On Mon, 13 Dec 2021 09:39:55 GMT, Сергей Цыпанов <d...@openjdk.java.net> wrote:
> Originally this was spotted by by Amir Hadadi in > https://stackoverflow.com/questions/70272651/missing-bounds-checking-elimination-in-string-constructor > > It looks like in the following code in `String(byte[], int, int, Charset)` > > while (offset < sl) { > int b1 = bytes[offset]; > if (b1 >= 0) { > dst[dp++] = (byte)b1; > offset++; // <--- > continue; > } > if ((b1 == (byte)0xc2 || b1 == (byte)0xc3) && > offset + 1 < sl) { > int b2 = bytes[offset + 1]; > if (!isNotContinuation(b2)) { > dst[dp++] = (byte)decode2(b1, b2); > offset += 2; > continue; > } > } > // anything not a latin1, including the repl > // we have to go with the utf16 > break; > } > > bounds check elimination is not executed when accessing byte array via > `bytes[offset]. > > The reason, I guess, is that offset variable is modified within the loop > (marked with arrow). > > Possible fix for this could be changing: > > `while (offset < sl)` ---> `while (offset >= 0 && offset < sl)` > > However the best is to invest in C2 optimization to handle all such cases. > > The following benchmark demonstrates good improvement: > > @State(Scope.Thread) > @BenchmarkMode(Mode.AverageTime) > @OutputTimeUnit(TimeUnit.NANOSECONDS) > public class StringConstructorBenchmark { > private byte[] array; > private String str; > > @Setup > public void setup() { > str = "Quizdeltagerne spiste jordbær med fløde, mens cirkusklovnen. > Я";//Latin1 ending with Russian > array = str.getBytes(StandardCharsets.UTF_8); > } > > @Benchmark > public String newString() { > return new String(array, 0, array.length, StandardCharsets.UTF_8); > } > > @Benchmark > public String translateEscapes() { > return str.translateEscapes(); > } > } > > Results: > > //baseline > Benchmark Mode Cnt Score Error Units > StringConstructorBenchmark.newString avgt 50 173,092 ± 3,048 ns/op > > //patched > Benchmark Mode Cnt Score Error Units > StringConstructorBenchmark.newString avgt 50 126,908 ± 2,355 ns/op > > The same is observed in String.translateEscapes() for the same String as in > the benchmark above: > > //baseline > Benchmark Mode Cnt Score Error Units > StringConstructorBenchmark.translateEscapes avgt 100 53,627 ± 0,850 ns/op > > //patched > Benchmark Mode Cnt Score Error Units > StringConstructorBenchmark.translateEscapes avgt 100 48,087 ± 1,129 ns/op > > Also I've looked into this with `LinuxPerfAsmProfiler`, full output for > baseline is available here > https://gist.github.com/stsypanov/d2524f98477d633fb1d4a2510fedeea6 and for > patched code here > https://gist.github.com/stsypanov/16c787e4f9fa3dd122522f16331b68b7. > > Here's the part of baseline assembly responsible for `while` loop: > > 3.62% ││ │ 0x00007fed70eb4c1c: mov %ebx,%ecx > 2.29% ││ │ 0x00007fed70eb4c1e: mov %edx,%r9d > 2.22% ││ │ 0x00007fed70eb4c21: mov (%rsp),%r8 > ;*iload_2 {reexecute=0 rethrow=0 return_oop=0} > ││ │ > ; - java.lang.String::<init>@107 (line 537) > 2.32% ↘│ │ 0x00007fed70eb4c25: cmp %r13d,%ecx > │ │ 0x00007fed70eb4c28: jge > 0x00007fed70eb5388 ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0} > │ │ > ; - java.lang.String::<init>@110 (line 537) > 3.05% │ │ 0x00007fed70eb4c2e: cmp 0x8(%rsp),%ecx > │ │ 0x00007fed70eb4c32: jae > 0x00007fed70eb5319 > 2.38% │ │ 0x00007fed70eb4c38: mov %r8,(%rsp) > 2.64% │ │ 0x00007fed70eb4c3c: movslq %ecx,%r8 > 2.46% │ │ 0x00007fed70eb4c3f: mov %rax,%rbx > 3.44% │ │ 0x00007fed70eb4c42: sub %r8,%rbx > 2.62% │ │ 0x00007fed70eb4c45: add $0x1,%rbx > 2.64% │ │ 0x00007fed70eb4c49: and > $0xfffffffffffffffe,%rbx > 2.30% │ │ 0x00007fed70eb4c4d: mov %ebx,%r8d > 3.08% │ │ 0x00007fed70eb4c50: add %ecx,%r8d > 2.55% │ │ 0x00007fed70eb4c53: movslq %r8d,%r8 > 2.45% │ │ 0x00007fed70eb4c56: add > $0xfffffffffffffffe,%r8 > 2.13% │ │ 0x00007fed70eb4c5a: cmp (%rsp),%r8 > │ │ 0x00007fed70eb4c5e: jae > 0x00007fed70eb5319 > 3.36% │ │ 0x00007fed70eb4c64: mov %ecx,%edi > ;*aload_1 {reexecute=0 rethrow=0 return_oop=0} > │ │ > ; - java.lang.String::<init>@113 (line 538) > 2.86% │ ↗│ 0x00007fed70eb4c66: movsbl > 0x10(%r14,%rdi,1),%r8d ;*baload {reexecute=0 rethrow=0 return_oop=0} > │ ││ > ; - java.lang.String::<init>@115 (line 538) > 2.48% │ ││ 0x00007fed70eb4c6c: mov %r9d,%edx > 2.26% │ ││ 0x00007fed70eb4c6f: inc %edx > ;*iinc {reexecute=0 rethrow=0 return_oop=0} > │ ││ > ; - java.lang.String::<init>@127 (line 540) > 3.28% │ ││ 0x00007fed70eb4c71: mov %edi,%ebx > 2.44% │ ││ 0x00007fed70eb4c73: inc %ebx > ;*iinc {reexecute=0 rethrow=0 return_oop=0} > │ ││ > ; - java.lang.String::<init>@134 (line 541) > 2.35% │ ││ 0x00007fed70eb4c75: test %r8d,%r8d > ╰ ││ 0x00007fed70eb4c78: jge > 0x00007fed70eb4c04 ;*iflt {reexecute=0 rethrow=0 return_oop=0} > ││ > ; - java.lang.String::<init>@120 (line 539) > > and this one is for patched code: > > 17.28% ││ 0x00007f6b88eb6061: mov %edx,%r10d > ;*iload_2 {reexecute=0 rethrow=0 return_oop=0} > ││ > ; - java.lang.String::<init>@107 (line 537) > 0.11% ↘│ 0x00007f6b88eb6064: test %r10d,%r10d > │ 0x00007f6b88eb6067: jl 0x00007f6b88eb669c > ;*iflt {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@108 (line 537) > 0.39% │ 0x00007f6b88eb606d: cmp %r13d,%r10d > │ 0x00007f6b88eb6070: jge 0x00007f6b88eb66d0 > ;*if_icmpge {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@114 (line 537) > 0.66% │ 0x00007f6b88eb6076: mov %ebx,%r9d > 13.70% │ 0x00007f6b88eb6079: cmp 0x8(%rsp),%r10d > 0.01% │ 0x00007f6b88eb607e: jae 0x00007f6b88eb6671 > 0.14% │ 0x00007f6b88eb6084: movsbl 0x10(%r14,%r10,1),%edi > ;*baload {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@119 (line 538) > 0.37% │ 0x00007f6b88eb608a: mov %r9d,%ebx > 0.99% │ 0x00007f6b88eb608d: inc %ebx > ;*iinc {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@131 (line 540) > 12.88% │ 0x00007f6b88eb608f: movslq %r9d,%rsi > ;*bastore {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@196 (line 548) > 0.17% │ 0x00007f6b88eb6092: mov %r10d,%edx > 0.39% │ 0x00007f6b88eb6095: inc %edx > ;*iinc {reexecute=0 rethrow=0 return_oop=0} > │ > ; - java.lang.String::<init>@138 (line 541) > 0.96% │ 0x00007f6b88eb6097: test %edi,%edi > 0.02% │ 0x00007f6b88eb6099: jl 0x00007f6b88eb60dc > ;*iflt {reexecute=0 rethrow=0 return_oop=0} > > Between instructions mapped to `if_icmpge` and `aload_1` in baseline we have > bounds check which is missing from patched code. As soon as the proper fix is provided in https://github.com/openjdk/jdk/pull/7034 I close this PR. ------------- PR: https://git.openjdk.java.net/jdk/pull/6812
