[LLVMdev] [LLVMDev] Does LLVM have a random number generator?

[Michael Spencer]

> On Sat, Oct 9, 2010 at 1:10 AM, Jeff Kunkel <jdkunk3 at gmail.com> wrote:
>> Hello, does LLVM already have a Random Number Generator built into
>> it's library somewhere?
>>
>> I know code generation is suppose to be deterministic, but when
>> producing a random number can be deterministic if the random number
>> generator is also deterministic.

What exactly is this for? Most heuristics in llvm passes are generated
by calculating an arbitrary "goodness" value by iterating over the
code in question and looking for indicators.

I have no opinion on using psudorandom numbers, but you have to be
careful to keep the results the same across platforms. Quite a few
stdlib algorithms don't say exactly how many times any given function
will be called.

> I am plugging this into my code. If someone wants to take it out and
> add it to the llvm library, it's a simple Linear Congruential
> Generator, but here it is:
>
>    typedef struct random_number_gen {
>      unsigned a, c, seed, m;
>      random_number_gen( unsigned seed, unsigned modulo ) :
> seed(seed), m(modulo) {
>        unsigned primes[] = { 2,   3,   5,   7,  11,  13,  17,  19,  23,  29 };
>        a = 1 + primes[ seed % ( sizeof(primes)/sizeof(primes[0]) ) ];
>        c = primes[ modulo % ( sizeof(primes)/sizeof(primes[0]) ) ];
>      }
>      unsigned rand() { return seed = (a*seed + c) % m; }
>    } random_number_gen;
>
> -Thanks
> -Jeff Kunkel

If we do add a psudorandom number generator to the Support library, I
would rather we use the same interface as C++0x <random> and use the
Mersenne twister.

- Michael Spencer