RFR: 8283094: Add Ideal transformation: x + (con - y) -> (x - y) + con [v5]

[Zhiqiang Zang]

On Tue, 29 Mar 2022 17:23:37 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Zhiqiang Zang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
>> 
>>  - Merge master.
>>  - rename test cases.
>>  - move tests to existing test classes.
>>  - comment out "x+(0-y)" -> "(x-y)" and "(0-y)+x" -> "(x-y)" because they
>>    are covered already.
>>  - Merge master.
>>  - include transformation from (con - y) + x into (x - y) + con and x + (con - y) into (x - y) + con.
>
> src/hotspot/share/opto/addnode.cpp line 314:
> 
>> 312:   // Convert x + (con - y) into "(x - y) + con"
>> 313:   if (op2 == Op_Sub(bt) && in2->in(1)->Opcode() == Op_ConIL(bt)) {
>> 314:     return AddNode::make(phase->transform(SubNode::make(in1, in2->in(2), bt)), in2->in(1), bt);
> 
> Both of proposed transformations are similar one in `AddNode::Ideal()` lines 181-214.
> There is exception there to avoid transformation if (y+con) could be increment for loop's variable. I wonder if we need the same check here.

Thanks for reviewing! That is indeed an interesting observation. I feel `con - y` could not be a loop increment so it should be sufficient to check `x` is not a loop induction variable?

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PR: https://git.openjdk.java.net/jdk/pull/7795