RFR: 8373613: PEXT/PDEP intrinsics cause performance regression on AMD pre-Zen 3 CPUs

[Emanuel Peter]

On Thu, 19 Feb 2026 03:22:16 GMT, Alessandro Autiero <duke at openjdk.org> wrote:

> The `Integer/Long.compress()` and `Integer/Long.expand()` intrinsics (added in JDK 19 by JDK-8283893) unconditionally use BMI2 PEXT/PDEP instructions on all BMI2-capable x86 CPUs. However, AMD processors before Zen 3 (Family < 0x19) and Zhaoxin CPUs implement PEXT/PDEP via microcode with ~18-cycle  latency, making the intrinsified path 1.4-2.4x slower than the Java software fallback.
> 
> This patch introduces a `CPU_FAST_BMI2` feature flag that distinguishes CPUs with native hardware PEXT/PDEP support (Intel Haswell+, AMD Zen 3+) from those with slow microcoded implementations. The `Op_CompressBits`/`Op_ExpandBits` match rules in `x86.ad` are gated on this new flag instead of the general `supports_bmi2()`, so CPUs without fast hardware support fall back to the Java implementation. C2 IR tests are updated to use the new `fast_bmi2` CPU feature predicate.

@Auties00 Is there a benchmark integrated that shows the performance impact of the regression and this patch?

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PR Comment: https://git.openjdk.org/jdk/pull/29809#issuecomment-3951674348