On Wednesday 29 February 2012 08:44:54 Bruce Evans wrote:
> 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.I'll have a closer look at the patch tomorrow, but the Intel documentation for the bswap instruction recommends to use xchg for bswap16.
signature.asc
Description: This is a digitally signed message part.
