svn commit: r232266 - in head/sys: amd64/include i386/include pc98/include x86/include

[Tijl Coosemans]

On Tuesday 28 February 2012 21:55:57 Dimitry Andric wrote:
> On 2012-02-28 20:39, Tijl Coosemans wrote:
>> Author: tijl
>> Date: Tue Feb 28 19:39:54 2012
>> New Revision: 232266
>> URL: http://svn.freebsd.org/changeset/base/232266
>> 
>> 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).
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