cmd/gc: relax address-of escape analysis

Make the loop nesting depth of &x depend on where x is declared,
not on where the &x appears. The latter is only a conservative
estimate of the former. Being more careful can avoid some
variables escaping, and it is easier to reason about.

It would have avoided issue 7313, although that was still a bug
worth fixing.

Not much effect in the tree: one variable in the whole tree
is saved from a heap allocation (something in x509 parsing).

LGTM=daniel.morsing
R=daniel.morsing
CC=golang-codereviews
https://golang.org/cl/62380043

1 files changed, 10 insertions, 0 deletions

diff --git a/test/escape2.go b/test/escape2.go
index 73342fd2bc..047adf5149 100644
--- a/test/escape2.go
+++ b/test/escape2.go
@@ -1389,3 +1389,13 @@ func foo148(l List) { // ERROR " l does not escape"
 	for p := &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
 	}
 }
+
+// related: address of variable should have depth of variable, not of loop
+
+func foo149(l List) { // ERROR " l does not escape"
+	var p *List
+	for {
+		for p = &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
+		}
+	}
+}