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, 18 insertions, 0 deletions

diff --git a/test/inline.go b/test/inline.go
index 2553230462..9428c1487b 100644
--- a/test/inline.go
+++ b/test/inline.go
@@ -11,6 +11,7 @@ package foo
 
 import (
 	"errors"
+	"runtime"
 	"unsafe"
 )
 
@@ -162,3 +163,20 @@ func k() (T, int, int) { return T{}, 0, 0 } // ERROR "can inline k"
 func _() { // ERROR "can inline _"
 	T.meth(k()) // ERROR "inlining call to k" "inlining call to T.meth"
 }
+
+func small1() { // ERROR "can inline small1"
+	runtime.GC()
+}
+func small2() int { // ERROR "can inline small2"
+	return runtime.GOMAXPROCS(0)
+}
+func small3(t T) { // ERROR "can inline small3"
+	t.meth2(3, 5)
+}
+func small4(t T) { // not inlineable - has 2 calls.
+	t.meth2(runtime.GOMAXPROCS(0), 5)
+}
+func (T) meth2(int, int) { // not inlineable - has 2 calls.
+	runtime.GC()
+	runtime.GC()
+}