On 07.12.2013 00:58, H. S. Teoh wrote:
On Sat, Dec 07, 2013 at 12:40:35AM +0100, bearophile wrote:
[...]
Regarding Java performance matters, from my experience another
significant source of optimization in the JavaVM that is often
overlooked is that the JavaVM is able to partially unroll even loops
with a statically-unknown number of cycles. Currently I think
GCC/DMD/LDC2 are not able or willing to do this.
[...]
Really? I've seen gcc/gdc unroll loops with unknown number of
iterations, esp. when you're using -O3. It just unrolls into something
like:
loop_start:
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
if (!loopCondition) goto end;
loopBody();
goto loop_start;
end: ...
I'm pretty sure I've seen gcc/gdc do this before.
The classic way of doing this (I have no idea if it's actually done by
any compilers) is Duff's Device[1], which turns this code:
do {
*to++ = *from++;
} while(--count > 0);}
into this:
int n = (count + 7) / 8;
switch(count % 8) {
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
case 1: *to++ = *from++;
} while(--n > 0);
[1]: http://en.wikipedia.org/wiki/Duff's_device
--
Simen
