mirror of https://gitee.com/openkylin/linux.git
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: Trent Piepho <xyzzy@speakeasy.org>
Cc: Francois-Rene Rideau <fare@tunes.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Trent Piepho
Linus Torvalds
parent
02fb6149f7
commit
8e2c20023f
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@ -10,6 +10,10 @@
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* separated swab functions from cpu_to_XX,
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* to clean up support for bizarre-endian architectures.
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*
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* Trent Piepho <xyzzy@speakeasy.org> 2007114
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* make constant-folding work, provide C versions that
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* gcc can optimize better, explain different versions
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*
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* See asm-i386/byteorder.h and suches for examples of how to provide
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* architecture-dependent optimized versions
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*
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@ -17,40 +21,66 @@
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#include <linux/compiler.h>
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/* Functions/macros defined, there are a lot:
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*
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* ___swabXX
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* Generic C versions of the swab functions.
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*
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* ___constant_swabXX
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* C versions that gcc can fold into a compile-time constant when
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* the argument is a compile-time constant.
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*
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* __arch__swabXX[sp]?
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* Architecture optimized versions of all the swab functions
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* (including the s and p versions). These can be defined in
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* asm-arch/byteorder.h. Any which are not, are defined here.
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* __arch__swabXXs() is defined in terms of __arch__swabXXp(), which
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* is defined in terms of __arch__swabXX(), which is in turn defined
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* in terms of ___swabXX(x).
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* These must be macros. They may be unsafe for arguments with
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* side-effects.
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*
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* __fswabXX
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* Inline function versions of the __arch__ macros. These _are_ safe
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* if the arguments have side-effects. Note there are no s and p
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* versions of these.
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*
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* __swabXX[sb]
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* There are the ones you should actually use. The __swabXX versions
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* will be a constant given a constant argument and use the arch
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* specific code (if any) for non-constant arguments. The s and p
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* versions always use the arch specific code (constant folding
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* doesn't apply). They are safe to use with arguments with
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* side-effects.
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*
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* swabXX[sb]
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* Nicknames for __swabXX[sb] to use in the kernel.
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*/
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/* casts are necessary for constants, because we never know how for sure
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* how U/UL/ULL map to __u16, __u32, __u64. At least not in a portable way.
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*/
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#define ___swab16(x) \
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({ \
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__u16 __x = (x); \
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((__u16)( \
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(((__u16)(__x) & (__u16)0x00ffU) << 8) | \
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(((__u16)(__x) & (__u16)0xff00U) >> 8) )); \
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})
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#define ___swab32(x) \
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({ \
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__u32 __x = (x); \
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((__u32)( \
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(((__u32)(__x) & (__u32)0x000000ffUL) << 24) | \
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(((__u32)(__x) & (__u32)0x0000ff00UL) << 8) | \
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(((__u32)(__x) & (__u32)0x00ff0000UL) >> 8) | \
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(((__u32)(__x) & (__u32)0xff000000UL) >> 24) )); \
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})
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#define ___swab64(x) \
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({ \
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__u64 __x = (x); \
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((__u64)( \
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(__u64)(((__u64)(__x) & (__u64)0x00000000000000ffULL) << 56) | \
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(__u64)(((__u64)(__x) & (__u64)0x000000000000ff00ULL) << 40) | \
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(__u64)(((__u64)(__x) & (__u64)0x0000000000ff0000ULL) << 24) | \
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(__u64)(((__u64)(__x) & (__u64)0x00000000ff000000ULL) << 8) | \
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(__u64)(((__u64)(__x) & (__u64)0x000000ff00000000ULL) >> 8) | \
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(__u64)(((__u64)(__x) & (__u64)0x0000ff0000000000ULL) >> 24) | \
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(__u64)(((__u64)(__x) & (__u64)0x00ff000000000000ULL) >> 40) | \
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(__u64)(((__u64)(__x) & (__u64)0xff00000000000000ULL) >> 56) )); \
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})
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static __inline__ __attribute_const__ __u16 ___swab16(__u16 x)
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{
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return x<<8 | x>>8;
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}
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static __inline__ __attribute_const__ __u32 ___swab32(__u32 x)
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{
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return x<<24 | x>>24 |
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(x & (__u32)0x0000ff00UL)<<8 |
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(x & (__u32)0x00ff0000UL)>>8;
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}
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static __inline__ __attribute_const__ __u64 ___swab64(__u64 x)
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{
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return x<<56 | x>>56 |
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(x & (__u64)0x000000000000ff00ULL)<<40 |
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(x & (__u64)0x0000000000ff0000ULL)<<24 |
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(x & (__u64)0x00000000ff000000ULL)<< 8 |
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(x & (__u64)0x000000ff00000000ULL)>> 8 |
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(x & (__u64)0x0000ff0000000000ULL)>>24 |
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(x & (__u64)0x00ff000000000000ULL)>>40;
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}
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#define ___constant_swab16(x) \
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((__u16)( \
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@ -77,13 +107,13 @@
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* provide defaults when no architecture-specific optimization is detected
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*/
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#ifndef __arch__swab16
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# define __arch__swab16(x) ({ __u16 __tmp = (x) ; ___swab16(__tmp); })
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# define __arch__swab16(x) ___swab16(x)
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#endif
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#ifndef __arch__swab32
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# define __arch__swab32(x) ({ __u32 __tmp = (x) ; ___swab32(__tmp); })
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# define __arch__swab32(x) ___swab32(x)
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#endif
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#ifndef __arch__swab64
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# define __arch__swab64(x) ({ __u64 __tmp = (x) ; ___swab64(__tmp); })
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# define __arch__swab64(x) ___swab64(x)
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#endif
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#ifndef __arch__swab16p
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@ -97,13 +127,13 @@
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#endif
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#ifndef __arch__swab16s
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# define __arch__swab16s(x) do { *(x) = __arch__swab16p((x)); } while (0)
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# define __arch__swab16s(x) ((void)(*(x) = __arch__swab16p(x)))
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#endif
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#ifndef __arch__swab32s
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# define __arch__swab32s(x) do { *(x) = __arch__swab32p((x)); } while (0)
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# define __arch__swab32s(x) ((void)(*(x) = __arch__swab32p(x)))
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#endif
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#ifndef __arch__swab64s
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# define __arch__swab64s(x) do { *(x) = __arch__swab64p((x)); } while (0)
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# define __arch__swab64s(x) ((void)(*(x) = __arch__swab64p(x)))
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#endif
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@ -113,15 +143,15 @@
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#if defined(__GNUC__) && defined(__OPTIMIZE__)
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# define __swab16(x) \
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(__builtin_constant_p((__u16)(x)) ? \
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___swab16((x)) : \
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___constant_swab16((x)) : \
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__fswab16((x)))
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# define __swab32(x) \
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(__builtin_constant_p((__u32)(x)) ? \
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___swab32((x)) : \
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___constant_swab32((x)) : \
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__fswab32((x)))
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# define __swab64(x) \
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(__builtin_constant_p((__u64)(x)) ? \
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___swab64((x)) : \
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___constant_swab64((x)) : \
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__fswab64((x)))
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#else
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# define __swab16(x) __fswab16(x)
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