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