RFR: 8362893: Improve performance for MemorySegment::getString

[simon]

On Sun, 27 Jul 2025 12:36:20 GMT, Philippe Marschall <duke at openjdk.org> wrote:

> Use `JavaLangAccess::uncheckedNewStringNoRepl` in `MemorySegment::getString` to avoid byte[] allocation in the String constructor.
> 
> Fall back to the old code in the case of malformed input to get replacement characters as per Javadoc API specification. The existing tests in `TestStringEncoding` seem sufficient to me.
> 
> I ran the tier1 test suite and it passes.
> 
> For performance analysis I ran.
> 
>     make test TEST="micro:org.openjdk.bench.java.lang.foreign.ToJavaStringTest" MICRO="OPTIONS=-prof gc"
> 
> on an AMD Ryzen 7 PRO 5750GE.
> 
> These are the formatted results, the current master is the line on top, this feature branch is the line below. We can see an improvement in throughput driven by a reduction in allocation.
> 
> 
> Benchmark                                              (size)  Mode  Cnt     Score    Error   Units
> 
> ToJavaStringTest.panama_readString                          5  avgt   30    18.996 ±  0.044   ns/op
> ToJavaStringTest.panama_readString                          5  avgt   30    13.851 ±  0.028   ns/op
> 
> ToJavaStringTest.panama_readString                         20  avgt   30    23.570 ±  0.050   ns/op
> ToJavaStringTest.panama_readString                         20  avgt   30    18.401 ±  0.069   ns/op
> 
> ToJavaStringTest.panama_readString                        100  avgt   30    32.094 ±  0.207   ns/op
> ToJavaStringTest.panama_readString                        100  avgt   30    24.427 ±  0.112   ns/op
> 
> ToJavaStringTest.panama_readString                        200  avgt   30    43.029 ±  0.185   ns/op
> ToJavaStringTest.panama_readString                        200  avgt   30    31.914 ±  0.064   ns/op
> 
> ToJavaStringTest.panama_readString                        451  avgt   30    81.145 ±  0.403   ns/op
> ToJavaStringTest.panama_readString                        451  avgt   30    58.975 ±  0.233   ns/op
> 
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm       5  avgt   30    72.000 ±  0.001    B/op
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm       5  avgt   30    48.000 ±  0.001    B/op
> 
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm      20  avgt   30   104.000 ±  0.001    B/op
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm      20  avgt   30    64.000 ±  0.001    B/op
> 
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm     100  avgt   30   264.000 ±  0.001    B/op
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm     100  avgt   30   144.000 ±  0.001    B/op
> 
> ToJavaStringTest.panama_readString:gc.alloc.rate.norm     200  avgt ...

src/java.base/share/classes/jdk/internal/foreign/StringSupport.java line 33:

> 31: import jdk.internal.util.Architecture;
> 32: import jdk.internal.util.ArraysSupport;
> 33: import jdk.internal.vm.annotation.ForceInline;

Nitpick: update copyright year 😄

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PR Review Comment: https://git.openjdk.org/jdk/pull/26493#discussion_r2234248805