RFR: 8323220: Reassociate loop invariants involved in Cmps and Add/Subs [v16]

[Emanuel Peter]

On Mon, 8 Apr 2024 18:42:43 GMT, Joshua Cao <duke at openjdk.org> wrote:

>> // inv1 == (x + inv2) => ( inv1 - inv2 ) == x
>> // inv1 == (x - inv2) => ( inv1 + inv2 ) == x
>> // inv1 == (inv2 - x) => (-inv1 + inv2 ) == x
>> 
>> 
>> For example,
>> 
>> 
>> fn(inv1, inv2)
>>   while(...)
>>       x = foobar()
>>       if inv1 == x + inv2
>>            blackhole()
>> 
>> 
>> We can transform this into
>> 
>> 
>> fn(inv1, inv2)
>>   t = inv1 - inv2
>>   while(...)
>>       x = foobar()
>>       if t == x
>>            blackhole()
>> 
>> 
>> Here is an example: https://github.com/openjdk/jdk/blob/b78896b9aafcb15f453eaed6e154a5461581407b/src/java.base/share/classes/java/lang/invoke/LambdaFormEditor.java#L910. LHS `1` and RHS `pos` are both loop invariant
>> 
>> Passes tier1 locally on Linux machine. Passes GHA on my fork.
>
> Joshua Cao has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/hotspot/share/opto/loopTransform.cpp
>   
>   formatting
>   
>   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>

@chhagedorn for the suggestions, I like the updates!
@caojoshua approved again :)

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

Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/17375#pullrequestreview-2000103864