| |
mhindsbo
Registered: Dec 2014
Posts: 51 |
fastest or smallest 'switch' statment
I use the following code a lot to switch between two values and was wondering what others do and if there is a faster, smaller or simply more elegant way someone has come up with.
lda #value1 ; default value: AR = value1
ldy switch ; get value of switch in YR
cpy #case1 ; compare switch
beq @cont
lda #value2 ; if switch != #case1 then AR = value2
@cont ... ; AR = value1/value2 depending on switch
|
|
... 14 posts hidden. Click here to view all posts....
|
| |
mhindsbo
Registered: Dec 2014
Posts: 51 |
thanks all! some good input. I'm glad I asked. |
| |
Fred
Registered: Feb 2003
Posts: 285 |
There is also a way of doing this without using any branch instruction or jump table.
Certainly not the fastest and also not the smallest code on a 6510 CPU:
lda switch
cmp #case1
php
pla
lsr
and #$01
eor #$ff
adc #$00
and #value1 - value2
clc
adc #value2 This can be optimized a bit by using an undocumented instruction:
lda switch
cmp #case1
php
pla
asr #$02
eor #$ff
adc #$01
and #value1 - value2
clc
adc #value2
For the 6510 cpu it doesn't matter a lot to avoid branches. For e.g. Intel based CPUs it does matter in certain cases. Some compilers will optimize the following:
if (condition) {
value = 12;
} else {
value = 34;
}
into this:
value = - Integer(condition) and (12 - 34) + 34;
Btw, the example of Bitbreaker using ARR #$00 will not work since the zero flag isn't taken into account. |
| |
Bitbreaker
Registered: Oct 2002
Posts: 508 |
Quoting FredBtw, the example of Bitbreaker using ARR #$00 will not work since the zero flag isn't taken into account.
As said, depending on the values: If value1 < value2 then carry will be either set on equal or cleared on not equal, just fair if you need to differ two cases with static values only. |
| |
Fred
Registered: Feb 2003
Posts: 285 |
True. With the right values it is a nice and short solution. |
| |
Bitbreaker
Registered: Oct 2002
Posts: 508 |
Quoting Fred
lda switch
cmp #case1
php
pla
asr #$02
eor #$ff
adc #$01
and #value1 - value2
clc
adc #value2
value = - Integer(condition) and (12 - 34) + 34;
Wouldn't this also be the same?
lda switch
cmp #case1
php
pla
asr #$02
sbc #$00 ;results in either $00 or $ff
and #value2 - value1
clc
adc #value1
Where we take:
value = (Integer(condition) - 1) and (34 - 12) + 12;
|
| |
Fred
Registered: Feb 2003
Posts: 285 |
Nice one.
Another optimization that can be done is when the AND uses a value of less than 128, the ANC instruction can be used instead so that the CLC afterwards can be removed. |
| |
lft
Registered: Jul 2007
Posts: 369 |
Or, you know, don't use addition in the first place.
; accumulator is either 00 or ff
and #value2 ^ value1
eor #value1
|
| |
soci
Registered: Sep 2003
Posts: 480 |
BB/Fred: Ok, great. As mentioned earlier there are no pipeline stalls to avoid here, and that switching construct is suboptimal in every way.
What's next, how to avoid cache line bouncing on large multi processor 6502 systems? Various synchronization primitives for my threaded code? Pre-fetching? How to optimize unaligned access? Use of barriers for memory mapped I/O? |
| |
Bitbreaker
Registered: Oct 2002
Posts: 508 |
Sure, but optimising is fun :-D |
| |
soci
Registered: Sep 2003
Posts: 480 |
Yes, no problem with that. But it seemed quite a bit pointless, and then it was pushed even further ;) |
| Previous - 1 | 2 | 3 - Next |
