Date: Wed, 29 Feb 2012 16:13:55 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Tijl Coosemans <tijl@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Dimitry Andric <dim@FreeBSD.org> Subject: Re: svn commit: r232266 - in head/sys: amd64/include i386/include pc98/include x86/include Message-ID: <20120229160522.W2514@besplex.bde.org> In-Reply-To: <201202282222.26343.tijl@freebsd.org> References: <201202281939.q1SJdtYr084858@svn.freebsd.org> <4F4D3F5D.70905@FreeBSD.org> <201202282222.26343.tijl@freebsd.org>
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On Tue, 28 Feb 2012, Tijl Coosemans wrote:
> On Tuesday 28 February 2012 21:55:57 Dimitry Andric wrote:
>> On 2012-02-28 20:39, Tijl Coosemans wrote:
>>> Log:
>>> Copy amd64 endian.h to x86 and merge with i386 endian.h. Replace
>>> amd64/i386/pc98 endian.h with stubs.
>>>
>>> In __bswap64_const(x) the conflict between 0xffUL and 0xffULL has been
>>> resolved by reimplementing the macro in terms of __bswap32(x). As a side
>>> effect __bswap64_var(x) is now implemented using two bswap instructions on
>>> i386 and should be much faster. __bswap32_const(x) has been reimplemented
>>> in terms of __bswap16(x) for consistency.
>> ...
>>> +#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))
>>> +
>>> +#define __bswap64_const(_x) \
>>> + (((__uint64_t)__bswap32(_x) << 32) | __bswap32((_x) >> 32))
>>
>> Hmm, shouldn't __bswap32_const() be implemented in terms of
>> __bswap16_const(), and similarly for __bswap64_const(), which should be
>> implemented in terms of __bswap32_const()?
>>
>> The whole reason for the difficult dance with __builtin_constant_p is to
>> allow for compile-time resolving of bswap'd constants. Invoking the
>> regular __bswap() macros doesn't gain you anything here.
>
> __bswap64_const is also used in __bswap64_var, so its argument isn't
> always a compile time constant and then __bswap32 becomes __bswap32_var.
> If it is a constant __bswap32 becomes __bswap32_const (even at -O0).
I cleaned this up a bit according to ideas in my previous mails, and
added a comment about the confusing use of __bswap64_const() (now
named __bswap64_gen()) in __bswap64_var():
% 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 29 Feb 2012 03:10:33 -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.
% @@ -68,32 +64,24 @@
% #endif
%
% -#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
% -
% -#define __bswap16_const(_x) (__uint16_t)((_x) << 8 | (_x) >> 8)
% -
% -#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))
% +#define ___bswap16(x) (__uint16_t)((x) << 8 | (x) >> 8)
% +#define __bswap16(x) (___bswap16((__uint16_t)(x)))
% +#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 __bswap64_const(_x) \
% - (((__uint64_t)__bswap32(_x) << 32) | __bswap32((_x) >> 32))
% -
% -#define __bswap64(_x) \
% - (__builtin_constant_p(_x) ? \
% - __bswap64_const((__uint64_t)(_x)) : __bswap64_var(_x))
% +#if defined(__GNUCLIKE_ASM) && defined(__GNUCLIKE_BUILTIN_CONSTANT_P)
%
% -static __inline __uint16_t
% -__bswap16_var(__uint16_t _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.
% + */
%
% - return (__bswap16_const(_x));
% -}
% +#define __bswap32(x) \
% + (__builtin_constant_p(x) ? \
% + __bswap32_gen((__uint32_t)(x)) : __bswap32_var(x))
%
% static __inline __uint32_t
% @@ -105,34 +93,35 @@
% }
%
% +#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
% +#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_ */
This passes simple tests, but I had to restore some messes for clang
that I originally left out to get clang to optimize __bswap64()
like it did before for the `var' case. clang apparently only
understands the general expression for __bswapN() up to N = 32.
Bruce
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