[jdk17] RFR: 8269230: C2: main loop in micro benchmark never executed

[Roland Westrelin]

On Wed, 30 Jun 2021 03:36:20 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> The benchmark loop is roughly:
>> 
>> for (int i = 0; i < limit; i++) {
>>   if (!(i < max_jint && i > min_jint)) {
>>     uncommon_trap();
>>   }
>> }
>> 
>> which IfNode::fold_compares() tranforms to:
>> 
>> for (int i = 0; i < limit; i++) {
>>   if (!(i - min_jint - 1 <u -2)) {
>>     uncommon_trap();
>>   }
>> }
>> 
>> Range elimination then adjusts loop limits so the unsigned comparison
>> can be removed. The new limit is:
>> 
>> min(limit, max(-2 + min_jint + 1, min_jint)) = min(limit, min_jint) = min_jint
>> 
>> and the main loop is never executed.
>> 
>> The logic that protects against overflows during range check
>> elimination is what causes the main loop to never be executed. As a
>> fix, I propose delaying IfNode::fold_compares() if the resulting test
>> compares with a potential negative number which could then cause RC
>> overflow code to trigger. I think that would preserve the most
>> interesting cases for IfNode::fold_compares() when it comes to range
>> check elimination:
>> 
>> i >= 0 && i < array.length
>
> tier1-3 testing passed. Performance results will be tomorrow.

@vnkozlov @veresov thanks for the reviews

-------------

PR: https://git.openjdk.java.net/jdk17/pull/156