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

[Dean Long]

On Thu, 8 Feb 2024 08:51:17 GMT, Andrew Haley <aph 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.
>
> src/hotspot/cpu/aarch64/aarch64.ad line 16188:
> 
>> 16186:     Label* L = $labl$$label;
>> 16187:     Assembler::Condition cond = (Assembler::Condition)$cmp$$cmpcode;
>> 16188:     if (cond == Assembler::EQ || cond == Assembler::LE)
> 
> So this is an explicitly unsigned comparison `CmpU` yet it has been converted into an explicitly signed `Assembler::LE`? Is that right?

I agree, this is confusing.  Why not fix cmpOpUEqNeLeGt so that it uses the unsigned condition code values?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16989#discussion_r1483675403