[AMD Official Use Only - AMD Internal Distribution Only] > -----Original Message----- > From: Bruce Richardson <bruce.richard...@intel.com> > Sent: Tuesday, June 10, 2025 8:37 PM > To: Varghese, Vipin <vipin.vargh...@amd.com> > Cc: dev@dpdk.org; Song, Keesang <keesang.s...@amd.com> > Subject: Re: [PATCH v4] build: reduce use of AVX compiler flags > > Caution: This message originated from an External Source. Use proper caution > when opening attachments, clicking links, or responding. > > > On Tue, Jun 10, 2025 at 02:52:24PM +0000, Varghese, Vipin wrote: > > [Public] > > > > Snipped > > > > > > > > > > In above log I get `2 instances of march`; logs `-march=native > > > > -mrtm -DALLOW_EXPERIMENTAL_API -DALLOW_INTERNAL_API -Wno- > format- > > > truncation -Wno-address-of-packed-member - > > > DRTE_LOG_DEFAULT_LOGTYPE=pmd.net.i40e -DCC_AVX512_SUPPORT > - > > > march=skylake-avx512`. > > > > > > > > Question-1: I think this is not expected right? The > > > > `-march=native` is populated > > > from `cflags` and `-march= skylake-avx512` is populated from ` > cc_avx512_flags`. > > > > > > The above command is correct. So long as the compiler supports > > > AVX-512 we will always compile the AVX-512 code paths for runtime > > > selection. In practice, all supported compilers have AVX-512 > > > support, so in reality we have the two scenarios you tested: > > > > > > * The target architecture e.g. znver3 in your case, doesn't support > > > avx512, > > > so the meson.build file adds on the necessary flags to add this support, > > > i.e. that file is compiled with -march=skylake=avx512, which is the > > > minimum ISA that gives you the necessary support. > > > * The target architecture, e.g. znver4, does support AVX-512, then no > > > additional flags are added and the files are compiled "as normal" > > > > > > In both these cases, whether the target architecture is specified as > > > "native" or explicitly makes no difference. > > > > So, for files which needs avx512 support like acl_avx512, fib_tire_avx512 > > and > others, we then only pass `cc_avx512_flags`. > > All other cases this is ignored. Thank you that makes sense. > > > > > > > > > Question-2: if the target is meet minimal ISA why not we use > > > > `-march=x86-64- > > > v4`? > > > > > > > > > > Good point, that would indeed be better. I'm just not sure whether > > > it is supported widely enough on our compilers. Do you know what gcc > > > and clang versions support that target? > > > > As I recollect we have been using `x86-64-v4` this from gcc 12.3 and > > clang-14. > > I am not sure if the older versions supports `avx512 that is > > x86-64-v4` > > > > Checking with godbolt.org, to see what versions give errors or not. > * Clang accepts the argument from v12 onwards > * GCC accepts the argument from v11 onwards. > > Unfortunately, that means that we need to at least keep some form of backward > compatibility for older compilers until such time as we have those versions as > minimum. > > If you are ok with the patch in general, I'll see if I can respin a version > where we > check for support for -march=x86-64-v4 and use that when we can, otherwise > falling back to skylake-avx512 as here. Does that seem a reasonable approach?
Thank you Bruce, that will be great. First try for `x86-86-v4` if it fails fall back to `skylake-avx512` > > /Bruce
