RFR: 8263038: Optimize String.format for simple specifiers

[Claes Redestad]

On Thu, 4 Mar 2021 17:20:40 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> This patch optimizes String.format expressions that uses trivial specifiers. In the JDK, the most common variation of String.format is a variation of format("foo: %s", s), which gets a significant speed-up from this.
> 
> Various other cleanups and minor improvements reduce overhead further and ensure we get a small gain also for more complex format strings.

Microbench results - baseline:
Benchmark                                                           Mode  Cnt     Score      Error   Units
StringFormat.complexFormat                                          avgt    5  8842.917 ±  658.269   ns/op
StringFormat.complexFormat:·gc.alloc.rate.norm                      avgt    5  2176.378 ±    0.483    B/op
StringFormat.stringFormat                                           avgt    5   859.863 ±   97.514   ns/op
StringFormat.stringFormat:·gc.alloc.rate.norm                       avgt    5   560.091 ±    0.011    B/op
StringFormat.stringIntFormat                                        avgt    5  1619.772 ±  147.646   ns/op
StringFormat.stringIntFormat:·gc.alloc.rate.norm                    avgt    5   728.132 ±    0.140    B/op
StringFormat.widthStringFormat                                      avgt    5  1060.200 ±  154.025   ns/op
StringFormat.widthStringFormat:·gc.alloc.rate.norm                  avgt    5   592.108 ±    0.093    B/op
StringFormat.widthStringIntFormat                                   avgt    5  2045.215 ±  246.189   ns/op
StringFormat.widthStringIntFormat:·gc.alloc.rate.norm               avgt    5   784.144 ±    0.121    B/op

Patched:
Benchmark                                                           Mode  Cnt     Score      Error   Units
StringFormat.complexFormat                                          avgt    5  8023.314 ± 1387.475   ns/op
StringFormat.complexFormat:·gc.alloc.rate.norm                      avgt    5  2120.399 ±    0.417    B/op
StringFormat.stringFormat                                           avgt    5   286.776 ±   30.645   ns/op
StringFormat.stringFormat:·gc.alloc.rate.norm                       avgt    5   256.044 ±    0.017    B/op
StringFormat.stringIntFormat                                        avgt    5   626.083 ±   68.652   ns/op
StringFormat.stringIntFormat:·gc.alloc.rate.norm                    avgt    5   432.073 ±    0.039    B/op
StringFormat.widthStringFormat                                      avgt    5  1061.631 ±  156.444   ns/op
StringFormat.widthStringFormat:·gc.alloc.rate.norm                  avgt    5   560.103 ±    0.106    B/op
StringFormat.widthStringIntFormat                                   avgt    5  1380.208 ±  267.445   ns/op
StringFormat.widthStringIntFormat:·gc.alloc.rate.norm               avgt    5   736.134 ±    0.144    B/op

-Xint similarly sees no change on complexString, but a 3-3.5x speed-up on stringFormat

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PR: https://git.openjdk.java.net/jdk/pull/2830