Simple bounds check hack

Many languages provide array bounds checking, typically through
an expression of the form:


(index >= 0 && index < size)


On a 2's complement machine where 'size' is non-negative this can be
easily replaced by


((unsigned)index < (unsigned)size)


saving a comparision and branch in the common case.


This optimization is simple enough that I'm embarassed to post
it, but the compilers I have looked at get it wrong ;-)


Regards,
Dawson Engler
engler@amsterdam.lcs.mit.edu
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