Date: Wed, 29 Feb 2012 18:44:54 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Bruce Evans <brde@optusnet.com.au> Cc: svn-src-head@FreeBSD.org, Tijl Coosemans <tijl@FreeBSD.org>, src-committers@FreeBSD.org, Dimitry Andric <dim@FreeBSD.org>, svn-src-all@FreeBSD.org Subject: Re: svn commit: r232266 - in head/sys: amd64/include i386/include pc98/include x86/include Message-ID: <20120229181608.S2887@besplex.bde.org> In-Reply-To: <20120229160522.W2514@besplex.bde.org> References: <201202281939.q1SJdtYr084858@svn.freebsd.org> <4F4D3F5D.70905@FreeBSD.org> <201202282222.26343.tijl@freebsd.org> <20120229160522.W2514@besplex.bde.org>
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On Wed, 29 Feb 2012, Bruce Evans wrote:
> 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():
A minor problem with only having a macro version for __bswap64() turned
up:
> % -#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 ___bswap16(x) (__uint16_t)((x) << 8 | (x) >> 8)
> % +#define __bswap16(x) (___bswap16((__uint16_t)(x)))
When x a non-volatile variable, gcc and clang produce the good code
"rolw $8,x" for "x = __bswap16(x);" on short x. But when x a a volatile
variable, gcc and clang produce fairly horrible code, with 2 loads of
x corresponding to the 2 accesses to x. This is probably required by
volatile semantics, and is a problem for all unsafe macros, especially
when their name says that they are safe (oops). When __bswap16 is
implemented as an inline function for the var case like it used to be,
it only loads x once and there are no problems with volatile variables.
Optimizing to "rolw $8,x" might still be possible iff x is not volatile,
but load-modify-store is probably better anyway.
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.
Bruce
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