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

[Vladimir Ivanov]

Hi Eric,

I was confused at first by what "signed extract" means.
It should be "sign extract".

Overall, the changes look good.

One comment:

   (i >> 31) >>> 31 ==> i >>> 31

The shift count value is irrelevant here, isn't it?

So, the transformation can be generalized to:

   (i >> n) >>> 31 ==> i >>> 31

Best regards,
Vladimir Ivanov

On 09.04.2020 15:17, Eric Liu wrote:
> 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
>