Re: [PATCH v2] x86: Fix backwards Prefer_No_VZEROUPPER check in ifunc-evex.h

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Jun 24, 2022 at 1:10 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Add third argument to X86_ISA_CPU_FEATURES_ARCH_P macro so the runtime
> CPU_FEATURES_ARCH_P check can be inverted if the
> MINIMUM_X86_ISA_LEVEL is not high enough to constantly evaluate
> the check.
>
> Use this new macro to correct the backwards check in ifunc-evex.h
> ---
>  sysdeps/x86/isa-ifunc-macros.h        | 29 +++++++++++++++++++++------
>  sysdeps/x86/isa-level.h               | 26 +++++++++---------------
>  sysdeps/x86_64/multiarch/ifunc-evex.h |  4 ++--
>  3 files changed, 35 insertions(+), 24 deletions(-)
>
> diff --git a/sysdeps/x86/isa-ifunc-macros.h b/sysdeps/x86/isa-ifunc-macros.h
> index ba6826d518..a3c98c841c 100644
> --- a/sysdeps/x86/isa-ifunc-macros.h
> +++ b/sysdeps/x86/isa-ifunc-macros.h
> @@ -56,15 +56,32 @@
>  # define X86_IFUNC_IMPL_ADD_V1(...)
>  #endif
>
> -#define X86_ISA_CPU_FEATURE_CONST_CHECK_ENABLED(name)                  \
> -  ((name##_X86_ISA_LEVEL) <= MINIMUM_X86_ISA_LEVEL)
> +/* Both X86_ISA_CPU_FEATURE_USABLE_P and X86_ISA_CPU_FEATURES_ARCH_P
> +   should only be used to check if a condition is true. I.e:
> +
> +        if (X86_ISA_CPU_FEATURE{S}_{USABLE|ARCH}_P (...)) // Good
> +        if (!X86_ISA_CPU_FEATURE{S}_{USABLE|ARCH}_P (...)) // Bad

If (X86_ISA_CPU_FEATURE{S}_{USABLE|ARCH}_P (...)) works,
if (!X86_ISA_CPU_FEATURE{S}_{USABLE|ARCH}_P (...)) should also
work.

> +
> +   There should be no need for inverting USABLE_P checks, but there is
> +   often need for inverting ARCH_P checks. If you want to get the not
> +   of an ARCH_P feature do:
> +
> +        if (X86_ISA_CPU_FEATURES_ARCH_P (..., !)) // Good
> + */
> +
>
>  #define X86_ISA_CPU_FEATURE_USABLE_P(ptr, name)                        \
> -  (X86_ISA_CPU_FEATURE_CONST_CHECK_ENABLED (name)                      \
> +  (((name##_X86_ISA_LEVEL) <= MINIMUM_X86_ISA_LEVEL)                   \
>     || CPU_FEATURE_USABLE_P (ptr, name))
>
> -#define X86_ISA_CPU_FEATURES_ARCH_P(ptr, name)                         \
> -  (X86_ISA_CPU_FEATURE_CONST_CHECK_ENABLED (name)                      \
> -   || CPU_FEATURES_ARCH_P (ptr, name))
> +
> +/* When using X86_ISA_CPU_FEATURES_ARCH_P a third argument must be
> +   provided to optionally invert the runtime CPU_FEATURES_ARCH_P
> +   check.  This is so we can consistently constant-evaluate conditions
> +   using Feature_X86_ISA_LEVEL <= MINIMUM_X86_ISA_LEVEL.  */
> +#define X86_ISA_CPU_FEATURES_ARCH_P(ptr, name, not)                    \
> +  (((name##_X86_ISA_LEVEL) <= MINIMUM_X86_ISA_LEVEL)                   \
> +   || not CPU_FEATURES_ARCH_P (ptr, name))
> +
>
>  #endif
> diff --git a/sysdeps/x86/isa-level.h b/sysdeps/x86/isa-level.h
> index 7cae11c228..bad9aba099 100644
> --- a/sysdeps/x86/isa-level.h
> +++ b/sysdeps/x86/isa-level.h
> @@ -65,12 +65,8 @@
>    (__X86_ISA_V1 + __X86_ISA_V2 + __X86_ISA_V3 + __X86_ISA_V4)
>
>
> -/*
> - * CPU Features that are hard coded as enabled depending on ISA build
> - *   level.
> - *    - Values > 0 features are always ENABLED if:
> - *          Value >= MINIMUM_X86_ISA_LEVEL
> - */
> +/* CPU Features that are default set depending on ISA build level.
> +   Feature is assumed set if: Value <= MINIMUM_X86_ISA_LEVEL.  */

This isn't accurate for Prefer_No_VZEROUPPER_X86_ISA_LEVEL.
I think this should be removed.  Each feature needs a comment to
describe the default.

>
>  /* ISA level >= 4 guaranteed includes.  */
> @@ -81,18 +77,16 @@
>  #define AVX2_X86_ISA_LEVEL 3
>  #define BMI2_X86_ISA_LEVEL 3
>
> -/*
> - * NB: This may not be fully assumable for ISA level >= 3. From
> - * looking over the architectures supported in cpu-features.h the
> - * following CPUs may have an issue with this being default set:
> - *      - AMD Excavator
> - */
> +/* NB: This feature is enabled when ISA level >= 3, which was disabled
> +   for the following CPUs:
> +        - AMD Excavator
> +   when ISA level < 3.  */
>  #define AVX_Fast_Unaligned_Load_X86_ISA_LEVEL 3
>
> -/*
> - * KNL (the only cpu that sets this supported in cpu-features.h)
> - * builds with ISA V1 so this shouldn't harm any architectures.
> - */
> +/* NB: This feature is disabled when ISA level >= 3, which was enabled
> +   for the following CPUs:
> +        - Intel KNL
> +   when ISA level < 3.  */
>  #define Prefer_No_VZEROUPPER_X86_ISA_LEVEL 3
>
>  #define ISA_SHOULD_BUILD(isa_build_level)                              \
> diff --git a/sysdeps/x86_64/multiarch/ifunc-evex.h b/sysdeps/x86_64/multiarch/ifunc-evex.h
> index 856c6261f8..310cfd269f 100644
> --- a/sysdeps/x86_64/multiarch/ifunc-evex.h
> +++ b/sysdeps/x86_64/multiarch/ifunc-evex.h
> @@ -37,7 +37,7 @@ IFUNC_SELECTOR (void)
>    if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX2)
>        && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, BMI2)
>        && X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> -                                     AVX_Fast_Unaligned_Load))
> +                                     AVX_Fast_Unaligned_Load, ))
>      {
>        if (X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512VL)
>           && X86_ISA_CPU_FEATURE_USABLE_P (cpu_features, AVX512BW))
> @@ -52,7 +52,7 @@ IFUNC_SELECTOR (void)
>         return OPTIMIZE (avx2_rtm);
>
>        if (X86_ISA_CPU_FEATURES_ARCH_P (cpu_features,
> -                                      Prefer_No_VZEROUPPER))
> +                                      Prefer_No_VZEROUPPER, !))
>         return OPTIMIZE (avx2);
>      }
>
> --
> 2.34.1
>


-- 
H.J.