RFR: 8267239: C1: RangeCheckElimination for % operator if divisor is IntConstant [v3]

[Yi Yang]

On Wed, 19 May 2021 02:42:03 GMT, Yi Yang <yyang at openjdk.org> wrote:

>> % operator follows from this rule that the result of the remainder operation can be negative only if the dividend is negative, and can be positive only if the dividend is positive. Moreover, the magnitude of the result is always less than the magnitude of the divisor(See [LS 15.17.3](https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.17.3)).
>>       
>> So if `y` is a constant integer and not equal to 0, then we can deduce the bound of remainder operation:
>> -  x % -y  ==> [0, y - 1] RCE
>> -  x % y   ==> [0, y - 1] RCE
>> -  -x % y  ==> [-y + 1, 0]
>> -  -x % -y ==> [-y + 1, 0]
>> 
>> Based on above rationale, we can apply RCE for the remainder operations whose dividend is constant integer and >= 0,  e.g.:
>> 
>> 
>> for(int i=0;i<1000;i++){
>>   int top5 = arr[i%5];  // Apply RCE if arr is a loop invariant
>>   ....
>> }
>> 
>> 
>> For more detailed RCE results, please check out the attachment on JBS, it was generated by ArithmeticRemRCE with additional flags -XX:+TraceRangeCheckElimination -XX:+PrintIR.
>> 
>> Testing:
>> - test/hotspot/jtreg/compiler/c1/(slowdebug)
>
> Yi Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   missing whitespace; more comment

Thank you Nils and Tobias for taking the time to review this patch! I've filed https://bugs.openjdk.java.net/browse/JDK-8267376 for investigating if it's possible to apply C2 RCE for % operator when divisor != 0.

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PR: https://git.openjdk.java.net/jdk/pull/4083