RFR: 8358179: Performance regression in Math.cbrt

[Sandhya Viswanathan]

On Tue, 24 Jun 2025 22:33:56 GMT, Mohamed Issa <missa at openjdk.org> wrote:

> The changes described below are meant to resolve the performance regression introduced by the **x86_64 cbrt** double precision floating point scalar intrinsic in #24470.
> 
> 1. Check for +0, -0, +INF, -INF, and NaN before any other input values.
> 2. If these special values are found, return immediately with minimal modifications to the result register.
> 
> The commands to run all relevant micro-benchmarks are posted below.
> 
> `make test TEST="micro:CbrtPerf.CbrtPerfRanges"`
> `make test TEST="micro:CbrtPerf.CbrtPerfSpecialValues"`
> 
> The results of all tests posted below were captured with an [Intel® Xeon 8488C](https://www.intel.com/content/www/us/en/products/sku/231730/intel-xeon-platinum-8480c-processor-105m-cache-2-00-ghz/specifications.html) using [OpenJDK v26-b1](https://github.com/openjdk/jdk/releases/tag/jdk-26%2B1) as the baseline version. The term _baseline1_ refers to runs with the intrinsic enabled and _baseline2_ refers to runs with the intrinsic disabled.
> 
> Each result is the mean of 8 individual runs, and the input ranges used match those from the original Java implementation. Overall, the changes provide a significant uplift over _baseline1_ except for a mild regression in the (**2^(-1022) <= |x| < INF**) input range, which is expected due to the extra checks. When comparing against _baseline2_, the modified intrinsic significantly still outperforms for the inputs (**-INF < x < INF**) that require heavy compute. However, the special value inputs that trigger fast path returns still perform better with _baseline2_.
> 
> | Input range(s)                                  | Baseline1 (ops/ms) | Change (ops/ms) | Change vs baseline1 (%) |
> | :-------------------------------------: | :-------------------: | :------------------: | :--------------------------: |
> | [-2^(-1022), 2^(-1022)]                   | 18470                     | 20847                   | +12.87                             |
> | (-INF, -2^(-1022)], [2^(-1022), INF) | 210538                   | 198925                 | -5.52                                |
> | [0]                                                     | 344990                  | 627561                 | +81.91                             |
> | [-0]                                                   | 291983                   | 629941                 | +115.75                           |
> | [INF]                                                 | 382685                   | 542211                 | +41.68                             |
> | [-INF...

src/hotspot/cpu/x86/stubGenerator_x86_64_cbrt.cpp line 51:

> 49: ATTRIBUTE_ALIGNED(16) static const juint _ABS_MASK[] =
> 50: {
> 51:     4294967295, 2147483647

This should be a 128 bit constant as we are using it with andpd.  Also please add in comments the hex value.

src/hotspot/cpu/x86/stubGenerator_x86_64_cbrt.cpp line 217:

> 215:   __ bind(B1_1);
> 216:   __ ucomisd(xmm0, ExternalAddress(ZERON), r11 /*rscratch*/);
> 217:   __ jcc(Assembler::zero, L_2TAG_PACKET_1_0_1); // Branch only if x is +/- zero or NaN

This could be Assembler::equal to be consistent.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/25962#discussion_r2170180412
PR Review Comment: https://git.openjdk.org/jdk/pull/25962#discussion_r2170181404