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

[Vladimir Kozlov]

On Tue, 29 Mar 2022 21:06:40 GMT, Zhiqiang Zang <duke at openjdk.java.net> wrote:

>> I think we need to check `y`. For example:
>> 
>> 
>> for (int i = 0; i < 10000; i++) {
>>     a[MAX - i] = b[MAX - i];
>> }
>
> But in this case it seems OK to me to make the transformation, for example:
> 
> for (int i = 0; i < 10000; i++) {
>     a[x + (MAX - i)] = b[x + (MAX - i)]; // a[(x - i) + MAX] = b[(x - i) + MAX]
> }
> 
> Is there something I overlooked?

In loopopts (and superword) we are loking for `(+-phi  +- const)` pattern for loop variable.
With your transformation we get `((+-phi +- x) + const)` which may prevent some optimizations.

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