Question

So I tried to test whether the D garbage collector works properly by running this program on Windows.

DMD 2.057 and 2.058 beta both give the same result, whether or not I specify -release, -inline, -O, etc.

The code:

import core.memory, std.stdio;

extern(Windows) int GlobalMemoryStatusEx(ref MEMORYSTATUSEX lpBuffer);

struct MEMORYSTATUSEX
{
    uint Length, MemoryLoad;
    ulong TotalPhys, AvailPhys, TotalPageFile, AvailPageFile;
    ulong TotalVirtual, AvailVirtual, AvailExtendedVirtual;
}

void testA(size_t count)
{
    size_t[] a;
    foreach (i; 0 .. count)
        a ~= i;
    //delete a;
}

void main()
{
    MEMORYSTATUSEX ms;
    ms.Length = ms.sizeof;

    foreach (i; 0 .. 32)
    {
        testA(16 << 20);
        GlobalMemoryStatusEx(ms);
        stderr.writefln("AvailPhys: %s MiB", ms.AvailPhys >>> 20);
    }
}

The output was:

AvailPhys: 3711 MiB
AvailPhys: 3365 MiB
AvailPhys: 3061 MiB
AvailPhys: 2747 MiB
AvailPhys: 2458 MiB
core.exception.OutOfMemoryError

When I uncommented the delete a; statement, the output was

AvailPhys: 3714 MiB
AvailPhys: 3702 MiB
AvailPhys: 3701 MiB
AvailPhys: 3702 MiB
AvailPhys: 3702 MiB
...

So I guess the question is obvious… does the GC actually work?

Answer 1

This looks like a regression – it doesn’t happen in D1 (DMD 1.069). David Simcha has been optimizing the GC lately, so it might have something to do with that. Please file a bug report.