<sys/platform/x86.h>: Remove the C preprocessor magic

In <sys/platform/x86.h>, define CPU features as enum instead of using
the C preprocessor magic to make it easier to wrap this functionality
in other languages.  Move the C preprocessor magic to internal header
for better GCC codegen when more than one features are checked in a
single expression as in x86-64 dl-hwcaps-subdirs.c.

1. Rename COMMON_CPUID_INDEX_XXX to CPUID_INDEX_XXX.
2. Move CPUID_INDEX_MAX to sysdeps/x86/include/cpu-features.h.
3. Remove struct cpu_features and __x86_get_cpu_features from
<sys/platform/x86.h>.
4. Add __x86_get_cpuid_feature_leaf to <sys/platform/x86.h> and put it
in libc.
5. Make __get_cpu_features() private to glibc.
6. Replace __x86_get_cpu_features(N) with __get_cpu_features().
7. Add _dl_x86_get_cpu_features to GLIBC_PRIVATE.
8. Use a single enum index for each CPU feature detection.
9. Pass the CPUID feature leaf to __x86_get_cpuid_feature_leaf.
10. Return zero struct cpuid_feature for the older glibc binary with a
smaller CPUID_INDEX_MAX [BZ #27104].
11. Inside glibc, use the C preprocessor magic so that cpu_features data
can be loaded just once leading to more compact code for glibc.

256 bits are used for each CPUID leaf.  Some leaves only contain a few
features.  We can add exceptions to such leaves.  But it will increase
code sizes and it is harder to provide backward/forward compatibilities
when new features are added to such leaves in the future.

When new leaves are added, _rtld_global_ro offsets will change which
leads to race condition during in-place updates. We may avoid in-place
updates by

1. Rename the old glibc.
2. Install the new glibc.
3. Remove the old glibc.

NB: A function, __x86_get_cpuid_feature_leaf , is used to avoid the copy
relocation issue with IFUNC resolver as shown in IFUNC resolver tests.

1 files changed, 2 insertions, 5 deletions

diff --git a/manual/platform.texi b/manual/platform.texi
index b67683aeb3..6caf68d796 100644
--- a/manual/platform.texi
+++ b/manual/platform.texi
@@ -142,13 +142,10 @@ reserved.
 Facilities specific to X86 that are not specific to a particular
 operating system are declared in @file{sys/platform/x86.h}.
 
-@deftypefun {const struct cpu_features *} __x86_get_cpu_features (unsigned int @var{max})
+@deftypefun {const struct cpuid_feature *} __x86_get_cpuid_feature_leaf (unsigned int @var{leaf})
 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
 Return a pointer to x86 CPU feature structure used by query macros for x86
-CPU features.  If @var{max} exceeds @code{COMMON_CPUID_INDEX_MAX} which
-is the limit of the CPUID leaves supported by @Theglibc{}, the function
-returns @code{NULL}, indicating that the queried processor feature is
-unsupported by @Theglibc{} run-time.
+CPU feature @var{leaf}.
 @end deftypefun
 
 @deftypefn Macro int HAS_CPU_FEATURE (@var{name})