A simple optimization proposal

Martin Grajcar maaartinus at gmail.com
Wed Feb 12 15:32:42 PST 2014

Hi John, Chris, and Vladimir,

I guess the Gist replacing

(x & array.length - 1) u< array.length
array.length != 0

can be easily changed to implement

(x & (m-1)) u< m, if and only if (m > 0)

in general, but I wonder if it will result in the optimal code for arrays,
which can look like

int mask = array.length - 1; // already there
if (mask < 0) goAway();

where the second line on i86 translates to simply jump if sign, i.e., a
conditional branch without any test as the flags has been already set.

This uses the equivalence m > 0 if and only if (m-1) >= 0 which holds with
the exception of m = Integer.MIN_VALUE. When checking array bounds, this
exception can't occur.


On Wed, Feb 12, 2014 at 11:00 PM, Vladimir Kozlov <
vladimir.kozlov at oracle.com> wrote:

> Looks reasonable. Kris, you need also look for other patterns listed in
> JDK-8003585.
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