Pavel Roskin wrote:
> On Tue, 2008-07-08 at 20:04 +0200, Christian Franke wrote:
>
> > With old gcc versions without the "rol" optimization, even the 16
> > bit swap should be a function:
> >
>
> Or better yet, an asm statement.
>
> We should consider optimized assembly vs function call. Even the
> 32-bit swap could be shorter:
>
> a: 86 c4 xchg %al,%ah
> c: c1 c0 10 rol $0x10,%eax
> f: 86 c4 xchg %al,%ah
> 11:
>
> That's 7 bytes! ...
But the function call in the 32-bit case requires only 5 bytes :-)
(The 16-bit case requires more due to short->int propagation)
Overall size from inline asm would only be smaller if there is any
benefit from additional optimizations.
Here is a possibly working draft for include/grub/i386/types.h (requires
a #ifdef in include/grub/types.h):
#define grub_swap_bytes32(x) \
({ \
grub_uint32_t _x = (x), _y;
asm ( \
"xchg %%al,%%ah\n" \
"roll $0x10,%%eax\n" \
"xchg %%al,%%ah\n" \
: "=a"(_y) : "0"(_x) \
); \
_y; \
})
Result with the test script from my last mail:
Debian gcc 4.1.2-7:
inline (portable)=357, inline (asm)=126, function=104
Cygwin gcc 3.4.4:
inline (portable)=340, inline (asm)=124, function=96
Function call is still better. The only candidate for inline is probably
grub_swap_bytes16().
> .... And if written properly, it could work with any of
> the registers that allow access to the lower two bytes (%eax, %ebx,
> %ecx and %edx), thus giving more flexibility to the optimizer.
>
This would require support to access the Rl and Rh parts of eRx for each
R in [a-d]. Something like:
asm (
"xchg %0:l,%0:h\n"
"roll $0x10,%0\n"
"xchg %0:l,%0:h\n"
: "=r"(_y) : "0"(_x) \
);
Do gcc or gas provide a syntax to do this?
Christian
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