RFR: 8278114: New addnode ideal optimization: converting "x + x" into "x << 1" [v9]

[Claes Redestad]

On Sun, 19 Dec 2021 20:28:07 GMT, Zhiqiang Zang <duke at openjdk.java.net> wrote:

>> A new ideal optimization can be introduced for addnode: converting "x + x" into "x << 1".
>> 
>> 
>> // Convert "x + x" into "x << 1"
>> if (in1 == in2) {
>>   return new LShiftINode(in1, phase->intcon(1));
>> }
>
> Zhiqiang Zang has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - refactor the ir test.
>  - rename tests.
>  - use compiler mode blackhole in microbenchmark to prevent function calling from dominating time.

Looks good. The further microbenchmark improvements suggested by @merykitty makes a lot of sense given the context. 

The comment about compiler blackhole is becoming a bit redundant with the upgrade to JMH 1.34 (#6955), which enables the compiler assisted blackholes by default.

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

Marked as reviewed by redestad (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6675