linux-kernel - Re: [PATCH] x86: do not expose CONFIG

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Date:	Tue, 3 Feb 2009 19:19:55 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Harvey Harrison <harvey.harrison@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...nel.org>,
	Jaswinder Singh Rajput <jaswinder@...nel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Jaswinder Singh Rajput <jaswinderrajput@...il.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] x86: do not expose CONFIG_BSWAP to userspace

On Thursday 29 January 2009, H. Peter Anvin wrote:
> Harvey Harrison wrote:
> > On Wed, 2009-01-28 at 15:27 -0800, H. Peter Anvin wrote:
> >> Harvey Harrison wrote:
> >>> Well, that's unfortunate, how about we just export the BSWAP version
> >>> unconditionally and hope pure i386 just goes away someday?
> >>>
> >> Well, we already have MOVBE coming up, too...
> >>
> > 
> > Is someone already working on an __arch_swab{16|32|64}p to use them?
> > 
> 
> Not that I know of, but it's trivial enough.  They can also be used for 
> all-register swapping, too, with the advantage that you get register 
> decoupling.

I just realized that gcc-4.3 and higher have the __builtin_bswap{16,32,64}
functions on x86, which are supposed to do the right thing on any
platform. Maybe a patch like the one below can solve this for both the
kernel and for other users of the byteorder headers. Unfortunately, I
could not find out whether the builtins are available on all other
platforms as well.

Signed-off-by: Arnd Bergmann <arnd@...db.de>

--- a/arch/x86/include/asm/swab.h
+++ b/arch/x86/include/asm/swab.h
@@ -4,9 +4,17 @@
 #include <linux/types.h>
 #include <linux/compiler.h>
 
+#ifdef __CHECKER__
+extern unsigned long __builtin_bswap_32(unsigned long x);
+extern unsigned long long __builtin_bswap_64(unsigned long long x);
+#endif
+
 static inline __attribute_const__ __u32 __arch_swab32(__u32 val)
 {
-#ifdef __i386__
+#if defined(__GNUC__) && ((__GNUC__ > 4) || \
+		((__GNUC__ == 4) && (__GNUC_MINOR__ >= 3)))
+	val = __builtin_bswap32(val);
+#elif defined (__i386__)
 # ifdef CONFIG_X86_BSWAP
 	asm("bswap %0" : "=r" (val) : "0" (val));
 # else
@@ -28,7 +36,10 @@ static inline __attribute_const__ __u32 __arch_swab32(__u32 val)
 
 static inline __attribute_const__ __u64 __arch_swab64(__u64 val)
 {
-#ifdef __i386__
+#if defined(__GNUC__) && ((__GNUC__ > 4) || \
+		((__GNUC__ == 4) && (__GNUC_MINOR__ >= 3)))
+	return __builtin_bswap64(val);
+#elif defined (__i386__)
 	union {
 		struct {
 			__u32 a;
--
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