Mail Archives: djgpp/1997/03/04/16:45:34
>> > 2) Cause your program to ignore FP exceptions by including the
>> > following somewhere at its beginning:
>> >
>> > #include <signal.h>
>> > ...
>> > signal (SIGFPE, SIG_IGN);
>>
>> i wasn't aware you could do this actually. does this perhaps mean that you
>> could use a memcopy with fld and fstp and just ignore errors like this? it
>> would be much faster than the fild fistp version obviously...
well, for the benefit of the djgpp community as a whole here's the result.
first the standard fpu memcopy which i use. this is 2 cycles faster than the
fastest i've ever seen anywhere else (the agner fog articles) and is 100%
accurate :
asm volatile ("1:\n\t"
"fildq (%%esi)\n\t" // load first qword 1 NP (2,3)
"fildq 8(%%esi)\n\t" // load second qword 2 NP (3,4)
"addl $16,%%esi\n\t" // update esi 3 uv
"addl $16,%%edi\n\t" // update edi 3 uv
"fistpq -8(%%edi)\n\t" // save 2nd qword 4 NP (-9)
"fistpq -16(%%edi)\n\t" // save 1st qword 10 NP (-15)
"decl %%ecx\n\t" // dec ecx 16 uv
"jnz 1b" // (loop) 16 v
:
: "S" (scr_buf), "D" (videoptr), "c" (no_to_move)
: "ecx", "esi", "edi" );
as you can see, the slow part is the fist which takes a fat 6NP :( but it
still manages 16 bytes in 16 cycles with 1/2 the normal write misses and
associated cache penalties.
now the fast (and theoretically not so accurate) version i came up with.
replace the flid and fist with fld and fst and set the flags as eli
described above. the result is an 8 cycle loop - twice as fast in fact.
the disadvantages is that this is a 'lossy' form of moving data about. there
are some sequences of numbers which cause errors and these show quite visibly
if you're using a blitz to screen for instance. my suggestion therefore is to
only use this for 24bit screen displays and to +-1 from the values that cause
fpu errors so that this never happens. the result is something that's visually
indistinguishable from what you want but twice as fast. (and 4 times faster
than the rep stos versions) so my question really is - does anyone know which
sequences cause fpu errors so i can avoid them? :) perhaps leath would know?
regards,
nik
--
Graham Tootell
nikki AT gameboutique DOT com
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