RFR: 8335182: Consolidate and streamline String concat code shapes

[Chen Liang]

On Thu, 27 Jun 2024 12:27:26 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> This PR attains a speed-up on some microbenchmarks by simplifying how we build up the MH combinator tree shape
> (only use prependers with prefix, always add a suffix to the newArray combinator), then simplifying/inlining some of the code in the helper functions. 
> 
> Many simple concatenation expressions stay performance neutral, while the win comes from enabling C2 to better optimize more complex shapes (concat13String, concatMix4String, concatConst6String see relatively large improvements):
> 
> 
> Name                                    Cnt     Base     Error      Test     Error  Unit  Change
> StringConcat.concat13String              50   66.380 ±   0.189    53.017 ±   0.241 ns/op   1.25x (p = 0.000*)
> StringConcat.concat4String               50   13.620 ±   0.053    12.382 ±   0.089 ns/op   1.10x (p = 0.000*)
> StringConcat.concat6String               50   17.168 ±   0.070    16.046 ±   0.064 ns/op   1.07x (p = 0.000*)
> StringConcat.concatConst2String          50    9.820 ±   0.081     8.158 ±   0.041 ns/op   1.20x (p = 0.000*)
> StringConcat.concatConst4String          50   14.938 ±   0.124    12.800 ±   0.049 ns/op   1.17x (p = 0.000*)
> StringConcat.concatConst6Object          50   56.115 ±   0.288    54.046 ±   0.214 ns/op   1.04x (p = 0.000*)
> StringConcat.concatConst6String          50   19.032 ±   0.073    16.213 ±   0.093 ns/op   1.17x (p = 0.000*)
> StringConcat.concatConstBoolByte         50    8.486 ±   0.066     8.556 ±   0.050 ns/op   0.99x (p = 0.004*)
> StringConcat.concatConstInt              50    8.942 ±   0.052     7.677 ±   0.029 ns/op   1.16x (p = 0.000*)
> StringConcat.concatConstIntConstInt      50   12.883 ±   0.070    12.431 ±   0.070 ns/op   1.04x (p = 0.000*)
> StringConcat.concatConstString           50    7.523 ±   0.050     7.486 ±   0.044 ns/op   1.00x (p = 0.058 )
> StringConcat.concatConstStringConstInt   50   11.961 ±   0.032    11.699 ±   0.049 ns/op   1.02x (p = 0.000*)
> StringConcat.concatEmptyConstInt         50    7.778 ±   0.038     7.723 ±   0.037 ns/op   1.01x (p = 0.000*)
> StringConcat.concatEmptyConstString      50    3.506 ±   0.026     3.505 ±   0.015 ns/op   1.00x (p = 0.930 )
> StringConcat.concatEmptyLeft             50    3.573 ±   0.075     3.518 ±   0.057 ns/op   1.02x (p = 0.044 )
> StringConcat.concatEmptyRight            50    3.713 ±   0.049     3.622 ±   0.053 ns/op   1.02x (p = 0.000*)
> StringConcat.concatMethodConstString     50    7.418 ±   0.030     7.478 ±   0.066 ns/op   0.99x (p = 0.005*)
> StringConcat.concatMix4String        ...

src/java.base/share/classes/java/lang/StringConcatHelper.java line 157:

> 155:             }
> 156:             index -= prefix.length();
> 157:             prefix.getBytes(buf, index, String.LATIN1);

Since we are now passing in a lot of empty prefix, I wonder how this call is elided; is there some specific JIT intrinsic? The java code has no shortcut for `length == 0`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19927#discussion_r1657409710