[Issue 17881] Provide mechanism to preallocate memory from the GC

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17881

--- Comment #4 from Steven Schveighoffer <schveiguy at yahoo.com> ---
Stevens-MacBook-Pro:testd steves$ cat testpreallocate.d
struct S
{
    S *next;
}

void main()
{
    version(freelist)
    {
        S* head = null;
        foreach(i; 0 .. 20_000_000)
            head = new S(head);
    }
    version(preallocate)
    {
        S* head = null;
        auto x = new S[20_000_000];
        foreach(i; 0 .. 20_000_000)
        {
            x[i] = S(head);
            head = &(x[i]);
        }
    }
}
Stevens-MacBook-Pro:testd steves$ dmd -O -release testpreallocate.d
-version=freelist
Stevens-MacBook-Pro:testd steves$ time ./testpreallocate

real    0m1.869s
user    0m1.750s
sys    0m0.114s
Stevens-MacBook-Pro:testd steves$ dmd -O -release testpreallocate.d
-version=preallocate
Stevens-MacBook-Pro:testd steves$ time ./testpreallocate

real    0m0.111s
user    0m0.045s
sys    0m0.062s


The point is that the GC is not well-equipped to handle the tight allocation
loop.

The second version has the drawback that all 20 million elements will remain in
memory as long as there is one element still alive.

What I'm looking for is something that has the performance (or similar, I
realize it won't be as good) of the second, but can collect each block
individually like the first.

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