A simple optimization proposal (original) (raw)

Martin Grajcar maaartinus at gmail.com
Wed Feb 12 09:40:30 PST 2014

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Most hash tables are power-of-two sized so that they can use masking for the access. It looks like the bounds check doesn't get eliminated, although it could be.

Based on the equivalence a[x & (a.length - 1)] throws if and only if a.length == 0, I'm proposing this simple algorithm:

• For each array access, check if the index has been computed via a bitwise and.
• If so, check if either of the operands was computed as length minus one.
• If so, replace the bounds check by a zero-length check.

This zero-length check can then be easily moved out of the loop by the existing optimizations.

I hope I'm not talking non-sense. For more details see http://stackoverflow.com/questions/21702939/why-the-bounds-check-doesnt-get-eliminated

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