cmd/gc: add way to specify 'noescape' for extern funcs

A new comment directive //go:noescape instructs the compiler
that the following external (no body) func declaration should be
treated as if none of its arguments escape to the heap.

Fixes #4099.

R=golang-dev, dave, minux.ma, daniel.morsing, remyoudompheng, adg, agl, iant
CC=golang-dev
https://golang.org/cl/7289048

1 files changed, 26 insertions, 0 deletions

diff --git a/test/escape2.go b/test/escape2.go
index 8e3aa4de74..9481619338 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1274,3 +1274,29 @@ func foo140() interface{} {
 		T: t,
 	}
 }
+
+//go:noescape
+
+func F1([]byte)
+
+func F2([]byte)
+
+//go:noescape
+
+func F3(x []byte) // ERROR "F3 x does not escape"
+
+func F4(x []byte)
+
+func G() {
+	var buf1 [10]byte
+	F1(buf1[:]) // ERROR "buf1 does not escape"
+	
+	var buf2 [10]byte // ERROR "moved to heap: buf2"
+	F2(buf2[:]) // ERROR "buf2 escapes to heap"
+
+	var buf3 [10]byte
+	F3(buf3[:]) // ERROR "buf3 does not escape"
+	
+	var buf4 [10]byte // ERROR "moved to heap: buf4"
+	F4(buf4[:]) // ERROR "buf4 escapes to heap"
+}