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, 4 insertions, 4 deletions

diff --git a/test/live_syscall.go b/test/live_syscall.go
index b7b85bcabf..7b44717350 100644
--- a/test/live_syscall.go
+++ b/test/live_syscall.go
@@ -17,23 +17,23 @@ import (
 
 func f(uintptr) // ERROR "f assuming arg#1 is unsafe uintptr"
 
-func g() {
+func g() { // ERROR "can inline g"
 	var t int
 	f(uintptr(unsafe.Pointer(&t))) // ERROR "live at call to f: .?autotmp" "g &t does not escape" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
-func h() {
+func h() { // ERROR "can inline h"
 	var v int
 	syscall.Syscall(0, 1, uintptr(unsafe.Pointer(&v)), 2) // ERROR "live at call to Syscall: .?autotmp" "h &v does not escape" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
-func i() {
+func i() { // ERROR "can inline i"
 	var t int
 	p := unsafe.Pointer(&t) // ERROR "i &t does not escape"
 	f(uintptr(p))           // ERROR "live at call to f: .?autotmp" "stack object .autotmp_[0-9]+ unsafe.Pointer$"
 }
 
-func j() {
+func j() { // ERROR "can inline j"
 	var v int
 	p := unsafe.Pointer(&v)              // ERROR "j &v does not escape"
 	syscall.Syscall(0, 1, uintptr(p), 2) // ERROR "live at call to Syscall: .?autotmp" "stack object .autotmp_[0-9]+ unsafe.Pointer$"