[jhc] strange crashes and changes in performance

[Henning Thielemann]

I'm playing around with a simple program that fills a buffer and writes 
that to disk.

The first variant is strange since it becomes much more than twice as 
slow, when the test runs twice:
  http://code.haskell.org/storablevector/speedtest/SpeedTestExpSlow.hs

The second variant is strange, because if I use allocaArray instead of 
allocaBytes then it crashes. However, when I run the test twice, it works.
  http://code.haskell.org/storablevector/speedtest/SpeedTestExpCrashAllocaArray.hs