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From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Wed, 15 Jul 2020 07:55:57 -0700
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Subject: PING: V7: [PATCH 2/2] x86: Install <sys/platform/x86.h> [BZ #26124]
To: GNU C Library <libc-alpha@sourceware.org>
Cc: Florian Weimer <fweimer@redhat.com>
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On Mon, Jun 29, 2020 at 9:35 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> Install <sys/platform/x86.h> so that programmers can do
>
>  #if __has_include(<sys/platform/x86.h>)
>  #include <sys/platform/x86.h>
>  #endif
>  ...
>
>    if (CPU_FEATURE_USABLE (SSE2))
>  ...
>    if (CPU_FEATURE_USABLE (AVX2))
>  ...
>
> <sys/platform/x86.h> exports only:
>
> enum
> {
>   COMMON_CPUID_INDEX_1 = 0,
>   COMMON_CPUID_INDEX_7,
>   COMMON_CPUID_INDEX_80000001,
>   COMMON_CPUID_INDEX_D_ECX_1,
>   COMMON_CPUID_INDEX_80000007,
>   COMMON_CPUID_INDEX_80000008,
>   COMMON_CPUID_INDEX_7_ECX_1,
>   /* Keep the following line at the end.  */
>   COMMON_CPUID_INDEX_MAX
> };
>
> struct cpuid_features
> {
>   struct cpuid_registers cpuid;
>   struct cpuid_registers usable;
> };
>
> struct cpu_features
> {
>   struct cpu_features_basic basic;
>   struct cpuid_features features[COMMON_CPUID_INDEX_MAX];
> };
>
> /* Get a pointer to the CPU features structure.  */
> extern const struct cpu_features *__x86_get_cpu_features
>   (unsigned int max) __attribute__ ((const));
>
> Since all feature checks are done through macros, programs compiled with
> a newer <sys/platform/x86.h> are compatible with the older glibc binaries
> as long as the layout of struct cpu_features is identical.  The features
> array can be expanded with backward binary compatibility for both .o and
> .so files.  When COMMON_CPUID_INDEX_MAX is increased to support new
> processor features, __x86_get_cpu_features in the older glibc binaries
> returns NULL and HAS_CPU_FEATURE/CPU_FEATURE_USABLE return falses on the
> new processor feature.  No new symbol version is neeeded.
>
> Both CPU_FEATURE_USABLE and HAS_CPU_FEATURE are provided.  HAS_CPU_FEATURE
> can be used to identify processor features.
>
> Note: Although GCC has __builtin_cpu_supports, it only supports a subset
> of <sys/platform/x86.h> and it is equivalent to CPU_FEATURE_USABLE.  It
> doesn't support HAS_CPU_FEATURE.

https://sourceware.org/pipermail/libc-alpha/2020-June/115546.html

-- 
H.J.