strconv: ParseInt, ParseFloat use two loops instead of one #33330
Description
What version of Go are you using (go version)?
$ go version go version devel +919594830f Tue Jul 23 23:42:43 2019 +0000 darwin/amd64
Does this issue reproduce with the latest release?
Yes, see version
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/Users/sarnold64/Library/Caches/go-build" GOENV="/Users/sarnold64/Library/Application Support/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GONOPROXY="" GONOSUMDB="" GOOS="darwin" GOPATH="/Users/sarnold64/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/Users/sarnold64/golang/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/Users/sarnold64/golang/go/pkg/tool/darwin_amd64" GCCGO="gccgo" AR="ar" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD="/Users/sarnold64/golang/go/src/go.mod" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/5s/zkkc95rs08731f9t6df3fvsc0000gr/T/go-build178433439=/tmp/go-build -gno-record-gcc-switches -fno-common"
What did you do?
I noticed that strconv takes more time than it needs to for ParseInt and ParseFloat because it loops over the input twice, once in underscoreOK and once to do the parsing.
What did you expect to see?
One loop
What did you see instead?
Two loops
I was playing around with pprof after the gophercon talks and saw this looking suspicious.
Some performance results on my laptop after fixing:
name old time/op new time/op delta
ParseInt/Pos/7bit-12 19.0ns ± 3% 12.1ns ± 0% -36.58% (p=0.000 n=6+5)
ParseInt/Pos/26bit-12 28.5ns ± 0% 18.4ns ± 2% -35.58% (p=0.002 n=6+6)
ParseInt/Pos/31bit-12 32.4ns ± 1% 20.9ns ± 1% -35.71% (p=0.002 n=6+6)
ParseInt/Pos/56bit-12 47.0ns ± 2% 29.3ns ± 1% -37.70% (p=0.002 n=6+6)
ParseInt/Pos/63bit-12 50.9ns ± 2% 32.0ns ± 1% -37.24% (p=0.002 n=6+6)
ParseInt/Neg/7bit-12 18.3ns ± 6% 12.2ns ± 1% -33.18% (p=0.002 n=6+6)
ParseInt/Neg/26bit-12 28.2ns ± 3% 18.2ns ± 2% -35.38% (p=0.002 n=6+6)
ParseInt/Neg/31bit-12 31.4ns ± 1% 20.7ns ± 1% -34.04% (p=0.004 n=6+5)
ParseInt/Neg/56bit-12 45.4ns ± 0% 29.4ns ± 1% -35.17% (p=0.004 n=5+6)
ParseInt/Neg/63bit-12 49.4ns ± 1% 32.0ns ± 1% -35.30% (p=0.002 n=6+6)
name old time/op new time/op delta
Atof64Decimal-12 30.1ns ± 1% 25.4ns ± 1% -15.75% (p=0.002 n=6+6)
Atof64Float-12 38.3ns ± 1% 31.5ns ± 0% -17.84% (p=0.000 n=6+5)
Atof64FloatExp-12 63.1ns ± 0% 53.9ns ± 0% -14.53% (p=0.002 n=6+6)
Atof64Big-12 113ns ± 0% 96ns ± 2% -14.85% (p=0.010 n=4+6)
Atof64RandomBits-12 180ns ±10% 160ns ±16% -11.36% (p=0.028 n=6+6)
Atof64RandomFloats-12 123ns ± 2% 102ns ± 1% -17.03% (p=0.002 n=6+6)
Atof32Decimal-12 31.4ns ± 0% 25.4ns ± 1% -18.86% (p=0.002 n=6+6)
Atof32Float-12 36.5ns ± 0% 29.4ns ± 0% -19.40% (p=0.008 n=5+5)
Atof32FloatExp-12 67.0ns ± 0% 56.6ns ± 0% -15.58% (p=0.008 n=5+5)
Atof32Random-12 90.6ns ± 1% 75.1ns ± 0% -17.15% (p=0.004 n=6+5)