BigInt memory usage

[bearophile]

On a 32 bit system this program has allocates about 100 MB of RAM:


import std.bigint;
int main() {
    static assert(BigInt.sizeof == 12);
    auto data = new BigInt[2_000_000];
    foreach (i; 0 .. data.length)
        data[i] = BigInt("9223372036854775808123");

    int count;
    foreach (i; 0 .. 2_000_000_000) { count++; }
    return count;
}



It means about 50 bytes/biginteger.

Every zero BigInt struct needs 12 bytes inside the 'data' array.

The represented numeral requires about 73 bits, this means 3 words (with lot of wasted space):

>>> l = 9223372036854775808123
>>> from math import log
>>> log(l, 2)
72.965784284662092


Using so much memory allows to store less numbers, and makes them slower too (I have hit both problems). Do you know why D BigInts use so much memory?

Bye,
bearophile