RFR: 8321308: AArch64: Fix matching predication for cbz/cbnz

[Fei Gao]

On Wed, 6 Dec 2023 01:54:59 GMT, Fei Gao <fgao at openjdk.org> wrote:

> For array length check like:
> 
>   if (a.length > 0) {
>     [Block 1]
>   } else {
>     [Block 2]
>   }
> 
> 
> Since `a.length` is unsigned, it's semantically equivalent to:
> 
>   if (a.length != 0) {
>     [Block 1]
>   } else {
>     [Block 2]
>   }
> 
> 
> On aarch64 port, we can do the conversion like above, during c2 compiler instruction matching, for certain unsigned integral comparisons.
> 
> For example,
> 
> cmpw  w11, #0 # unsigned
> bls   label   # unsigned
> [Block 1]
> 
> label:
> [Block 2]
> 
> 
> can be converted to:
> 
> cbz  w11, label
> [Block 1]
> 
> label:
> [Block 2]
> 
> 
> Currently, we have some matching rules to do the conversion [[1]](https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64.ad#L16179). But the predicate here [[2]](https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64.ad#L6140) matches wrong `BoolTest` masks, so these rules fail to convert. I guess it's a typo introduced in [JDK-8160006](https://bugs.openjdk.org/browse/JDK-8160006). The patch fixes it.

Suppose the GHA failure of `java/util/stream/GathererTest` on `linux-x86` is not caused by the patch :)

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

PR Comment: https://git.openjdk.org/jdk/pull/16989#issuecomment-1842050060