Optimize StringSequence.startsWith

[dreis2211]

Hi,

I noticed that a vanilla Boot app spends some time in StringSequence.startsWith or respectively StringSequence.charAt:

This PR optimizes startsWith in cases where the length doesn't differ from the source length, which essentially means that StringSequence is "equal" to the source. In those cases, we can use startsWith on the actual source and by that avoid the repeated boundary checks inside charAt. A little benchmark of old vs. new approach shows the following results.

Benchmark                                Mode  Cnt   Score    Error   Units
MyBenchmark.testNew                      avgt   10   6,270 ±  0,255   ns/op
MyBenchmark.testNew:·gc.alloc.rate       avgt   10  ≈ 10⁻⁴           MB/sec
MyBenchmark.testNew:·gc.alloc.rate.norm  avgt   10  ≈ 10⁻⁶             B/op
MyBenchmark.testNew:·gc.count            avgt   10     ≈ 0           counts
MyBenchmark.testOld                      avgt   10   9,029 ±  0,330   ns/op
MyBenchmark.testOld:·gc.alloc.rate       avgt   10  ≈ 10⁻⁴           MB/sec
MyBenchmark.testOld:·gc.alloc.rate.norm  avgt   10  ≈ 10⁻⁶             B/op
MyBenchmark.testOld:·gc.count            avgt   10     ≈ 0           counts

Let me know what you think.
Cheers,
Christoph

[philwebb]

Nice!

[philwebb]

Moving the parameter type to String opens the door for even more optimizations I think. @dreis2211 and @wilkinsona I'd appreciate it if you could look at commit 4a8492d and see if it makes sense.

[philwebb]

Reopening to ensure we get at least one other pair of eyes on this before we release.

[dreis2211]

Looks good to me. In reality it won't change much though, because there are actually no other cases in the code where the length differs from the original string (apart from tests). So it's a good polishing commit, but no real performance improvement anymore ;)

[wilkinsona]

Thanks, @dreis2211. FWIW, it looks good to me too.

[philwebb]

Thanks for the reviews!