math/big: reduce allocations in writeMultiple()

[pierrre]

Go version

go version go1.23.5 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/pierre/.cache/go-build'
GOENV='/home/pierre/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/pierre/go/pkg/mod'
GONOPROXY='xxx'
GONOSUMDB='xxx'
GOOS='linux'
GOPATH='/home/pierre/go'
GOPRIVATE='xxx'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.5'
GODEBUG=''
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/pierre/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build458815232=/tmp/go-build -gno-record-gcc-switches'

What did you do?

I'm trying to write a big.Int to a io.Writer.
I'm using fmt.FPrintf, and it calls big.Int.Format().

What did you see happen?

I did a benchmark, and noticed that many allocations are done in writeMultiple():

func writeMultiple(s fmt.State, text string, count int) {
	if len(text) > 0 {
		b := []byte(text)
		for ; count > 0; count-- {
			s.Write(b)
		}
	}
}

=> The string to []byte conversion.

What did you expect to see?

I think we can replace

s.Write(b)

with

io.WriteString(w, text)

and remove

b := []byte(text)

because fmt.State implementations implements WriteString.

[gabyhelp]

Related Issues

• io: optimize WriteString with a pool of buffer #28311

Related Code Changes

• fmt: simplify buffer write methods and adjust calls to them

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[cagedmantis]

@griesemer @golang/security

[gopherbot]

Change https://go.dev/cl/645215 mentions this issue: math/big: optimize writeMultiple to use StringWriter and ByteWriter

[Jorropo]

I've submitted a patch and seeing where it is used I've overhauled it:

• return without trying anything if called with a count of 0.
• if writing one byte (which happens often when padding with 0 or ) try ByteWriter
• try StringWriter
• fallback to Writer

Unlike what is suggested here I did not used io.WriteString as it would make it much worst if for some reason someone passed in a fmt.State that does not implement io.WriteString because io.WriteString would allocate and copy []byte for each iteration of count.

[pierrre]

Unlike what is suggested here I did not used io.WriteString as it would make it much worst if for some reason someone passed in a fmt.State that does not implement io.WriteString because io.WriteString would allocate and copy []byte for each iteration of count.

I'm just wondering: are there several implementations of fmt.State ?

[Jorropo]

Idk
This is part of the public API https://pkg.go.dev/math/big#Int.Format anyone is free to plugin their own implementation.
Doing the correct thing is easier than reviewing the open source corpus so I did not tried.