RFR: 8263058: Optimize vector shift with zero shift count

[Eric Liu]

On Wed, 10 Mar 2021 10:45:05 GMT, Eric Liu <github.com+10482586+theRealELiu at openjdk.org> wrote:

> Like scalar shift, vector shift could do nothing when shift count is
> zero.
> 
> This patch implements the 'Identity' method for all kinds of vector
> shift nodes to optimize out 'ShiftVCntNode 0', which is typically a
> redundant 'mov' in final generated code like below:
> 
>         add     x17, x12, x14
>         ldr     q16, [x17, #16]
>         mov     v16.16b, v16.16b
>         add     x14, x13, x14
>         str     q16, [x14, #16]
> 
> With this patch, the code above could be optimized as below:
> 
>         add     x17, x12, x14
>         ldr     q16, [x17, #16]
>         add     x14, x13, x14
>         str     q16, [x14, #16]
> 
> [TESTS]
> compiler/vectorapi/TestVectorShiftImm.java, jdk/incubator/vector,
> hotspot::tier1 passed without new failure.

I'm now fixing the build failure...

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PR: https://git.openjdk.java.net/jdk/pull/2911