Re: V3: [PATCH] x86: Install <sys/platform/x86.h> [BZ #26124]

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu:

> I changed the manual to
>
> @deftypefn Macro int HAS_CPU_FEATURE (@var{name})
> This macro returns a nonzero value (true) if the processor has the feature
> @var{name}.
> @end deftypefn
>
> @deftypefn Macro int CPU_FEATURE_USABLE (@var{name})
> This macro returns a nonzero value (true) if the processor feature
> @var{name} is supported by the operating system.

Does this mean that it's necessary to check both before using the
feature?  This is what the description implies to me.

If CPU_FEATURE_USABLE implies HAS_CPU_FEATURE (so it's not necessary to
check both), then I don't see the use case for HAS_CPU_FEATURE.  To me,
exposing both liks like a trap for programmers: they might check CPU
support only, but not operating system support.  That's trap that we
have fallen into with glibc itself at least once.

>> >> struct cpu_features (even in its reduced form) is fairly large.  We will
>> >> never be able to reduce its size again if it becomes public ABI.
>> >
>> > Fixed by
>> >
>> > struct cpu_features
>> > {
>> >   struct cpu_features_basic basic;
>> >   unsigned int *usable_p;
>> >   struct cpuid_registers cpuid[COMMON_CPUID_INDEX_MAX];
>> > };
>>
>> I think the cpuid member is the fat part.  But the pointer indirection
>> allows us to grow the *usable_p part without having to duplicate the
>> backing storage for __x86_get_cpu_features, so it is an improvement.
>>
>> > __builtin_cpu_supports is equivalent to CPU_FEATURE_USABLE and it
>> > doesn't support HAS_CPU_FEATURE which does provide useful information.
>>
>> I'm still puzzled as to why you aren't extending the existing function.
>>
>
> I am working on it:
>
> https://gcc.gnu.org/pipermail/gcc-patches/2020-May/546522.html
>
> But it is very unlikely to support HAS_CPU_FEATURE and
> <sys/platform/x86.h> works with all GCCs.

On the other hand, it's easier for our users to upgrade GCC than to
update glibc.

Thanks,
Florian