com>]

On Tue, Jul 20, 2021 at 11:19:48AM +0100, Bruce Richardson wrote:
> On Mon, Jul 19, 2021 at 11:34:33PM +0100, Liang Ma wrote:
> > From: Liang Ma <liangma@bytedance.com>
> > 
> > GCC 6.3.0 has a known bug which related to _mm512_extracti64x4_epi64.
> > Please reference https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82887
> > 
> > Some DPDK PMD avx512 version heavily use _mm512_extracti64x4_epi6,
> > which cause building failure with debug buildtype.
> > 
> > Therefore, it's helpful to check if compiler work with
> > _mm512_extracti64x4_epi6.
> > 
> > This patch check the compiler compile result against the test code
> > snippet. If the checking is failed then disable avx512.
> > 
> > Bugzilla ID: 717
> > Fixes: e6a6a138919f (net/i40e: add AVX512 vector path)
> > Fixes: 808a17b3c1e6 (net/ice: add Rx AVX512 offload path)
> > Fixes: 4b64ccb328c9 (net/iavf: fix VLAN extraction in AVX512 path)
> > Cc: stable@dpdk.org
> > 
> > Reported-by: Liang Ma <liangma@liangbit.com>
> > Signed-off-by: Liang Ma <liangma@bytedance.com>
> > ---
> 
> Looks generally ok, but some comments below.
> 
> >  config/x86/meson.build | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/config/x86/meson.build b/config/x86/meson.build
> > index b9348c44de..77370a91f7 100644
> > --- a/config/x86/meson.build
> > +++ b/config/x86/meson.build
> > @@ -10,6 +10,19 @@ if not is_windows
> >      endif
> >  endif
> >  
> > +#check if compiler is working with _mm512_extracti64x4_epi64
> > +#Ref https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82887
> > +if not is_windows
> 
> Don't think this is needed here. The reason for it in the previous check
> for avx512 is because that check is done by a shell script which won't work
> on windows. Since we use meson functions for this check, it will work
> anywhere.
> 
> > +    code = '''#include <immintrin.h>
> > +    void test(__m512i zmm){
> > +        __m256i ymm = _mm512_extracti64x4_epi64(zmm, 0);}'''
> > +    result = cc.compiles(code, args : '-mavx512f', name : 'avx512 checking')
> > +    if result == false and cc.has_argument('-mno-avx512f')
> 
> Rather than checking for -mno-avx512f here, the whole block should probably
> be in an avx512 block itself. If the compiler doesn't have the "-mavx512f"
> flag, there may be problems with the "cc.compiles" command (or maybe it
> just counts as an error case?).
> 
> I'd suggest changing the "is_windows" condition to a cc.has_argument() one
> for avx512.
agree, I will fix that on v3.
> 
> > +        machine_args += '-mno-avx512f'
> > +        warning('Broken _mm512_extracti64x4_epi64, disabling AVX512 support')
> > +    endif
> > +endif
> > +
> >  # we require SSE4.2 for DPDK
> >  if cc.get_define('__SSE4_2__', args: machine_args) == ''
> >      message('SSE 4.2 not enabled by default, explicitly enabling')
> > -- 
> > 2.17.1
> >