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Message-ID: <20070215143441.GA18155@linux-mips.org>
Date: Thu, 15 Feb 2007 14:34:41 +0000
From: Ralf Baechle <ralf@...ux-mips.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Atsushi Nemoto <anemo@....ocn.ne.jp>, linux-mips@...ux-mips.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
On Wed, Feb 14, 2007 at 08:39:03PM -0800, Andrew Morton wrote:
> Can someone please tell us how this magic works? (And it does appear to
> work).
>
> It seems to assuming that the compiler will assume that members of packed
> structures can have arbitrary alignment, even if that alignment is obvious.
>
> Which makes sense, but I'd like to see chapter-and-verse from the spec or
> from the gcc docs so we can rely upon it working on all architectures and
> compilers from now until ever more.
>
> IOW: your changlogging sucks ;)
It was my entry for the next edition of the C Puzzle Book ;-)
The whole union thing was only needed to get rid of a warning but Marcel's
solution does the same thing by attaching the packed keyword to the entire
structure instead, so this patch is now using his macros but using __packed
instead.
Ralf
Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>
diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
index 09ec447..60d94fc 100644
--- a/include/asm-generic/unaligned.h
+++ b/include/asm-generic/unaligned.h
@@ -1,122 +1,27 @@
-#ifndef _ASM_GENERIC_UNALIGNED_H_
-#define _ASM_GENERIC_UNALIGNED_H_
-
-/*
- * For the benefit of those who are trying to port Linux to another
- * architecture, here are some C-language equivalents.
- *
- * This is based almost entirely upon Richard Henderson's
- * asm-alpha/unaligned.h implementation. Some comments were
- * taken from David Mosberger's asm-ia64/unaligned.h header.
- */
-
-#include <linux/types.h>
-
-/*
- * The main single-value unaligned transfer routines.
- */
-#define get_unaligned(ptr) \
- __get_unaligned((ptr), sizeof(*(ptr)))
-#define put_unaligned(x,ptr) \
- __put_unaligned((__u64)(x), (ptr), sizeof(*(ptr)))
-
/*
- * This function doesn't actually exist. The idea is that when
- * someone uses the macros below with an unsupported size (datatype),
- * the linker will alert us to the problem via an unresolved reference
- * error.
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License. See the file "COPYING" in the main directory of this archive
+ * for more details.
*/
-extern void bad_unaligned_access_length(void) __attribute__((noreturn));
-
-struct __una_u64 { __u64 x __attribute__((packed)); };
-struct __una_u32 { __u32 x __attribute__((packed)); };
-struct __una_u16 { __u16 x __attribute__((packed)); };
-
-/*
- * Elemental unaligned loads
- */
-
-static inline __u64 __uldq(const __u64 *addr)
-{
- const struct __una_u64 *ptr = (const struct __una_u64 *) addr;
- return ptr->x;
-}
-
-static inline __u32 __uldl(const __u32 *addr)
-{
- const struct __una_u32 *ptr = (const struct __una_u32 *) addr;
- return ptr->x;
-}
-
-static inline __u16 __uldw(const __u16 *addr)
-{
- const struct __una_u16 *ptr = (const struct __una_u16 *) addr;
- return ptr->x;
-}
-
-/*
- * Elemental unaligned stores
- */
-
-static inline void __ustq(__u64 val, __u64 *addr)
-{
- struct __una_u64 *ptr = (struct __una_u64 *) addr;
- ptr->x = val;
-}
-
-static inline void __ustl(__u32 val, __u32 *addr)
-{
- struct __una_u32 *ptr = (struct __una_u32 *) addr;
- ptr->x = val;
-}
+#ifndef __ASM_GENERIC_UNALIGNED_H
+#define __ASM_GENERIC_UNALIGNED_H
-static inline void __ustw(__u16 val, __u16 *addr)
-{
- struct __una_u16 *ptr = (struct __una_u16 *) addr;
- ptr->x = val;
-}
+#include <linux/compiler.h>
-#define __get_unaligned(ptr, size) ({ \
- const void *__gu_p = ptr; \
- __u64 val; \
- switch (size) { \
- case 1: \
- val = *(const __u8 *)__gu_p; \
- break; \
- case 2: \
- val = __uldw(__gu_p); \
- break; \
- case 4: \
- val = __uldl(__gu_p); \
- break; \
- case 8: \
- val = __uldq(__gu_p); \
- break; \
- default: \
- bad_unaligned_access_length(); \
- }; \
- (__typeof__(*(ptr)))val; \
+#define get_unaligned(ptr) \
+({ \
+ struct __packed { \
+ typeof(*(ptr)) __v; \
+ } *__p = (void *) (ptr); \
+ __p->__v; \
})
-#define __put_unaligned(val, ptr, size) \
-do { \
- void *__gu_p = ptr; \
- switch (size) { \
- case 1: \
- *(__u8 *)__gu_p = val; \
- break; \
- case 2: \
- __ustw(val, __gu_p); \
- break; \
- case 4: \
- __ustl(val, __gu_p); \
- break; \
- case 8: \
- __ustq(val, __gu_p); \
- break; \
- default: \
- bad_unaligned_access_length(); \
- }; \
+#define put_unaligned(val, ptr) \
+do { \
+ struct __packed { \
+ typeof(*(ptr)) __v; \
+ } *__p = (void *) (ptr); \
+ __p->__v = (val); \
} while(0)
-#endif /* _ASM_GENERIC_UNALIGNED_H */
+#endif /* __ASM_GENERIC_UNALIGNED_H */
-
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