strings: smarter growth of temporal buffer and avoid copying on return

The implementation for single strings had two optimization opportunities:
1. Grow the temporary buffer by known size before appending.
2. Avoid a full copy of the result since the underlying buffer won't be mutated afterward.
Both things were leveraged by using a Builder instead of a byte slice.

Relevant benchmark results:

        name           old time/op    new time/op    delta
        SingleMatch-8    32.0µs ± 3%    26.1µs ± 3%  -18.41%  (p=0.000 n=9+10)

        name           old speed      new speed      delta
        SingleMatch-8   469MB/s ± 3%   574MB/s ± 3%  +22.56%  (p=0.000 n=9+10)

        name           old alloc/op   new alloc/op   delta
        SingleMatch-8    81.3kB ± 0%    49.0kB ± 0%  -39.67%  (p=0.000 n=10+10)

        name           old allocs/op  new allocs/op  delta
        SingleMatch-8      19.0 ± 0%      11.0 ± 0%  -42.11%  (p=0.000 n=10+10)

Change-Id: I23af56a15875206c0ff4ce29a51bec95fd48bb11
GitHub-Last-Rev: 403cfc3c2794b5da27792c51999417a2a052b365
GitHub-Pull-Request: golang/go#47766
Reviewed-on: https://go-review.googlesource.com/c/go/+/343089
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Robert Griesemer <gri@golang.org>
1 file changed
tree: 370a1847d84bd36d596af360ebdd37a3af6cba58
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. codereview.cfg
  12. CONTRIBUTING.md
  13. CONTRIBUTORS
  14. LICENSE
  15. PATENTS
  16. README.md
  17. SECURITY.md
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