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

[Vladimir Kozlov]

On Fri, 25 Mar 2022 21:16:03 GMT, Zhiqiang Zang <duke at openjdk.java.net> wrote:

>> Hello,
>> 
>> `x + (con - y)  -> (x - y) + con` is a widely seen pattern; however it is missing in current implementation, which prevents some obvious constant folding from happening, such as `x + (1 - y) + 2` will be not optimized at all, rather than into `x - y + 3`.
>> 
>> This pull request adds this transformation.
>
> 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.

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