Commit 8e2c2002 authored by Trent Piepho's avatar Trent Piepho Committed by Linus Torvalds

Fix constant folding and poor optimization in byte swapping code

Constant folding does not work for the swabXX() byte swapping functions,
and the C versions optimize poorly.

Attempting to initialize a global variable to swab16(0x1234) or put
something like "case swab32(42):" in a switch statement will not compile.
It can work, swab.h just isn't doing it correctly.  This patch fixes that.

Contrary to the comment in asm-i386/byteorder.h, gcc does not recognize the
"C" version of swab16 and turn it into efficient code.  gcc can do this,
just not with the current code.  The simple function:

u16 foo(u16 x) { return swab16(x); }

Would compile to:
        movzwl  %ax, %eax
        movl    %eax, %edx
        shrl    $8, %eax
        sall    $8, %edx
        orl     %eax, %edx

With this patch, it will compile to:
        rolw    $8, %ax

I also attempted to document the maze different macros/inline functions
that are used to create the final product.
Signed-off-by: default avatarTrent Piepho <xyzzy@speakeasy.org>
Cc: Francois-Rene Rideau <fare@tunes.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 02fb6149
......@@ -10,6 +10,10 @@
* separated swab functions from cpu_to_XX,
* to clean up support for bizarre-endian architectures.
*
* Trent Piepho <xyzzy@speakeasy.org> 2007114
* make constant-folding work, provide C versions that
* gcc can optimize better, explain different versions
*
* See asm-i386/byteorder.h and suches for examples of how to provide
* architecture-dependent optimized versions
*
......@@ -17,40 +21,66 @@
#include <linux/compiler.h>
/* Functions/macros defined, there are a lot:
*
* ___swabXX
* Generic C versions of the swab functions.
*
* ___constant_swabXX
* C versions that gcc can fold into a compile-time constant when
* the argument is a compile-time constant.
*
* __arch__swabXX[sp]?
* Architecture optimized versions of all the swab functions
* (including the s and p versions). These can be defined in
* asm-arch/byteorder.h. Any which are not, are defined here.
* __arch__swabXXs() is defined in terms of __arch__swabXXp(), which
* is defined in terms of __arch__swabXX(), which is in turn defined
* in terms of ___swabXX(x).
* These must be macros. They may be unsafe for arguments with
* side-effects.
*
* __fswabXX
* Inline function versions of the __arch__ macros. These _are_ safe
* if the arguments have side-effects. Note there are no s and p
* versions of these.
*
* __swabXX[sb]
* There are the ones you should actually use. The __swabXX versions
* will be a constant given a constant argument and use the arch
* specific code (if any) for non-constant arguments. The s and p
* versions always use the arch specific code (constant folding
* doesn't apply). They are safe to use with arguments with
* side-effects.
*
* swabXX[sb]
* Nicknames for __swabXX[sb] to use in the kernel.
*/
/* casts are necessary for constants, because we never know how for sure
* how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.
*/
#define ___swab16(x) \
({ \
__u16 __x = (x); \
((__u16)( \
(((__u16)(__x) & (__u16)0x00ffU) << 8) | \
(((__u16)(__x) & (__u16)0xff00U) >> 8) )); \
})
#define ___swab32(x) \
({ \
__u32 __x = (x); \
((__u32)( \
(((__u32)(__x) & (__u32)0x000000ffUL) << 24) | \
(((__u32)(__x) & (__u32)0x0000ff00UL) << 8) | \
(((__u32)(__x) & (__u32)0x00ff0000UL) >> 8) | \
(((__u32)(__x) & (__u32)0xff000000UL) >> 24) )); \
})
#define ___swab64(x) \
({ \
__u64 __x = (x); \
((__u64)( \
(__u64)(((__u64)(__x) & (__u64)0x00000000000000ffULL) << 56) | \
(__u64)(((__u64)(__x) & (__u64)0x000000000000ff00ULL) << 40) | \
(__u64)(((__u64)(__x) & (__u64)0x0000000000ff0000ULL) << 24) | \
(__u64)(((__u64)(__x) & (__u64)0x00000000ff000000ULL) << 8) | \
(__u64)(((__u64)(__x) & (__u64)0x000000ff00000000ULL) >> 8) | \
(__u64)(((__u64)(__x) & (__u64)0x0000ff0000000000ULL) >> 24) | \
(__u64)(((__u64)(__x) & (__u64)0x00ff000000000000ULL) >> 40) | \
(__u64)(((__u64)(__x) & (__u64)0xff00000000000000ULL) >> 56) )); \
})
static __inline__ __attribute_const__ __u16 ___swab16(__u16 x)
{
return x<<8 | x>>8;
}
static __inline__ __attribute_const__ __u32 ___swab32(__u32 x)
{
return x<<24 | x>>24 |
(x & (__u32)0x0000ff00UL)<<8 |
(x & (__u32)0x00ff0000UL)>>8;
}
static __inline__ __attribute_const__ __u64 ___swab64(__u64 x)
{
return x<<56 | x>>56 |
(x & (__u64)0x000000000000ff00ULL)<<40 |
(x & (__u64)0x0000000000ff0000ULL)<<24 |
(x & (__u64)0x00000000ff000000ULL)<< 8 |
(x & (__u64)0x000000ff00000000ULL)>> 8 |
(x & (__u64)0x0000ff0000000000ULL)>>24 |
(x & (__u64)0x00ff000000000000ULL)>>40;
}
#define ___constant_swab16(x) \
((__u16)( \
......@@ -77,13 +107,13 @@
* provide defaults when no architecture-specific optimization is detected
*/
#ifndef __arch__swab16
# define __arch__swab16(x) ({ __u16 __tmp = (x) ; ___swab16(__tmp); })
# define __arch__swab16(x) ___swab16(x)
#endif
#ifndef __arch__swab32
# define __arch__swab32(x) ({ __u32 __tmp = (x) ; ___swab32(__tmp); })
# define __arch__swab32(x) ___swab32(x)
#endif
#ifndef __arch__swab64
# define __arch__swab64(x) ({ __u64 __tmp = (x) ; ___swab64(__tmp); })
# define __arch__swab64(x) ___swab64(x)
#endif
#ifndef __arch__swab16p
......@@ -97,13 +127,13 @@
#endif
#ifndef __arch__swab16s
# define __arch__swab16s(x) do { *(x) = __arch__swab16p((x)); } while (0)
# define __arch__swab16s(x) ((void)(*(x) = __arch__swab16p(x)))
#endif
#ifndef __arch__swab32s
# define __arch__swab32s(x) do { *(x) = __arch__swab32p((x)); } while (0)
# define __arch__swab32s(x) ((void)(*(x) = __arch__swab32p(x)))
#endif
#ifndef __arch__swab64s
# define __arch__swab64s(x) do { *(x) = __arch__swab64p((x)); } while (0)
# define __arch__swab64s(x) ((void)(*(x) = __arch__swab64p(x)))
#endif
......@@ -113,15 +143,15 @@
#if defined(__GNUC__) && defined(__OPTIMIZE__)
# define __swab16(x) \
(__builtin_constant_p((__u16)(x)) ? \
___swab16((x)) : \
___constant_swab16((x)) : \
__fswab16((x)))
# define __swab32(x) \
(__builtin_constant_p((__u32)(x)) ? \
___swab32((x)) : \
___constant_swab32((x)) : \
__fswab32((x)))
# define __swab64(x) \
(__builtin_constant_p((__u64)(x)) ? \
___swab64((x)) : \
___constant_swab64((x)) : \
__fswab64((x)))
#else
# define __swab16(x) __fswab16(x)
......
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