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flat assembler > Blog > Division by multiplication - revisited

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Tomasz Grysztar
Assembly Artist

Joined: 16 Jun 2003
Posts: 7066
Location: Kraków, Poland
Furs wrote:
I'm a bit confused. So I need to iterate all shift values, but how can I check if the condition holds if I don't know the value of N? Should I assume the highest value possible for it that it can have? I mean, N is a runtime value, only the divisor is at compile time.
Yes, you can consider just the highest possible value of N, if inequality holds for it, then it holds for all the smaller values too.

For example if P = 1 then NP<2^n is true as long as N<2^n. If P = 2 then to have 2N<2^n we need N<2^{n-1}, the allowed range of input becomes smaller.

Furs wrote:
And also if I divide 2^32 by 641, the remainder is 640, not 1, so how come it works?
EDIT: I'm guessing P is not the remainder, but rather D-remainder, am I right? I looked before at your example with 10, and you said P = 4, which fits just as well as on 641. (i.e. 10-6 = 4, and 641-640 = 1)
Oh, you're right, I confused the terminology, P is the complement of the remainder, P = D - R. In that post above where I gave all the formulas it is explained correctly.
Post 28 Feb 2018, 23:56
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Joined: 04 Mar 2016
Posts: 1351
Ok, the thing I gathered together works great (with all that I got from here, including your first method if the 0xFFFF...FF value is possible, I bet nobody else uses it Razz). Now I can feel at peace knowing the stupid thing can spit out the most optimal constants/method. Thanks Wink

A related question. What's the best (fastest, in general) way to check if a number is divisible by a constant? Sure, I can use the division here, and see if the result multiplied with the divisor is equal to the original value... but that's 2 muls. So I found this here: https://math.stackexchange.com/questions/1251327/is-there-a-fast-divisibility-check-for-a-fixed-divisor

For odd divisors it's fantastic, one mul and one cmp, literally. But it needs two branches if the divisor is even, which sucks (or a lot of extra overhead code/registers usage, unless you can find a better way, problem is the ZF can't be used except with cmov, no adc/sbb tricks). Any better way for even divisors? (I'm not a math guru so I don't really understand how it works, lol)

I did implement them both in C++ with constexpr (compile-time) forcing, and it works fine, but it'd be nice to have a better way for even divisors. (by "better way" I mean in the resulting asm of course, not having two branches is a good win unless I need 3-4 extra instructions, then it's not so good)

(well I have them in C++ so far for HLL usage, which I just use it to compile a simple function if I want the constants for asm usage by looking at the output... like what I was originally doing, except GCC's optimizer sucks, though it's a bit ugly since I had to use internal constexpr functions to force it at compile-time, GCC is too dumb with loops otherwise; I don't know if anyone is interested in my C++ code with them)

For example, let's say checking if number is divisible by 6. So far my code spits this out:
test al, 1
jnz .no
imul eax, eax, 0xAAAAAAAB
cmp eax, 0x55555555
ja .no    
Post 10 Mar 2018, 16:31
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