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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