tree 5957a21baa0a3a087d908d4a222f41df82fd713a
parent be03d4f470ed7887405ab93f751a8591aef9b6f9
author Katharine Berry <ktbry@google.com> 1559148969 -0700
committer Daniel Martí <mvdan@mvdan.cc> 1571903092 +0000

cover: significantly improve the speed of cover.ParseProfiles

ParseProfiles currently uses a regex to parse each line. This is not
very fast, and can lead to ParseProfiles being excessively slow on
certain pathological inputs.

This change substantially improves the performance by parsing manually
instead. On an input of about 3 GB of data containing about 36 million
lines, the time spent in ParseProfiles drops from 72 seconds to 11
seconds, with actual string parsing time dropping from 61 seconds to 2
seconds.

Since this change completely changes the parsing, it also adds some
tests for ParseProfiles to help ensure the new parsing is correct.

A benchmark for parseLine is also included. Here is a comparison of the old
regex implementation versus the new manual one:

name          old time/op    new time/op      delta
ParseLine-12    2.43µs ± 2%      0.05µs ± 8%    -97.98%  (p=0.000 n=10+9)

name          old speed      new speed        delta
ParseLine-12  42.5MB/s ± 2%  2103.2MB/s ± 7%  +4853.14%  (p=0.000 n=10+9)

Fixes golang/go#32211

Change-Id: If8f91ecbda776c08243de4e423de4eea55f0082b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/179377
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>