RFR: 8332856: C2: Add new transform for bool eq/ne (cmp (and (urshift X const1) const2) 0)

[Tobias Hotz]

On Mon, 20 May 2024 14:15:46 GMT, Tobias Hotz <duke at openjdk.org> wrote:

> This PR adds a new ideal optimization for the following pattern:
> 
> public boolean testFunc(int a) {
>     int mask = 0b101;
>     int shift = 12;
>     return ((a >> shift) & mask) == 0;
> }
> 
> Where the mask and shift are constant values and a is a variable. For this optimization to work, the right shift has to be idealized to a unsinged right shift earlier in the pipeline, which here: https://github.com/openjdk/jdk/blob/b92bd671835c37cff58e2cdcecd0fe4277557d7f/src/hotspot/share/opto/mulnode.cpp#L731
> If the shift is already an unsiged bit shift, it works as well.
> On AMD64 CPUs, this means that this whole line computation can be reduced to a simple `test` instruction.

Here are some numbers of the newly added benchmark:

Baseline: (min, avg, max) = (20,223, 20,302, 20,411), stdev = 0,058
With patch: (min, avg, max) = (29,668, 29,765, 29,941), stdev = 0,098

As you can see, the time per iteration was reduced by roughly 30% in this microbenchmark

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PR Comment: https://git.openjdk.org/jdk/pull/19310#issuecomment-2136022313