strings: Builder copy check causes a dynamic memory allocation #23382
Description
What version of Go are you using (go version)?
go version go1.9.1 darwin/amd64
Does this issue reproduce with the latest release?
N/A
What operating system and processor architecture are you using (go env)?
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/achilleroussel/dev"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/0p/tfjq063j4hdcjnm0_9qb4bww0000gn/T/go-build417403397=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
What did you do?
Regarding 3058d38
The change causes the strings.Builder value to always escape to the heap if any of its methods is called, which means there will be 2 dynamic memory allocations to build a string (one for the byte slice and one for the builder).
The intent of this type was to save on memory allocations caused by the use of bytes.Buffer, now it seems like it's just as inefficient.
Here's a quick benchmark to highlight what I'm talking about:
package main
import "testing"
type A struct{ self *A }
func (a *A) F() { a.self = a }
func BenchmarkStruct(b *testing.B) {
for i := 0; i != b.N; i++ {
a := A{}
a.F()
}
}$ go test -v -run _ -bench . -benchmem
goos: darwin
goarch: amd64
BenchmarkStruct-4 50000000 24.6 ns/op 8 B/op 1 allocs/op