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# mod operator efficiency

Paul Fenerty
Greenhorn
Posts: 23
Hello,

There is a technique for implementing a multiplicative linear congruential random number generator

x[n+1] = ( a * x[n] ) mod m (eq. 1)

in 32 bit integers that eliminates the potential for overflow in the intermediate product (a * x[n]), known as Schrage's method, and popularized in Numerical Recipes in C

http://www.library.cornell.edu/nr/bookcpdf/c7-1.pdf

Briefly:

*********************************
Schrage�s algorithm is based on an approximate factorization of m,

m = aq + r, i.e., q = [m/a], r = m mod a (7.1.4)

with square brackets denoting integer part. If r is small, specifically

r < q, and 0 < z < m - 1,

it can be shown that both a(z mod q) and r[z/q]lie in the range

0,...,m - 1, and that

az mod m = a(z mod q) - r[z/q] if it is = 0, (7.1.5)
a(z mod q) - r[z/q] + m otherwise
********************************

Applying Schrage to the LCG, in C code, Numerical Recipes achieves

x = a * (x - (x / q) q) - r (x / q) (eq. 2)

by substituting the modulo arithmetic equivalence

x mod q = x - q[x/q]

into a preceding step, which is

x = a * (x % q) - r * (x / q) (eq. 3)

which you get by applying Numerical Recipe's (7.1.5) to the LCG (eq. 1)

So, why keep going past (eq. 3) ??

Doing so replaces the mod operator in (eq. 3) with a subtraction, a multiplication, and a division, in (eq. 2).

Is mod that expensive??

Which expression is faster in Java? Note that this code can be iterated up to near 10^18 repetitions, with a combined pair of certain such LCGs, so every cycle counts bigtime.

Thanks!!
Paul

Paul Fenerty
Greenhorn
Posts: 23
They also replace (x/q), which appears twice in (eq. 2) with another variable, and so there is a savings there, evaluating (x/q) only the once per iteration, in a separate expression.

Javaworld claims:

"divide remains the slow operation on most if not all architectures."

in

http://www.javaworld.com/javaworld/jw-04-1997/jw-04-optimize.html

Anybody know the relative efficiency of * / and mod ??

Thanks again!

William Brogden
Author and all-around good cowpoke
Rancher
Posts: 13071
6
If that question was bothering me, as a first step I would write some test code and then use the javap tool to look at the bytecodes written by the compiler.
Bill

Paul Fenerty
Greenhorn
Posts: 23
Thanks for that.

The new info that javap provides is the following:

* -> imul
/ -> idiv
% -> irem

Digging around, I (only) found a lisp implementation (ACL2), by J. Strother Moore:

http://www.cs.utexas.edu/users/moore/

that looks like this:

http://www.cs.utexas.edu/users/moore/publications/marktoberdorf-02/m5.lisp
-----------------------------------------------------------------------------

; (IDIV) Instruction

(defun execute-IDIV (inst th s)
(modify th s
c (+ (inst-length inst) (pc (top-frame th s)))
:stack (push (int-fix
(truncate (top (pop (stack (top-frame th s))))
(top (stack (top-frame th s)))))
(pop (pop (stack (top-frame th s)))))))

; -----------------------------------------------------------------------------

; (IMUL) Instruction

(defun execute-IMUL (inst th s)
(modify th s
c (+ (inst-length inst) (pc (top-frame th s)))
:stack (push (int-fix
(* (top (pop (stack (top-frame th s))))
(top (stack (top-frame th s)))))
(pop (pop (stack (top-frame th s)))))))

; -----------------------------------------------------------------------------

; (IREM) Instruction

(defun execute-IREM (inst th s)
(let* ((val1 (top (pop (stack (top-frame th s)))))
(val2 (top (stack (top-frame th s))))
(result (- val1 (* (truncate val1 val2) val2))))
(modify th s
c (+ (inst-length inst) (pc (top-frame th s)))
:stack (push result
(pop (pop (stack (top-frame th s))))))))

; -----------------------------------------------------------------------------

Don't know how typical this is among JVMs, but irem here is busier than the other two.

Taking some data on a P3, three samples of 2 x 10^10 iterations across each of the three versions of Schrage's version of Lehmer's mLCG come in as follows, and confirm that the modulo expression is the slowest approach:

**************

x = a * (x - (x / q) * q) - r * (x / q); // (eq. 2)
8m 44s

**************

int k = x / q;
x = a * (x - k * q) - r * k; // (eq. 2b)
8m 43s

**************

x = a * (x % q) - r * (x / q); // (eq. 3)
9m 50s

... so looks like they got it right in Numerical Recipes.

Paul Fenerty
Greenhorn
Posts: 23
um, make that 2 x 10^9 iterations ...