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Re: [PATCH] 64bit unaligned access on 32bit kernel

To: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] 64bit unaligned access on 32bit kernel
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 7 Mar 2006 18:09:07 +0000
Cc: Atsushi Nemoto <anemo@mba.ocn.ne.jp>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
In-reply-to: <20060306170552.0aab29c5.akpm@osdl.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>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.4.2.1i
On Mon, Mar 06, 2006 at 05:05:52PM -0800, Andrew Morton wrote:

> I worry about what impact that change might have on code generation. 
> Hopefully none, if gcc is good enough.
> 
> But I cannot think of a better fix.

Below's fix results in exactly the same code size on all compilers and
configurations I've tested it.

I also have another more elegant fix which as a side effect makes
get_unaligned work for arbitrary data types but it that one results in a
slight code bloat:

gcc 4.1.0 ip22 64-bit
   text    data     bss     dec     hex filename
2717213  337920  167968 3223101  312e3d vmlinux
2717277  337920  167968 3223165  312e7d vmlinux         unaligned-4.patch

  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 4dc8ddb..9a63564 100644
--- a/include/asm-generic/unaligned.h
+++ b/include/asm-generic/unaligned.h
@@ -26,35 +26,13 @@
  * the linker will alert us to the problem via an unresolved reference
  * error.
  */
-extern void bad_unaligned_access_length(void) __attribute__((noreturn));
+extern int 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 
  */
 
@@ -76,26 +54,16 @@ static inline void __ustw(__u16 val, __u
        ptr->x = val;
 }
 
-#define __get_unaligned(ptr, size) ({          \
-       const void *__gu_p = ptr;               \
-       __typeof__(*(ptr)) 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();  \
-       };                                      \
-       val;                                    \
+#define __get_unaligned(ptr, size)                                             
\
+({                                                                             
\
+       const void *__gu_p = ptr;                                               
\
+       int __sz = size;                                                        
\
+                                                                               
\
+       ((__sz == 1) ? (__typeof__(*(ptr)))*(const __u8 *)__gu_p                
\
+       : ((__sz == 2) ? (__typeof__(*(ptr)))((struct __una_u16 *)__gu_p)->x    
\
+       : ((__sz == 4) ? (__typeof__(*(ptr)))((struct __una_u32 *)__gu_p)->x    
\
+       : ((__sz == 8) ? (__typeof__(*(ptr)))((struct __una_u64 *)__gu_p)->x    
\
+       : bad_unaligned_access_length()))));                                    
\
 })
 
 #define __put_unaligned(val, ptr, size)                \

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