HomeForumsWhat's newResources 
 
 
fast memset
vbt - Jul 29, 2012
    vbt Jul 29, 2012 
I'm still playing to find speed on emulators and i find memset slow, anybody tried to write a fast memset, i only found this :

_memclrwh: ! terulet torlo: memclrwh(byte* buf, long width, long height, long step)

! step = map width - brush width

xor r2,r2

lswh_1:

mov r5,r1

lswh_2:

mov.b r2,@r4

add #1,r4

dt r1

bf lswh_2

add r7,r4

dt r6

bf lswh_1

rts

nop

i'd like to replace such thing :

memset(SpriteOnScreenMap, 255, 0x10000);

    mrkotfw Jul 29, 2012 
I'm not sire if that's as fast as you think it is. It writes in units of 1 byte and doesn't even look to eliminate some trivial pipeline stalls. Newlib's implementation might be faster.

    antime Jul 29, 2012 
If possible, use DMA. If you have to use CPU transfers, try the existing library code first. Smart people have spent a lot of time optimizing it. Other than that, write in larger chunks, to reduce the amount of time spent waiting on the (S)DRAM. Use all the normal optimizing tricks, like unrolling your loops and taking advantage of the instruction set where possible. Also test on real hardware if you can, emulators rarely take things like memory wait states or pipeline stalls into account.

A naive attempt at a 2-way unrolled version (still writing only a single byte at a time):

Code: 
  
 .align 4 _memclrwh: mov #0, r1 cmp/eq r1, r5 bt leave cmp/eq r1, r6 bt leave mov r5, r0 mov r4, r3 add #1, r3 add r5, r4 outer: tst #1, r0 bf/s odd mov r4, r2 inner: mov.b r1, @-r2 odd: cmp/hi r3, r2 bt/s inner mov.b r1, @-r2 add r7, r3 dt r6 bf/s outer add r7, r4 leave: rts nop