Re: [tip:x86/fpu] x86: Add new static_cpu_has() function using alternatives

["H. Peter Anvin" <hpa@zytor.com>]

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On 05/18/2010 01:10 PM, Eric Dumazet wrote:
> 
> Might be time to change Documentation/Changes about gcc requirements ...
> 
> # make
>   CHK     include/linux/version.h
>   CHK     include/generated/utsrelease.h
>   CALL    scripts/checksyscalls.sh
>   CHK     include/generated/compile.h
>   CC      arch/x86/kernel/process_32.o
> /usr/src/linux-2.6/arch/x86/include/asm/cpufeature.h: In function
> `prepare_to_copy':
> /usr/src/linux-2.6/arch/x86/include/asm/cpufeature.h:315: warning: asm
> operand 1 probably doesn't match constraints
> /usr/src/linux-2.6/arch/x86/include/asm/cpufeature.h:315: error:
> impossible constraint in `asm'
> /usr/src/linux-2.6/arch/x86/include/asm/cpufeature.h:313: warning:
> 'flag' might be used uninitialized in this function
> make[2]: *** [arch/x86/kernel/process_32.o] Error 1
> make[1]: *** [arch/x86/kernel] Error 2
> make: *** [arch/x86] Error 2
> # gcc -v
> Reading specs from /usr/lib/gcc/i386-redhat-linux/3.4.6/specs
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
> --infodir=/usr/share/info --enable-shared --enable-threads=posix
> --disable-checking --with-system-zlib --enable-__cxa_atexit
> --disable-libunwind-exceptions --enable-java-awt=gtk
> --host=i386-redhat-linux
> Thread model: posix
> gcc version 3.4.6 20060404 (Red Hat 3.4.6-10)
> 

Here is the fallback patch if you want to test it.

	-hpa

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diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index dca9c54..4681459 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -332,6 +332,7 @@ static __always_inline __pure bool __static_cpu_has(u8 bit)
 #endif
 }
 
+#if __GNUC__ >= 4
 #define static_cpu_has(bit)					\
 (								\
 	__builtin_constant_p(boot_cpu_has(bit)) ?		\
@@ -340,6 +341,12 @@ static __always_inline __pure bool __static_cpu_has(u8 bit)
 		__static_cpu_has(bit) :				\
 		boot_cpu_has(bit)				\
 )
+#else
+/*
+ * gcc 3.x is too stupid to do the static test; fall back to dynamic.
+ */
+#define static_cpu_has(bit) boot_cpu_has(bit)
+#endif
 
 #endif /* defined(__KERNEL__) && !defined(__ASSEMBLY__) */