cmd/compile: Go 1.19 might make generic types slower

[kscooo]

What version of Go are you using (go version)?

$ go version
go version go1.19 linux/amd64

Does this issue reproduce with the latest release?

It reproduces on the 1.19

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ksco/.cache/go-build"
GOENV="/home/ksco/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/ksco/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/ksco/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
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 -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3292360144=/tmp/go-build -gno-record-gcc-switches"

What did you do?

playground: https://go.dev/play/p/6UxOn6pftVs

main_test.go

package main

import (
	"testing"
)

func addInt(a, b int) int {
	return a + b
}

func addString(a, b string) string {
	return a + b
}

func BenchmarkWithoutGenerics(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = addInt(1, 2)
		_ = addString("foo", "bar")
	}
}

type Addable interface {
	int | string
}

func add[T Addable](a, b T) T {
	return a + b
}

func BenchmarkWithGenerics(b *testing.B) {
	for i := 0; i < b.N; i++ {
		_ = add(1, 2)
		_ = add("foo", "bar")
	}
}

What did you expect to see?

1.19 should be similar to the 1.18 benchmark test, not much slower.

What did you see instead?

goversion: 1.19

goos: linux
goarch: amd64
pkg: generics/ch13
cpu: AMD Ryzen 7 5800H with Radeon Graphics         
BenchmarkWithoutGenerics
BenchmarkWithoutGenerics-16    	71445846	        16.49 ns/op
BenchmarkWithGenerics
BenchmarkWithGenerics-16       	32776173	        36.30 ns/op
PASS

goversion: 1.18

goos: linux
goarch: amd64
pkg: generics/ch13
cpu: AMD Ryzen 7 5800H with Radeon Graphics         
BenchmarkWithoutGenerics
BenchmarkWithoutGenerics-16    	64931702	        16.91 ns/op
BenchmarkWithGenerics
BenchmarkWithGenerics-16       	70497928	        17.01 ns/op
PASS

The assembly generated by 1.19 is found to have more

main..dict.add[int](SB), AX
main..dict.add[string](SB), AX

These lines are directly inlined in 1.18 (there may be a mistake in this expression, forgive me for not being familiar with the principles of assembly).

/cc @ianlancetaylor

[OneOfOne]

does export GOEXPERIMENT=unified make it better?

[randall77]

This was caused by https://go-review.googlesource.com/c/go/+/395854 . It was a deliberate change to avoid inlining in cases where we were running into bugs.
I think we will leave this as is for 1.19. unified IR should fix this for 1.20.

[mknyszek]

Since unified IR is expected to fix this, I'm going to assign this @mdempsky for visibility, but feel free to unassign.