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Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementatio

To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
From: Ralf Baechle <ralf@linux-mips.org>
Date: Thu, 15 Feb 2007 14:34:41 +0000
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20070214203903.8d013170.akpm@linux-foundation.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20050830104056.GA4710@linux-mips.org> <20060306.203218.69025300.nemoto@toshiba-tops.co.jp> <20060306170552.0aab29c5.akpm@osdl.org> <20070214214226.GA17899@linux-mips.org> <20070214203903.8d013170.akpm@linux-foundation.org>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.4.2.2i
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@linux-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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