Re: PING: V7: [PATCH 2/2] x86: Install <sys/platform/x86.h> [BZ #26124]

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

On Wed, Jul 15, 2020 at 7:55 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> 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
>

The first patch has been checked into master branch.   The second patch is at:

https://sourceware.org/pipermail/libc-alpha/2020-June/115547.html

-- 
H.J.