PING^3: V8 [PATCH 2/2] x86: Install <sys/platform/x86.h> [BZ #26124]

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Wed, Aug 19, 2020 at 5:52 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Wed, Aug 12, 2020 at 7:00 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Wed, Aug 5, 2020 at 4:01 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 false 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.
>
> PING:
>
> https://sourceware.org/pipermail/libc-alpha/2020-August/116865.html
>

If there are no more comments, I will check it next Friday,
Sept 11.


-- 
H.J.