Array Appending and DRuntime

[dsimcha]

== Quote from dsimcha (dsimcha at yahoo.com)'s article
> 2.  gcx.d appears to cache the last block size query.  This means that
> repeatedly querying the same block in a single threaded program (where
> thread_needLock() returns false and no lock is necessary) is very fast.  This
> is true in both the old Phobos GC and the druntime GC.  I wonder if this was
> somehow bypassed by the ~= operator when druntime was integrated with DMD in
> its early days.

On further examination, this is clearly somehow related to caching.  Here is a
very similar test program, that appends to tow arrays instead of one:

import std.stdio, std.perf;

void main() {
    scope pc = new PerformanceCounter;
    pc.start;
    uint[] foo, bar;
    foreach(i; 0..1_000_000) {
        foo ~= i;
		bar ~= i;
    }
    pc.stop;
    writeln(pc.milliseconds);
}

Timings:
DMD 2.019:  ~1800 ms
DMD 2.029:  ~2300 ms (Note:  Still slower but not by as much even in absolute terms)
DMD 2.029 (Using Appender instead of ~=):  49 ms

By appending to two arrays, we screw up the caching scheme, hence much poorer
performance.  However most use cases probably involve appending to only one array
at a time.  Since this is a clear regression, I'll file a bug report.