Update list of i686-class processors in sysdeps/x86/cpu-features.h
Commit Message
On Wed, 19 Sep 2018, Florian Weimer wrote:
> > We have a general principle that it's best for compile-time selection of
> > subarchitecture variants of functions to be based on how the compiler
> > behaves rather than configuring for a variant host triplet or using
> > --with-cpu. We could e.g. select SSE functions by default based on
> > __SSE2__ (even if we don't do so at present). But testing whether the
> > compiler produces a CMOV instruction for particular code seems more
> > fragile than testing #if conditions for processors without CMOV.
>
> I agree about CMOV because it is extremely unlikely that we will see
> another useful x86 CPU that doesn't have CMOV.
So how about this patch that makes sysdeps/x86/cpu-features.h use short
lists of processors without certain features instead of long lists of
those with them?
Invert sense of list of i686-class processors in sysdeps/x86/cpu-features.h.
I noticed that sysdeps/x86/cpu-features.h had conditionals on whether
to define HAS_CPUID, HAS_I586 and HAS_I686 with a long list of
preprocessor macros for i686-and-later processors which however was
out of date. This patch avoids the problem of the list getting out of
date by instead having conditionals on all the (few, old) pre-i686
processors for which GCC has preprocessor macros, rather than the
(many, expanding list) i686-and-later processors. It seems HAS_I586
and HAS_I686 are unused so the only effect of these macros being
missing is that 32-bit glibc built for one of these processors would
end up doing runtime detection of CPUID availability.
i386 builds are prevented by a configure test so there is no need to
allow for them here. __geode__ (no long nops?) and __k6__ (no CMOV,
at least according to GCC) are conservatively handled as i586, not
i686, here (as noted above, this is a theoretical distinction at
present in that only HAS_CPUID appears to be used).
Tested for x86.
2018-09-19 Joseph Myers <joseph@codesourcery.com>
* sysdeps/x86/cpu-features.h [__geode__ || __k6__]: Handle like
[__i586__ || __pentium__].
[__i486__]: Handle explicitly.
(HAS_CPUID): Define to 1 if above macros are undefined.
(HAS_I586): Likewise.
(HAS_I686): Likewise.
Comments
* Joseph Myers:
> 2018-09-19 Joseph Myers <joseph@codesourcery.com>
>
> * sysdeps/x86/cpu-features.h [__geode__ || __k6__]: Handle like
> [__i586__ || __pentium__].
> [__i486__]: Handle explicitly.
> (HAS_CPUID): Define to 1 if above macros are undefined.
> (HAS_I586): Likewise.
> (HAS_I686): Likewise.
Looks okay to me, thanks.
Florian
@@ -257,30 +257,19 @@ extern const struct cpu_features *__get_cpu_features (void)
#ifdef __x86_64__
# define HAS_CPUID 1
-#elif defined __i586__ || defined __pentium__
+#elif (defined __i586__ || defined __pentium__ \
+ || defined __geode__ || defined __k6__)
# define HAS_CPUID 1
# define HAS_I586 1
# define HAS_I686 HAS_ARCH_FEATURE (I686)
-#elif (defined __i686__ || defined __pentiumpro__ \
- || defined __pentium4__ || defined __nocona__ \
- || defined __atom__ || defined __core2__ \
- || defined __corei7__ || defined __corei7_avx__ \
- || defined __core_avx2__ || defined __nehalem__ \
- || defined __sandybridge__ || defined __haswell__ \
- || defined __knl__ || defined __bonnell__ \
- || defined __silvermont__ \
- || defined __k6__ || defined __k8__ \
- || defined __athlon__ || defined __amdfam10__ \
- || defined __bdver1__ || defined __bdver2__ \
- || defined __bdver3__ || defined __bdver4__ \
- || defined __btver1__ || defined __btver2__)
-# define HAS_CPUID 1
-# define HAS_I586 1
-# define HAS_I686 1
-#else
+#elif defined __i486__
# define HAS_CPUID 0
# define HAS_I586 HAS_ARCH_FEATURE (I586)
# define HAS_I686 HAS_ARCH_FEATURE (I686)
+#else
+# define HAS_CPUID 1
+# define HAS_I586 1
+# define HAS_I686 1
#endif
#endif /* cpu_features_h */