8389111: C2: Use BMI2 BZHI for variable int low-bit masks on x86

[aprilivy]

C2 currently lowers variable int low-bit masks of the form:

value & ((1 << bits) - 1)

to a five-instruction sequence on x86. This change adds a BMI2 matcher rule that uses BZHI, reducing it to three instructions.

The shift count is explicitly masked with 31 because Java shift distances wrap modulo 32, while BZHI treats counts greater than or equal to 32 differently.

A scalar JMH throughput benchmark on a Ryzen 9 9950X3D improved from approximately 0.199 ns/op to 0.185 ns/op (6.9%).

Testing:

• HotSpot release and fastdebug builds
• compiler/intrinsics/bmi: 26/26 tests passed
• Verified generated assembly with UseBMI2Instructions enabled and disabled

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Issue

• JDK-8389111: C2: Use BMI2 BZHI for variable int low-bit masks on x86 (Enhancement - P4)

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[bridgekeeper]

👋 Welcome back aivy! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

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[aprilivy]

Hi @sviswa7 and @jatin-bhateja, would either of you be willing to take a look at this? Thanks!

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[mlbridge]

Webrevs

• 01: Full - Incremental (dcd66a97)
• 00: Full (8c87e3ec)

[SirYwell]

Two questions:

1. This should also work with long, right? Is there a specific reason why this change is int only?
2. Would it make sense to only emit the and instruction if necessary?

[aprilivy]

Two questions:

1. This should also work with long, right? Is there a specific reason why this change is int only?
2. Would it make sense to only emit the and instruction if necessary?

Thanks!

1. Yes, the analogous long rule is semantically valid, using a count mask of 63. I prototyped it, but it regressed the scalar benchmark on my system, so I left it out rather than include a rule without a demonstrated improvement. It could be investigated separately though.

2. Yes. The andl 31 is required for an unrestricted count to preserve shift semantics, but it can be omitted when C2 proves that the count is already in [0, 31]. I’ll investigate adding a range-guarded form for that case.

[aprilivy]

I’ve now added the bounded-count form discussed above. When C2 proves the count is within [0, 31], it selects a BZHI rule without the extra mask, unrestricted counts retain the andl 31 fallback. I also added IR coverage for both paths. The bounded benchmark improved from 0.207 to 0.197 ns/op (~4.8%)

[symious]

Better to change to

instruct bzhiI_rReg_rReg_bounded(rRegI dst, rRegI src, immI_1 one, rRegI shift,
                                 immI_M1 minus_one, rFlagsReg cr)
  ...
  match(Set dst (AndI src (AddI (LShiftI one shift) minus_one)));