68000 programming optimization tips? (for speed)

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GManiac
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Re: 68000 programming optimization tips? (for speed)

Post by GManiac » Wed Apr 19, 2017 8:22 pm

By the way, a lot of people ignore Scc commands (ST, SF, Sxx), but they are pretty useful to calculate complex boolean expressions (with addition of NOT, AND, OR, EOR) rather than doing a massive branching.

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Sik
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Re: 68000 programming optimization tips? (for speed)

Post by Sik » Thu Apr 20, 2017 9:50 pm

Can confirm, once I was looking at the pathfinding code in Star Chaser and ran into a SHS instruction and was wondering what the heck was going on because of how uncommon it is. (pretty ironic since I wrote that code...)

These days I'm exploiting Scc a lot for quickly getting 0/1 values from a condition (Scc followed by AND). Or just outright 0/-1 (no AND) if I just want a boolean to be used later.
Sik is pronounced as "seek", not as "sick".

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Re: 68000 programming optimization tips? (for speed)

Post by Natsumi » Fri Apr 21, 2017 10:53 am

also it can be very useful for something like this;

Code: Select all

.cloop		tst.b	(a1)+			; check if entry is null
		seq	d2			; if is, set d2 to $FF
		add.b	d2,d3			; and then subtract 1 from total count if so
		dbf	d1,.cloop		; loop til all cards are checked
Over branching conditionally, this actually saves 4 cycles per loop. Quite nifty optimization since this loop is ran many times over.

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Sik
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Re: 68000 programming optimization tips? (for speed)

Post by Sik » Sat Apr 22, 2017 12:35 am

Not to mention the part where you avoid an extra label just for the branch. The best kind of optimizations are those that also make the code look cleaner =O) (like using MOVEP instead of bit shifting for generating VDP commands for DMA)
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Miquel
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Re: 68000 programming optimization tips? (for speed)

Post by Miquel » Mon Jul 09, 2018 5:43 am

Sik wrote:
Thu Apr 20, 2017 9:50 pm
These days I'm exploiting Scc a lot for quickly getting 0/1 values from a condition (Scc followed by AND). Or just outright 0/-1 (no AND) if I just want a boolean to be used later.
Or just do a: "neg.b" to get the appropriate value.

Hard to remember all those 68k tricks ;)
Yeah, I see you have to hive hope for the “believers” and the “non- believers”. Don’t worry I know.
“How much you have yet to learn”, is usually what them tell me when I fail to comprehend.

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Sik
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Re: 68000 programming optimization tips? (for speed)

Post by Sik » Mon Jul 09, 2018 7:35 am

The Scc instructions only come in byte size so if I want a word or long result I actually need to use AND if I want 1 instead of -1 (I mean, I could use EXT and NEG together, but it's not better and takes up more lines of code).

EDIT: also using AND you can get other values like 2 or 4. Convenient for offsets :v
Sik is pronounced as "seek", not as "sick".

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Miquel
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Re: 68000 programming optimization tips? (for speed)

Post by Miquel » Wed Jul 11, 2018 9:40 am

You are right ;)
Yeah, I see you have to hive hope for the “believers” and the “non- believers”. Don’t worry I know.
“How much you have yet to learn”, is usually what them tell me when I fail to comprehend.

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Re: 68000 programming optimization tips? (for speed)

Post by gasega68k » Mon Aug 13, 2018 4:52 am

I don't know if this should be called programmig optimization or programming trick, but is someting that I did recently to quick save/restore X flag:

Code: Select all

    subx.b  d0,d0  ;set d0 to $ff if X is set, otherwise 0
then to restore X flag:

Code: Select all

    add.b  d0,d0
This can also work with subx.w, add.w, in this case d0 will be set to $ffff, or 0.

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Stef
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Re: 68000 programming optimization tips? (for speed)

Post by Stef » Sun Aug 19, 2018 6:09 pm

I rarely need to save X but neat way of doing it :)

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flamewing
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Re: 68000 programming optimization tips? (for speed)

Post by flamewing » Mon Aug 27, 2018 9:32 am

Speaking of X, there is a neat thing you can do with it:

Code: Select all

add.w    d0,d0
subx.w    d1,d1
negx.w    d0
addx.w    d1,d1
The result: d1 = sign(d0).

You can also use subx and addx to compute the min, max, and minmax of 2 numbers, but it is slower than the branch versions (but might be useful if you need the functions to run in constant time).

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Stef
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Re: 68000 programming optimization tips? (for speed)

Post by Stef » Tue Aug 28, 2018 2:58 pm

what sign(x) is intended to return ? 1 if signed, 0 if not ?

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Sik
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Re: 68000 programming optimization tips? (for speed)

Post by Sik » Tue Aug 28, 2018 8:49 pm

I think it's meant to return -1, 0 or +1. At least that's what sign functions usually do.

Still trying to get my head around it, mind you (mainly the NEGX).
Sik is pronounced as "seek", not as "sick".

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flamewing
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Re: 68000 programming optimization tips? (for speed)

Post by flamewing » Wed Aug 29, 2018 10:49 am

Sik wrote:
Tue Aug 28, 2018 8:49 pm
I think it's meant to return -1, 0 or +1. At least that's what sign functions usually do.
You guessed right.
Sik wrote:
Tue Aug 28, 2018 8:49 pm
Still trying to get my head around it, mind you (mainly the NEGX).
It's only purpose is to set X if d0 was nonzero. Note that subx d1,d1 preserves X, which is important for the next if d0 started out as $8000.

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