your-dentist

6th July 2004 10:49 UTC

A question of speed.
When I build superscopes with individual coords for each points, I mostly use bnot() to do this.
Example: superscope has three points.
Coords are: x1, x2, x3 and y1, y2, y3
In perpoint textfield:
cnt=i*(n-1);
x=bnot(cnt)*x1+bnot(cnt-1)*x2+bnot(cnt-2)*x2;
y=bnot(cnt)*y1+bnot(cnt-1)*y2+bnot(cnt-2)*y2;

So I recently read that multiplications are not good for this, because they're too slow.
Are multiplication with 1 or 0 as slow as, let's say .693482*1.593782?
I mean 'whatever'*1 is 'whatever' and 'whatever'*0 is 0.
Is it faster to use IFs in this case?

piR

6th July 2004 13:14 UTC

For only three points in a superscope, I don't think any optimization is really needed ;)

About multiplication, unless the compiler tries to make an optimization, the time needed to make a compute will be the same whatever the values multiplied.

For a real optimization, you should better use a series of if(), because AVS only executes the necessary code (in you example, it always executes 3 bnot() and 3 multiplications, using if() it can only execute from 1 to 3 bnot(), and that's all).

Jaak

6th July 2004 16:24 UTC

why not use ifs?
i think they the fastest. for less than... less say 200 points you really dont have to worry if you use ifs or multiplications
:p
just dont use divisions =)

your-dentist

6th July 2004 16:41 UTC

Is there no case differentiation in the multiplication algo, that makes multiplication with whole numbers faster?
I prefer the bnot() method, because it's easier to read the source.
Having tons of if(b,if(1-c,if(a,0,d),1),5); things sux.
I gave up using division too.
Though /3 is shorter than *.333. :)

Raz

6th July 2004 22:35 UTC

if you want easy to read just do this:
frame:
a=0;
point:
a=a+1;
x1=if(equal(a,1),point position on x axis,x1);
y1=if(equal(a,1),point position on y axis,y1);
x1=if(equal(a,2),point position on x axis,x1);
etc
x=x1;
y=y1;

NemoOrange

7th July 2004 20:45 UTC

I've been experimenting along these lines & I've been using i & if statements.

For your scope, you have 3 points, so n=3. The i "points" are located at 0, .5 and 1

With this in mind, using a if statement short cut, you can skip the equal or above conditions & just use values of i.

I'm having trouble explaining it, but in the end you'll come up with:

x=if(i,if(i-.5,x3,x2),x1);
y=if(i,if(i-.5,y3,y2),y1);

I've done larger statements like these where n=9;

x=if(i,if(i-1/8,if(i-2/8,if(i-3/8,if(i-4/8,if(i-5/8,if(i-6/8,if(i-7/8,x9,x8),x7),x6),x5),x4),x3),x2),x1)

Raz

7th July 2004 21:09 UTC

Looks interesting nemo, but get rid of all those needless divisions. Makes it much slower.

1/8 = 0.125
2/8 = 0.25
3/8 = 0.375
etc

piR

8th July 2004 12:46 UTC

Originally posted by your-dentist
Is there no case differentiation in the multiplication algo, that makes multiplication with whole numbers faster?

NemoOrange

8th July 2004 16:31 UTC

Originally posted by Raz
Looks interesting nemo, but get rid of all those needless divisions. Makes it much slower.

1/8 = 0.125
2/8 = 0.25
3/8 = 0.375
etc

your-dentist

8th July 2004 21:50 UTC

Anthoer way using megabuf().
I doubt that it's faster, because it needs to access the cache.

All coords are put into the megabuf.
Like this:
assign(megabuf(0),x1);assign(megabuf(1),x2);
assign(megabuf(0+yoffset),y1);assign(megabuf(1+yoffset),y2);

Then used like this:
Per point:
cnt=i*(n-1);
x=megabuf(cnt);
y=megabuf(cnt+yoffset);

The var 'yoffset' can be chosen freely, though it must be bigger than the amount of individual point coords.

Jaak

9th July 2004 16:28 UTC

Originally posted by NemoOrange
Although, are you sure this is correct? I remember somebody mentioning how 1/3 is faster than .3333. Maybe I'm wrong. [/B]

loop(2000,
 loop(2000,
  assign(a,a/3)
 )
);

loop(2000,
 loop(2000,
  assign(a,a*.333)
 )
);

dirkdeftly

9th July 2004 17:57 UTC

1/3 is more accurate than .3333, but not faster

*however*

unless you're going over a division 4,000,000 times, you're not going to lose 3.3 fps over it. and it would take running over a division 100,000 times to even lose .1 fps. it's really not that big of a deal, and in many cases using the division is a lot more convenient than the decimal.
so jaak, for the love of god, stop obsessing over divisions. there are more important optimisations.


your-dentist: avs is really quite shitty and not well optimised, and afaik x*1 and x*0 are no faster than x*1.23456789. the bottom line is, ifs are faster than mults, and the difference in speed is not quite as negligible as that between a division and a multiplication.
(so yeah, this is one of those important optimisations)

Jaak

10th July 2004 09:13 UTC

Originally posted by Atero
so jaak, for the love of god, stop obsessing over divisions. there are more important optimisations.

TomyLobo

13th July 2004 20:44 UTC

about the speed difference between using megabuf() and if/equal/bnot/whatever:
megabuf() is most probably faster in terms of memory access because it accesses memory only 4 times:
1. fetch the index (if it isnt already in the fpu from previous calculations) (prolly fld [e*x])

2. store the index to a temporary integer (fist [ebp+/-something])

3. put the index into a cpu register (mov e*x, [ebp+/-something])

4. get the value from the megabuf using that index and the base address of the megabuf (fld [e*x+e*x])

that would be a complexity of 4 which is a constant cost while using if()s, equal()s or bnot()s would require a linear amount of time which would be slower if you have more than 3 or 4 or so...

your-dentist

14th July 2004 15:42 UTC

Sounds plausible.
So for high amounts of points, using megabuf() is faster.
Whoa, you really seem to have become an expert at AVS code!

TomyLobo

17th July 2004 15:40 UTC

a little ASM knowledge helps ;)

jheriko

28th July 2004 20:01 UTC

btw, if you want to know whats fastest you should get the winamp sdk and look at the evallib source. the actual functions are in nseel-cfunc.c, the pro/epilogs are in the nseel-addfuncs.h file

its all pretty interesting but you need to be a real tech head to understand 90% of the rest of it cos its written in c and asm and in a code style almost totally unreadable to me (and therefore by logical extension everyone :P).