RFR: 8270366: C2: Add associative rule to add/sub node [v2]

[Vladimir Kozlov]

On Wed, 14 Jul 2021 14:04:41 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Please review this small patch that add associative rule to add/sub node, that eliminates a multiply instruction.
>> 
>> e.g.
>> 
>> private static int assocInt(int a, int b, int c) {
>>   return a * c + b * c;
>> }
>> 
>> 
>> x86_64:
>> Before:
>> 
>>   0x00007fda1506152c:   imul   %ecx,%esi
>>   0x00007fda1506152f:   imul   %ecx,%edx
>>   0x00007fda15061532:   mov    %esi,%eax
>>   0x00007fda15061534:   add    %edx,%eax                    ;*iadd {reexecute=0 rethrow=0 return_oop=0}
>>                                                             ; - TestAssoc::assocInt at 6 (line 9)
>> 
>> 
>> After:
>> 
>>   0x00007fc1c078d52c:   add    %edx,%esi
>>   0x00007fc1c078d52e:   mov    %ecx,%eax
>>   0x00007fc1c078d530:   imul   %esi,%eax                    ;*iadd {reexecute=0 rethrow=0 return_oop=0}
>>                                                             ; - TestAssoc::assocInt at 6 (line 9)
>
> Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Missing case pointed out by Andrew Dinn
>  - Merge branch 'master' into JDK-8270366-associative
>  - sub
>  - add

https://docs.oracle.com/javase/specs/jls/se16/html/jls-15.html#jls-15.17.1

If an integer multiplication overflows, then the result is the low-order bits of the mathematical product as represented in some sufficiently large two's-complement format. As a result, if overflow occurs, then the sign of the result may not be the same as the sign of the mathematical product of the two operand values.


I am interpreting this as using long arithmetic for integer overflow and taking low 32-bits of result. In this sense suggested optimization is legal.

*But* we may have bugs in VM. I suggest to add a regression test to compare golden (from Interpreter) values with results from optimized compiled code for all interesting/corner cases.

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

PR: https://git.openjdk.java.net/jdk/pull/4765