Patchwork Update list of i686-class processors in sysdeps/x86/cpu-features.h

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Submitter Joseph Myers
Date Sept. 19, 2018, 9:42 p.m.
Message ID <alpine.DEB.2.21.1809192140360.4536@digraph.polyomino.org.uk>
Download mbox | patch
Permalink /patch/29478/
State New
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Comments

Joseph Myers - Sept. 19, 2018, 9:42 p.m.
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out of date.  This patch avoids the problem of the list getting out of
date by instead having conditionals on all the (few, old) pre-i686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.
Florian Weimer - Sept. 20, 2018, 8:45 a.m.
* 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

Patch

diff --git a/sysdeps/x86/cpu-features.h b/sysdeps/x86/cpu-features.h
index d342664c64..fb22d7b9d6 100644
--- a/sysdeps/x86/cpu-features.h
+++ b/sysdeps/x86/cpu-features.h
@@ -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 */