cmd/compile: encourage inlining of functions with single-call bodies

This is a simple tweak to allow a bit more mid-stack inlining.
In cases like this:

func f() {
    g()
}

We'd really like to inline f into its callers. It can't hurt.

We implement this optimization by making calls a bit cheaper, enough
to afford a single call in the function body, but not 2.
The remaining budget allows for some argument modification, or perhaps
a wrapping conditional:

func f(x int) {
    g(x, 0)
}
func f(x int) {
    if x > 0 {
        g()
    }
}

Update #19348

Change-Id: Ifb1ea0dd1db216c3fd5c453c31c3355561fe406f
Reviewed-on: https://go-review.googlesource.com/c/147361
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/test/closure3.dir/main.go b/test/closure3.dir/main.go
index 59c36e3218..ae4bef79a6 100644
--- a/test/closure3.dir/main.go
+++ b/test/closure3.dir/main.go
@@ -238,6 +238,8 @@ func main() {
 				if c != 4 {
 					ppanic("c != 4")
 				}
+				for i := 0; i < 10; i++ { // prevent inlining
+				}
 			}()
 		}()
 		if c != 4 {