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changes.mady.by.user Changpeng Fang

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...

original loop

for (int i = init,  i < limit;  i += stride) {  // loop with array range check
    ...
    if (scale*i + offset < a.length) {  // array range check
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="650022d0087cf8f8-15e6a121-4f314a0f-b45382fb-0ca208eb546899980f145f05"><ac:plain-text-body><![CDATA[        ... a[scale*i+offset] ...
]]></ac:plain-text-body></ac:structured-macro>
    } else raise_uncommon_trap();
    ...
}

after loop predication

if ( scale * imax + offset < a.length ) {  // loop predicate. imax is the maximum value of i where init <= i < limit
    for (int i = init,  i < limit;  i += stride) {  // loop without array range check
         ...
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="c8ef1b9e212f50b7-a1a00060-4c07469f-a3cdb771-97a7a1d14f6de044c0f2c976"><ac:plain-text-body><![CDATA[         ... a[scale*i+offset] ...
]]></ac:plain-text-body></ac:structured-macro>
         ...
    }
} else raise_uncommon_trap();

...

Loop Predication Advantages

Source Code

1. The parser inserts a dummy loop predicate on each path to the loop head block (Parse::add_predicate in parse1.cpp)

2. The loop optimizer analyzes the loop and promote the checks outside the loop (IdealLoopTree::loop_predication in loopTransformation.cpp)

3. Loop predication is controlled by the -XX flag UseLoopPredicate. -XX:+TraceLoopPredicate turns on the debug engine for loop predication in debug VM.