RFR: 8307351: (CmpI/L(AndI/L reg1 reg2)) on x86 can be optimized [v2]

[Tobias Hartmann]

On Tue, 9 May 2023 09:24:11 GMT, Tobias Hotz <duke at openjdk.org> wrote:

>> This patch aims to optimize a case where a And-Node followed by a Cmp-Node would not be converted into a single "test" instruction. The Architecture Description file currently only handles the cases where the right operand of the And-Node is a constant, but not if both are a register.
>> Before this patch, a "and" followed by a "test" would be emitted, so the removed "and" means 2 bytes less have to be emitted.
>> I've attached a JMH Benchmark to demonstrate the performance improvements. Here are the numbers of my Windows 11 machine:
>> Before:
>> 
>> Benchmark                                                   Mode  Cnt   Score   Error  Units
>> AndCmpTestInstruction.benchmarkOpaqueAndCmpEqualsInt        avgt    8  26,736 ± 0,131  ns/op
>> AndCmpTestInstruction.benchmarkOpaqueAndCmpEqualsLong       avgt    8  24,305 ± 0,610  ns/op
>> AndCmpTestInstruction.benchmarkStaticAndCmpEqualsInt        avgt    8  33,052 ± 0,056  ns/op
>> AndCmpTestInstruction.benchmarkStaticLargeAndCmpEqualsLong  avgt    8  18,355 ± 0,030  ns/op
>> AndCmpTestInstruction.benchmarkStaticSmallAndCmpEqualsLong  avgt    8  25,587 ± 0,107  ns/op
>> 
>> After:
>> 
>> Benchmark                                                   Mode  Cnt   Score   Error  Units  Improvement
>> AndCmpTestInstruction.benchmarkOpaqueAndCmpEqualsInt        avgt    8  22,665 ± 0,170  ns/op  (~18%)
>> AndCmpTestInstruction.benchmarkOpaqueAndCmpEqualsLong       avgt    8  18,880 ± 0,123  ns/op  (~29%)
>> AndCmpTestInstruction.benchmarkStaticAndCmpEqualsInt        avgt    8  33,198 ± 0,126  ns/op  (unchanged)
>> AndCmpTestInstruction.benchmarkStaticLargeAndCmpEqualsLong  avgt    8  18,427 ± 0,079  ns/op  (unchanged)
>> AndCmpTestInstruction.benchmarkStaticSmallAndCmpEqualsLong  avgt    8  25,641 ± 0,168  ns/op  (unchanged)
>> 
>> As you can see, the cases with a small static mask have not improved as they have already been covered by another match rule. The test with the large static mask (benchmarkStaticLargeAndCmpEqualsLong) has a sorted instruction sequence as the value is moved to a register and it is not being used directly in the instruction, but there is no measurable performance uplift here.
>> I've tested my changes using the Tier1 jtreg Tests on Windows.
>
> Tobias Hotz has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update benchmark copyright and remove invalid copypasted comment

Looks good to me too. Testing passed.

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13587#pullrequestreview-1419816581