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

[Vladimir Kozlov]

On Tue, 29 Mar 2022 20:46:31 GMT, Zhiqiang Zang <duke at openjdk.java.net> wrote:

>> 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?

I think we need to check `y`. For example:


for (int i = 0; i < 10000; i++) {
    a[MAX - i] = b[MAX - i];
}

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

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