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

[Andrew Haley]

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.

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?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16989#discussion_r1482613251