cmd/compile: inlining budget excludes cross-package calls

[mdempsky]

In the example below, p.G and q.H are identical, except that p.G is in the same package as p.F, yet cmd/compile decides that q.H is inlineable whereas p.G is not.

This is because cmd/compile currently only computes Inlcost for locally defined functions. Calls to functions in external packages are effectively treated as free.

$ go tool compile -m p.go
p.go:3:6: can inline F
p.go:13:3: inlining call to F

$ go tool compile -m q.go
q.go:5:6: can inline H
q.go:6:5: inlining call to p.F

$ cat p.go
package p

func F() {
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
}

func G() {
    F()
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
}

$ cat q.go
package q

import "./p"

func H() {
    p.F()
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
}

[ALTree]

This is #19261, see josh comment: #19261 (comment)

[mdempsky]

@ALTree Indeed, thanks! Closing as duplicate.