On Mon, 8 Jun 2026 11:46:10 GMT, Ehsan Behrangi <[email protected]> wrote:
>> This change improves the AArch64 implementation of String.equals by
>> introducing SIMD-based fast paths using SVE and NEON.
>>
>> SVE implementation:
>> - Uses predicated loads and comparisons for short lengths (len < VL)
>> - Uses a full predicated loop for longer inputs
>> - Handles the tail via an overlapped compare at (base + len - VL)
>>
>> NEON implementation:
>> - Uses an 8-byte pre-read to simplify tail handling and eliminate 4/2/1-byte
>> scalar branches
>> - Processes 16-byte chunks using LDP pair loads
>> - Uses CMP/CCMP to collapse comparisons into a single branch on mismatch
>>
>> These changes reduce branch pressure and improve throughput for both short
>> and long strings.
>>
>> Correctness:
>> - The implementation preserves existing semantics and matches behavior for
>> all lengths
>>
>> Testing:
>> - Updated and extended intrinsic tests to cover boundary conditions and
>> mismatch positions
>>
>> Benchmark:
>> Across evaluated macrobenchmarks (DaCapo and Renaissance), most workloads
>> spend <0.5% of CPU time in String.equals. DaCapo biojava is a notable
>> exception (~8–9%). In biojava, most String.equals calls are on very short
>> strings (1–2 bytes), where SVE shows ~1% end-to-end improvement, while NEON
>> is largely neutral or shows a small regression (~1%).
>>
>> Measured using JMH on AArch64 (Arm Neoverse V2 CPU). Values are relative (%)
>> vs baseline. Negative values indicate regressions. Mismatch results are
>> reported across first(DF), middle(DM), and last(DL) difference positions.
>>
>> SVE results:
>>
>> Length | L1_EQ L1_DF L1_DM L1_DL | U16_EQ U16_DF U16_DM U16_DL | Avg
>> -------+----------------------------+-----------------------------+------
>> 0 | 19.63 | 20.05 | 19.84
>> 1 | 16.59 17.81 16.57 18.34 | 16.02 0.71 0.42 1.39 | 10.98
>> 2 | 16.44 1.32 0.30 -0.16 | 15.90 -5.17 -4.55 -1.09 | 2.87
>> 3 | 26.58 1.60 1.43 27.07 | 30.34 -8.86 -7.06 14.08 | 10.65
>> 7 | 41.47 -2.94 -3.37 39.82 | 24.02 -8.82 -6.27 20.48 | 13.05
>> 8 | 19.08 -1.16 -3.50 -0.90 | 22.49 -9.75 17.50 13.13 | 7.11
>> 9 | 20.17 -4.12 -5.17 19.03 | 9.25 -2.24 21.35 3.39 | 7.71
>> 15 | 19.48 -3.83 -4.50 19.01 | 29.26 -10.06 11.76 17.07 | 9.77
>> 16 | 19.04 -3.15 16.41 16.85 | 38.37 -11.12 13.18 27.70 | 14.66
>> 17 | 8.95 -2.40 5.68 6.38 | 16.32 -1.61 7.49 11.44 | 6.53
>> 31 | 28.87 -0.01 19.79 23.37 | 41.43 -7.57 23.85 35.89 | 20.70
>> 32 | 32.58...
>
> Ehsan Behrangi has updated the pull request incrementally with one additional
> commit since the last revision:
>
> 8381560: Relax cnt and result register constraints for SVE String.equals
test/micro/org/openjdk/bench/java/lang/StringEquals.java line 66:
> 64: @Setup
> 65: public void setup() {
> 66: if(size > 0) {
Suggestion:
if (size > 0) {
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/31400#discussion_r3373297778
