RFR(S):8242429:Better implementation for signed extract

[Eric Liu]

Hi,

This is a small enhancement for C2 compiler.


For java code "(i >> 31) >>> 31", it can be optimized to "i >>> 31".
AArch64 has implemented this in back-end match rules, while AMD64
hasn't.

Indeed, this pattern can be optimized in mid-end by adding some simple
transformations. Besides, "0 - (i >> 31)" could also be optimized to
"i >>> 31".

This patch adds two conversions:

        1. URShiftINode:        (i >> 31) >>> 31 ==> i >>> 31

        +------+   +----------+
        | Parm |   | ConI(31) |
        +------+   +----------+
           |       /       |
           |      /        |
           |     /         |
      +---------+          |
      | RShiftI |          |
      +---------+          |
               \           |
                \          |
                 \         |
                  +----------+
                  |  URShiftI|
                  +----------+

        2. SubINode:            0 - (i >> 31) ==> i >>> 31

        +------+    +----------+
        | Parm |    | ConI(31) |
        +------+    +----------+
               \           |
                \          |
                 \         |
                  \        |
   +---------+      +---------+
   | ConI(0) |      | RShiftI |
   +---------+      +---------+
               \        |
                \       |
                 \      |
                   +------+
                   | SubI |
                   +------+

With this patch, these two graghs above both can be optimized to below:

        +------+   +----------+
        | Parm |   | ConI(31) |
        +------+   +----------+
            |         /
            |        /
            |       /
            |      /
        +----------+
        | URShiftI |
        +----------+

This patch solved the same issue for long type and also removed the
relevant match rules in "aarch64.ad" which become useless now.


JBS: https://bugs.openjdk.java.net/browse/JDK-8242429
Webrev: http://cr.openjdk.java.net/~yzhang/ericliu/8242429/webrev.00/

[Tests]
Jtreg hotspot::hotspot_all_no_apps, jdk::jdk_core and langtools::tier1.
No new failure found.


--
Thanks,
Eric