RFR: 8261008: Optimize Xor

[Bernardo Sulzbach]

On Mon, 1 Mar 2021 10:40:51 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> This patch canonicalizes 'Xor' to constant zero when both inputs are the
>> same.
>> 
>> It's not quite easy to measure the performance change between 'xor' and
>> constant zero, the later is typically a single 'mov' in generated code.
>> But given by this transformation, c2 may perform some other more
>> powerful optimizations.
>> 
>> This was tested with the micro benchmark below. Loop in this case is
>> properly removed and the performance increases significantly.
>> 
>> public void xorTheSame(MyState s, Blackhole bh) {
>>   int x = s.in1;
>>   int y = s.in2;
>>   for (int i = 0; i < 5000; i++) {
>>     y = x ^ x;
>>     x = y ^ y;
>>   }
>>   bh.consume(x);
>> }
>> 
>> [Test]
>> All jtreg tests passed without new failure.
>
> Just wondering, why is `x ^ x` special? Shouldn't we then also optimize `x - x`, `x | x` and others?

If we agree that this optimization is desired, then it makes sense to also write optimizations for `x - x` and `x | x`.

However, how often do these patterns occur in real code? Can you provide some examples (of source code from OSS projects) where `x ^ x`, `x - x`, or `x | x` occur?

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

PR: https://git.openjdk.java.net/jdk/pull/2776