cmd/compile: low function inline cost

[subtle-byte]

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?

Yes

What did you do?

I write the following code:

package main
import "encoding/binary"
func main() {
}

func A(b []byte) uint64 {
	return binary.LittleEndian.Uint64(b)
}

// Copy-paste of binary.LittleEndian.Uint64
func Uint64(b []byte) uint64 {
	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
}

func B(b []byte) uint64 {
	return Uint64(b)
}

Then I run the command go build -gcflags -m=2 . and I see:

can inline A with cost 5 as: func([]byte) uint64 { return binary.littleEndian.Uint64(binary.LittleEndian, b) }
inlining call to binary.littleEndian.Uint64
can inline B with cost 63 as: func([]byte) uint64 { return Uint64(b) }
inlining call to Uint64

So A calls Uint64 from another module and B calls Uint64 from the same module, and A and B have different costs.

What did you expect to see?

I expected to see the same cost no matter where the called function is located.

[subtle-byte]

It is the effect of #42958. encoding/binary.LittleEndian.Uint64 is considered cheap for inline.