Try harder to reclaim memory in fmpz_single

[fredrik-johansson]

The fmpz mpz cache is supposed to free slots with more than 64 limbs allocated, but does so with an mpz_realloc which isn't guaranteed to actually reclaim the memory.

This results in soft memory leaks for complex fmpz workloads with big coefficients where more and more coefficients get cached and the huge allocations stay in the pool. The memory can be reclaimed by calling flint_cleanup_master(), but this shouldn't be necessary.

The fix is to clear+init instead of realloc.

Demonstration:

#include "profiler.h"
#include "fmpz.h"
#include "fmpz_poly.h"

int main()
{
    flint_rand_t state;
    slong i, n, bits;

    flint_rand_init(state);

    fmpz_poly_t a, b, c;

    for (i = 0; i <= 200; i++)
    {
        n = 1 + n_randint(state, 1 + 1000 * (i % 10));
        bits = 1 + n_randint(state, 1 + 1000 * i);

        fmpz_poly_init(a);
        fmpz_poly_init(b);
        fmpz_poly_init(c);

        fmpz_poly_randtest(a, state, n, bits);
        fmpz_poly_randtest(b, state, n, bits);
        fmpz_poly_add(c, a, b);

        fmpz_poly_clear(a);
        fmpz_poly_clear(b);
        fmpz_poly_clear(c);

        if (i % 10 == 0)
            print_memory_usage();
    }

    flint_rand_clear(state);
    flint_cleanup_master();
    return 0;
}

Before (note the growing rss):

virt/peak/rss/peak: 19.5 MB / 19.5 MB / 2.61 MB / 2.61 MB
virt/peak/rss/peak: 26.8 MB / 26.8 MB / 12.0 MB / 12.0 MB
virt/peak/rss/peak: 32.9 MB / 32.9 MB / 17.9 MB / 17.9 MB
virt/peak/rss/peak: 54.7 MB / 54.7 MB / 39.8 MB / 39.8 MB
virt/peak/rss/peak: 54.9 MB / 54.9 MB / 40.1 MB / 40.1 MB
virt/peak/rss/peak: 60.8 MB / 60.9 MB / 46.1 MB / 46.1 MB
virt/peak/rss/peak: 74.6 MB / 74.6 MB / 59.8 MB / 59.8 MB
virt/peak/rss/peak: 75.9 MB / 76.0 MB / 61.2 MB / 61.2 MB
virt/peak/rss/peak:  102 MB /  103 MB / 87.8 MB / 87.8 MB
virt/peak/rss/peak:  102 MB /  103 MB / 87.7 MB / 87.8 MB
virt/peak/rss/peak:  106 MB /  106 MB / 91.6 MB / 91.6 MB
virt/peak/rss/peak:  117 MB /  117 MB /  102 MB /  102 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB
virt/peak/rss/peak:  212 MB /  212 MB /  197 MB /  197 MB

After:

virt/peak/rss/peak: 19.5 MB / 19.5 MB / 2.69 MB / 2.69 MB
virt/peak/rss/peak: 26.8 MB / 26.8 MB / 12.0 MB / 12.0 MB
virt/peak/rss/peak: 32.6 MB / 32.6 MB / 17.6 MB / 17.6 MB
virt/peak/rss/peak: 32.6 MB / 54.3 MB / 17.9 MB / 39.3 MB
virt/peak/rss/peak: 32.6 MB / 54.3 MB / 17.9 MB / 39.3 MB
virt/peak/rss/peak: 32.6 MB / 54.3 MB / 17.9 MB / 39.3 MB
virt/peak/rss/peak: 36.7 MB / 57.6 MB / 22.0 MB / 42.7 MB
virt/peak/rss/peak: 38.5 MB / 65.5 MB / 23.8 MB / 50.7 MB
virt/peak/rss/peak: 37.3 MB / 86.9 MB / 22.6 MB / 72.0 MB
virt/peak/rss/peak: 41.3 MB / 86.9 MB / 26.6 MB / 72.0 MB
virt/peak/rss/peak: 41.2 MB / 92.1 MB / 26.5 MB / 77.2 MB
virt/peak/rss/peak: 40.4 MB / 92.1 MB / 25.7 MB / 77.2 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB
virt/peak/rss/peak: 37.3 MB /  205 MB / 22.6 MB /  190 MB