Date: Tue, 28 Feb 2012 22:22:20 +0100 From: Tijl Coosemans <tijl@freebsd.org> To: Dimitry Andric <dim@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r232266 - in head/sys: amd64/include i386/include pc98/include x86/include Message-ID: <201202282222.26343.tijl@freebsd.org> In-Reply-To: <4F4D3F5D.70905@FreeBSD.org> References: <201202281939.q1SJdtYr084858@svn.freebsd.org> <4F4D3F5D.70905@FreeBSD.org>
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[-- Attachment #1 --] 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). [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (FreeBSD) iF4EABEIAAYFAk9NRZIACgkQfoCS2CCgtitN6AD8DQEnzXs8/yF/R0V/q8r/QqKT 4LPpT2qnabaPO9u4XPkA/2THbeoN9I7c1DAty5t4EBlZw31uf8dThiNUkl1d5HIq =XmI7 -----END PGP SIGNATURE-----
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