compress/flate: add Reset() to allow reusing large buffers to compress multiple buffers
This adds a Reset() to compress/flate's decompressor and plumbs that through
to compress/zlib and compress/gzip's Readers so callers can avoid large
allocations when performing many inflate operations. In particular this
preserves the allocation of the decompressor.hist buffer, which is 32kb and
overwritten as needed while inflating.
On the benchmark described in issue 6317, produces the following speedup on
my 2.3ghz Intel Core i7 MBP with go version devel +
6b696a34e0af Sun Aug 03
15:14:59 2014 -0700 darwin/amd64:
blocked.text w/out patch vs blocked.text w/ patch:
benchmark old ns/op new ns/op delta
BenchmarkGunzip
8371577533 7927917687 -5.30%
benchmark old allocs new allocs delta
BenchmarkGunzip 176818 148519 -16.00%
benchmark old bytes new bytes delta
BenchmarkGunzip
292184936 12739528 -95.64%
flat.text vs blocked.text w/patch:
benchmark old ns/op new ns/op delta
BenchmarkGunzip
7939447827 7927917687 -0.15%
benchmark old allocs new allocs delta
BenchmarkGunzip 90702 148519 +63.74%
benchmark old bytes new bytes delta
BenchmarkGunzip
9959528 12739528 +27.91%
Similar speedups to those bradfitz saw in https://golang.org/cl/
13416045.
Fixes #6317.
Fixes #7950.
LGTM=nigeltao
R=golang-codereviews, bradfitz, dan.kortschak, adg, nigeltao, jamesr
CC=golang-codereviews
https://golang.org/cl/
97140043