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Loop Predication
Loop Predication is an optimization in C2. The general idea is to insert a predicate on the entry path to a loop,
and raise a uncommon trap if the check of the condition fails. The condition checks are promoted from inside the
loop body, and thus the checks inside the loop could be eliminated. Currently, loop predication optimization has
been applied to remove array range check and loop invariant checks (such as null checks and array checks).
Array Range Check Elimination
Use loop predicate to remove array range checks in a loop.
original loop |
|---|
for (int i = init, i < limit; i += stride) { // loop with array range check |
after loop predication |
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if ( scale * imax + offset < a.length ) { // loop predicate. imax is the maximum value of i where init <= i < limit |
From the above example, the requirements to perform loop predication for array range check elimination are that
init, limit, offset and array a are loop invariants, and stride and scale are compile time constants.
Loop Invariant Check Elimination
Similar to array range check elimination, but the loop invariant check elimination is much simple. The check inside the
loop is loop invariant, and can be promoted outside the loop directly. Current implementation of loop invariant check
elimination includes null check elimination and array check elimination, etc.
Loop Predication Advantages
Overview
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