GMP integer size limit reached?

Question

I have written an implementation of Toom multiplication with GMP. I tested multiplication of 2 numbers of around 7000 digits (23,000 bits) and it worked perfectly. However when I tried it with 70000 digits (230,000 bits) the program started producing wrong answers.

I am using a 32 bit system. Could GMP run out of memory to use? I did not get any error so I think this is unlikely. If this is not the case I suspect some loss of precision somewhere.

Your question doesn't make any sense. It reads like "my program produces wrong answers, could it be because GMP is running out of memory?" That seems like a totally random diagnosis that bears no resemblance whatsoever to your description of the problem. You should put much more effort into describing the problem and, if you want to propose a diagnosis, provide some evidence to back that diagnosis up. Otherwise, let other people diagnose the problem. — David Schwartz, Aug 25 '14 at 04:09
@DavidSchwartz I don't understand how my question doesn't make sense. The program explicitly works for small values but not large; that's why I believe there is an issue of precision. That is my evidence, in addition to the fact that the entire program makes extensive usage of GMP features. — qwr, Aug 25 '14 at 04:39
Isn't it far more likely that the bug is in your code than that's in a widely-used and extensively-reviewed library like GMP? — David Schwartz, Aug 25 '14 at 05:17
What is the size of limb i.e. `sizeof(mp_limb_t)` for your configuration (even if you have 32-bit OS, the limb might be defined as `unsigned long long` as your CPU might have 64-bit registers) ? Why you are implementing Toom multiplication if it's implemented by library itself (that is, `mpz_mul` "redirects" to proper algorithm based essentially on number of limbs of operands) ? The library has defined multiple thresholds for various mul implementations (defined in gmp-impl.h as number of limbs). For very large operands (thousends limbs) FFT multiplication might be preffered. — Grzegorz Szpetkowski, Aug 25 '14 at 16:30

score 3 · Accepted Answer · answered Aug 25 '14 at 16:40

In general GMP allocates data on heap with "malloc and friends". If data cannot be allocated properly, then it invokes standard abort() function (so process should be simply aborted), see memory.c:

ret = malloc (size);
if (ret == 0)
  {
    fprintf (stderr, "GNU MP: Cannot allocate memory (size=%lu)\n", (long) size);
    abort ();
  }

It's rather impossible to tell what went wrong in your calculations, without taking much more details (see my comment).

GMP integer size limit reached?

1 Answers1