[Issue 20438] New: [Reg 2.086] GC: memory not reusable when calling GC.collect after GC.free

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20438

          Issue ID: 20438
           Summary: [Reg 2.086] GC: memory not reusable when calling
                    GC.collect after GC.free
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: regression
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: r.sagitario at gmx.de

Reported in the learn forum: This program shows increasing memory usage even
though allocated memory is freed manually:

    version=FREE;
    version=COLLECT;
    import std.stdio;
    import core.memory;
    immutable int[] intZ = [1,2,3,4,4,6,6,8,8,65,8,23];
    void main() {
        writeln(GC.stats);
        enum max = 100000;
        foreach (i; 0 .. max) {
            bool doprint = !(i % (max/10));
            int[] z = intZ.dup;
            if (doprint) writef("%7d  ", GC.stats.usedSize);
            version(FREE) GC.free(cast(void*) z.ptr);
            version(COLLECT) GC.collect();
            if (doprint) writefln("%7d", GC.stats.usedSize);
        }
    }

This doesn't happen when either version FREE or COLLECT is not defined.

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