On Friday 02 March 2012 12:52:41 Bruce Evans wrote:
> On Fri, 2 Mar 2012, Bruce Evans wrote:
>
> > On Thu, 1 Mar 2012, Tijl Coosemans wrote:
> >
> >> On Wednesday 29 February 2012 08:44:54 Bruce Evans wrote:
> >>> ...
> >>> So any macro version must use gcc features to be safe. The following
> >>> seems to work:
> >>>
> >>> #define __bswap16(x) __extension__ ({ __uint16_t __x = x;
> >>> (__uint16_t)(__x << 8 | __x >> 8); })
> >>>
> >>> clang now produces "rolw $8,x" when x is volatile. This seems to
> >>> violate volatile semantics. gcc produces load-rolw-store then. Both
> >>> produce "rolw $8,x" when x is not volatile.
> >>
> >> I'll have a closer look at the patch tomorrow, but the Intel
> >> documentation for the bswap instruction recommends to use xchg for
> >> bswap16.
> >
> > That's what i386 used to do, using an asm (see for example the RELENG_4
> > version), but the asm was replaced by C code and the compiler apparently
> > knows better. This might depend on -mtune. I tested with the default,
> > ...
>
> Here is another version. It has now been tested a bit at runtime.
> I had to restore almost all of the complications, so the following
> reduces to mostly style changes including comments about surprising
> details. It has 1 fix for clang (the comitted version doesn't
> work for clang).
>
> % Index: endian.h
> % ===================================================================
> % RCS file: /home/ncvs/src/sys/x86/include/endian.h,v
> % retrieving revision 1.1
> % diff -u -2 -r1.1 endian.h
> % --- endian.h 28 Feb 2012 19:39:54 -0000 1.1
> % +++ endian.h 2 Mar 2012 10:43:53 -0000
> % @@ -37,8 +37,4 @@
> % #include <sys/_types.h>
> %
> % -#ifdef __cplusplus
> % -extern "C" {
> % -#endif
> % -
> % /*
> % * Define the order of 32-bit words in 64-bit words.
> % @@ -68,25 +64,17 @@
> % #endif
> %
> % -#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
> % -
> % -#define __bswap16_const(_x) (__uint16_t)((_x) << 8 | (_x) >> 8)
> % +#define __bswap16_gen(x) (__uint16_t)((x) << 8 | (x) >> 8)
> % +#define __bswap32_gen(x) \
> % + (((__uint32_t)__bswap16(x) << 16) | __bswap16((x) >> 16))
> % +#define __bswap64_gen(x) \
> % + (((__uint64_t)__bswap32(x) << 32) | __bswap32((x) >> 32))
> %
> % -#define __bswap16(_x) \
> % - (__builtin_constant_p(_x) ? \
> % - __bswap16_const((__uint16_t)(_x)) : __bswap16_var(_x))
> % -
> % -#define __bswap32_const(_x) \
> % - (((__uint32_t)__bswap16(_x) << 16) | __bswap16((_x) >> 16))
> % -
> % -#define __bswap32(_x) \
> % - (__builtin_constant_p(_x) ? \
> % - __bswap32_const((__uint32_t)(_x)) : __bswap32_var(_x))
> % +#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
> %
> % -#define __bswap64_const(_x) \
> % - (((__uint64_t)__bswap32(_x) << 32) | __bswap32((_x) >> 32))
> % +/* The following mess is to avoid multiple evaluation of x. */
> %
> % -#define __bswap64(_x) \
> % - (__builtin_constant_p(_x) ? \
> % - __bswap64_const((__uint64_t)(_x)) : __bswap64_var(_x))
> % +#define __bswap16(x) \
> % + (__builtin_constant_p(x) ? \
> % + __bswap16_gen((__uint16_t)(x)) : __bswap16_var((__uint16_t)(x)))
>
> For clang, x must be cast in the `var' clause too, since when x is
> something like 0x12345678 (having been passed here without conversion
> by __bswap32()), clang parses the `var' clause and warns about the
> implicit truncation of 0x12345678 to 0x5678 in it. This is a bug
> in clang -- when warning about fine details, it is important not to
> do it for non-problems.
>
> %
> % static __inline __uint16_t
> % @@ -94,7 +82,19 @@
> % {
> %
> % - return (__bswap16_const(_x));
> % + return (__bswap16_gen(_x));
> % }
> %
> % +/*
> % + * The following messes are old optimizations for gcc. The messes have
> % + * been reduced significantly, but I don't know how to implement the
> % + * preferred way of defining defaults above and overriding them cleanly
> % + * here. Even clang needs help for the 64-bit case, so to keep the
> % + * ifdefs sane we use this for the 32-bit case with clang too.
> % + */
> % +
> % +#define __bswap32(x) \
> % + (__builtin_constant_p(x) ? \
> % + __bswap32_gen((__uint32_t)(x)) : __bswap32_var(x))
> % +
> % static __inline __uint32_t
> % __bswap32_var(__uint32_t _x)
> % @@ -105,34 +105,37 @@
> % }
> %
> % +#define __bswap64(x) \
> % + (__builtin_constant_p(x) ? \
> % + __bswap64_gen((__uint64_t)(x)) : __bswap64_var(x))
> % +
> % static __inline __uint64_t
> % __bswap64_var(__uint64_t _x)
> % {
> % -#ifdef _LP64
> % +
> % +#ifdef _LP64
> % __asm ("bswap %0" : "+r" (_x));
> % return (_x);
> % #else
> % - return (__bswap64_const(_x));
> % + /*
> % + * It is important for the optimizations that the following is not
> % + * really generic, but expands to 2 __bswap32_var()'s.
> % + */
> % + return (__bswap64_gen(_x));
> % #endif
> % }
> %
> % -#define __htonl(x) __bswap32(x)
> % -#define __htons(x) __bswap16(x)
> % -#define __ntohl(x) __bswap32(x)
> % -#define __ntohs(x) __bswap16(x)
> % +#else /* !(__GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P */
> %
> % -#else /* !(__GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P) */
> % -
> % -/*
> % - * No optimizations are available for this compiler. Fall back to
> % - * non-optimized functions by defining the constant usually used to prevent
> % - * redefinition.
> % - */
> % -#define _BYTEORDER_FUNC_DEFINED
> % +/* XXX these are broken, since they evaluate x more than once. */Maybe make them static inline functions. That won't be very efficient at -O0 but it'll be correct and this implementation probably isn't very efficient anyway. Otherwise the patch looks good to me. > % +#define __bswap16(x) (__bswap16_gen((__uint16_t)(x))) > % +#define __bswap32(x) (__bswap32_gen((__uint32_t)(x))) > % +#define __bswap64(x) (__bswap64_gen((__uint64_t)(x))) > % > % #endif /* __GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P */ > % > % -#ifdef __cplusplus > % -} > % -#endif > % +#define __htonl(x) __bswap32(x) > % +#define __htons(x) __bswap16(x) > % +#define __ntohl(x) __bswap32(x) > % +#define __ntohs(x) __bswap16(x) > % > % #endif /* !_MACHINE_ENDIAN_H_ */
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