RFR: 8315098: Improve URLEncodeDecode microbenchmark

[Claes Redestad]

The `URLEncodeDecode` microbenchmark accidentally generates strings with a lot of `'\u0000'` chars, heavily skewing towards strings that need to be encoded in a rather unrealistic what. To be more realistic the benchmark should test a mix of inputs.

This patch fixes these inadvertent cases, and sets up the benchmark for a healthier mix by default - adding controls to allow testing some mixed scenarios.

#15354 explore a few optimizations to `URLEncoder`, but due the nature of this microbenchmark a trivial fast-path scan for chars that need no encoding shows underwhelming results. With the modifications to this benchmark then a simple fast-path to `URLEncode.encode` shows a decent win when some or all the inputs remain unchanged:


Name                           (encodeChars) (maxLength) (unchanged) Cnt  Base   Error   Test   Error  Unit  Diff%
URLEncodeDecode.testDecodeUTF8             6        1024           0  15 3,307 ± 0,507  3,010 ± 0,048 ms/op   9,0% (p = 0,030 )
URLEncodeDecode.testDecodeUTF8             6        1024          75  15 2,296 ± 0,003  2,313 ± 0,017 ms/op  -0,7% (p = 0,001*)
URLEncodeDecode.testDecodeUTF8             6        1024         100  15 0,812 ± 0,010  0,819 ± 0,017 ms/op  -0,8% (p = 0,201 )
URLEncodeDecode.testDecodeUTF8            35        1024           0  15 6,909 ± 0,065  7,192 ± 0,415 ms/op  -4,1% (p = 0,014 )
URLEncodeDecode.testDecodeUTF8            35        1024          75  15 3,346 ± 0,206  3,320 ± 0,270 ms/op   0,8% (p = 0,753 )
URLEncodeDecode.testDecodeUTF8            35        1024         100  15 0,794 ± 0,034  0,818 ± 0,015 ms/op  -3,0% (p = 0,016 )
URLEncodeDecode.testEncodeUTF8             6        1024           0  15 2,434 ± 0,019  2,579 ± 0,120 ms/op  -6,0% (p = 0,000*)
URLEncodeDecode.testEncodeUTF8             6        1024          75  15 1,764 ± 0,014  0,937 ± 0,012 ms/op  46,9% (p = 0,000*)
URLEncodeDecode.testEncodeUTF8             6        1024         100  15 1,227 ± 0,008  0,401 ± 0,001 ms/op  67,4% (p = 0,000*)
URLEncodeDecode.testEncodeUTF8            35        1024           0  15 6,177 ± 0,062  6,057 ± 0,199 ms/op   1,9% (p = 0,029 )
URLEncodeDecode.testEncodeUTF8            35        1024          75  15 2,716 ± 0,023  1,876 ± 0,012 ms/op  30,9% (p = 0,000*)
URLEncodeDecode.testEncodeUTF8            35        1024         100  15 1,220 ± 0,003  0,401 ± 0,001 ms/op  67,2% (p = 0,000*)


A potential future improvement would be to extend test data with varying amounts of surrogate pairs, e.g. Chinese or emojis

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Commit messages:
 - 8315098: Improve URLEncodeDecode microbenchmark

Changes: https://git.openjdk.org/jdk/pull/15448/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15448&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315098
  Stats: 48 lines in 1 file changed: 23 ins; 2 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/15448.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15448/head:pull/15448

PR: https://git.openjdk.org/jdk/pull/15448