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 I agree that this change is correct. Clearly when we have an unsigned compare an `LS` test is never going to be true and ought never to be generated. So this rule is actually missing the relevant case of an `LE` test.

As you correctly note, the error was introduced by the change I made as part of [JDK-8160006](https://bugs.openjdk.org/browse/JDK-8160006) where the original in-rule occurrences of the predicate tested for `BoolTest::le`. This was mistranscribed into the operand name as Lt and into the operand predicate as a check against constant `BoolTest::lt`. The `BoolTest::lt` constants was later replaced by the equivalent value `Assembler::LS`.

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PR Comment: https://git.openjdk.org/jdk/pull/16989#issuecomment-1933829971