mod & div...

[crystalsquid]

Curious question - how does the VM actually do an integer mod operation?
There does not appear to be an 'imod' bytecode, and the 'idiv' doesn't return the modulus (although I don't know of any processor that doesnt return the mod when it does an integer divide).

I just so happen to want to do a divide and a mod at the same time, and I don't particularly wish to do:

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int div = a / b; int mod = a - (div*b);

Any suggestions?

- Dom

PS: This is in a loop I am looking at running >5000 times a frame, so I would _really_ like to do it quite quickly

[blahblahblahh]

Well, the first question is: what happens if you do

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c1 = a % b; c2 = a / b;

[blahblahblahh]

PS what are you doing 5k divs and mods for in one frame? Is there possibly a better algo?

In particular, there are some interesting identities when you start working in modulus arithmetic (which are heavily used in crypto), and there's a small chance some of those could help you rearrange your algo...

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[Mark Thornton]

Curious question - how does the VM actually do an integer mod operation?
There does not appear to be an 'imod' bytecode,

Remainder: irem, lrem, frem, drem.

[crystalsquid]

Ah - rem for remainder!
There I was looking for the conventional one used by most of the known computing world!

Still - does it reuse the result from a divide once the bytecode is compiled?

As to what I am doing - its a good old fashioned software renderer to get 3D running for Java 1.1. The mod/div is for generating DDA values for the edge gradients - although I may not be doing that now

- Dom

[phazer]

It's up to the JVM to make the div followed by mod optimization. I doubt they make this optimization because it occurs very seldom.

Here is a code example and the generated byte code:

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class Test {   public void test(int a, int b) {     int y = a / b;     int x = a % b;   } } javap -c Test ... Method void test(int, int)    0 iload_1    1 iload_2    2 idiv    3 istore_3    4 iload_1    5 iload_2    6 irem    7 istore 4    9 return