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

[Andrew Dinn]

On Wed, 6 Dec 2023 04:05:55 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 :)

@fg1417 Regarding the rework, see my response to @dean-long which explains how the interface for `cmpOpUEqNeLeGt` should be redefined (also how the rules can be retained as currently defined).

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

PR Comment: https://git.openjdk.org/jdk/pull/16989#issuecomment-1938813886