RFR: 8173585: Intrinsify StringLatin1.indexOf(char) [v5]

[Jason Tatton]

> This is an implementation of the indexOf(char) intrinsic for StringLatin1 (1 byte encoded Strings). It is provided for
> x86 and ARM64. The implementation is greatly inspired by the indexOf(char) intrinsic for StringUTF16. To incorporate it
> I had to make a small change to StringLatin1.java (refactor of functionality to intrisified private method) as well as
> code for C2.  Submitted to: hotspot-compiler-dev and core-libs-dev as this patch contains a change to hotspot and
> java/lang/StringLatin1.java  https://bugs.openjdk.java.net/browse/JDK-8173585
> 
> Details of testing:
> ============
> I have created a jtreg test “compiler/intrinsics/string/TestStringLatin1IndexOfChar” to cover this new intrinsic. Note
> that, particularly for the x86 implementation of the intrinsic, the code path taken is dependent upon the length of the
> input String. Hence the test has been designed to cover all these cases. In summary they are:
> - A “short” string of < 16 characters.
> - A SIMD String of 16 – 31 characters.
> - A AVX2 SIMD String of 32 characters+.
> 
> Hardware used for testing:
> -----------------------------
> 
> - Intel Xeon CPU E5-2680 (JVM did not recognize this as having AVX2 support) • Intel i7 processor (with AVX2 support).
> - AWS Graviton 2 (ARM 64 processor).
> 
> I also ran; ‘run-test-tier1’ and ‘run-test-tier2’ for: x86_64 and aarch64.
> 
> Possible future enhancements:
> ====================
> For the x86 implementation there may be two further improvements we can make in order to improve performance of both
> the StringUTF16 and StringLatin1 indexOf(char) intrinsics:
> 1. Make use of AVX-512 instructions.
> 2. For “short” Strings (see below), I think it may be possible to modify the existing algorithm to still use SSE SIMD
> instructions instead of a loop.
> Benchmark results:
> ============
> **Without** the new StringLatin1 indexOf(char) intrinsic:
> 
> | Benchmark  | Mode | Cnt | Score | Error | Units |
> | ------------- | ------------- |------------- |------------- |------------- |------------- |
> | IndexOfBenchmark.latin1_mixed_char | avgt | 5 | **26,389.129** | ± 182.581 | ns/op |
> | IndexOfBenchmark.utf16_mixed_char | avgt | 5 | 17,885.383 | ± 435.933  | ns/op |
> 
> 
> **With** the new StringLatin1 indexOf(char) intrinsic:
> 
> | Benchmark  | Mode | Cnt | Score | Error | Units |
> | ------------- | ------------- |------------- |------------- |------------- |------------- |
> | IndexOfBenchmark.latin1_mixed_char | avgt | 5 | **17,875.185** | ± 407.716  | ns/op |
> | IndexOfBenchmark.utf16_mixed_char | avgt | 5 | 18,292.802 | ± 167.306  | ns/op |
> 
> 
> The objective of the patch is to bring the performance of StringLatin1 indexOf(char) in line with StringUTF16
> indexOf(char) for x86 and ARM64. We can see above that this has been achieved. Similar results were obtained when
> running on ARM.

Jason Tatton has updated the pull request incrementally with one additional commit since the last revision:

  8173585: Intrinsify StringLatin1.indexOf(char)
  
  Added new unit test: findOneItem. This will test strings of varying
  length ensuring that for all lengths one instance of the search char can
  be found. We check what happens when the search character is in each
  position of the search string (including first and last positions).

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/71/files
  - new: https://git.openjdk.java.net/jdk/pull/71/files/c8a2849e..8ead02ab

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=71&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=71&range=03-04

  Stats: 34 lines in 1 file changed: 26 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/71/head:pull/71

PR: https://git.openjdk.java.net/jdk/pull/71