[MLton] s->alignment considered harmful

[Florian Weimer]

Storing s->alignment in a variable means that the alignment code in
util/align.h uses full integer divisions to perform the alignment.
Consequently, any allocation-intense program mainly stresses the
integer divider.  A simple s/s->alignment/4/ (assuming that the
default alignment value is 8 on i386) leads to a 60% speed increase
for examples/thread-switch.sml on a slightly dated Opteron.

The best option seems to be to compile the GC multiple times, for each
alignment choice.  Alternatively, the util/align.h could be tweaked to
assume power-of-two alignment, or the alignment could be stored as a
mask.

I think this accounts for the slowdown that has been observed after
the x86_64 branch merge.

-- 
Florian Weimer                <fweimer at bfk.de>
BFK edv-consulting GmbH       http://www.bfk.de/
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