cmd/gc: apply escape analysis results to closures.

This avoids an allocation when closures are used
as "macros", in Walk idioms, or as argument to defer.

benchmark                old ns/op    new ns/op    delta
BenchmarkSearchWrappers       1171          354  -69.77%
BenchmarkCallClosure             3            3  -12.54%
BenchmarkCallClosure1          119            7  -93.95%
BenchmarkCallClosure2          183           74  -59.18%
BenchmarkCallClosure3          187           75  -59.57%
BenchmarkCallClosure4          187           76  -58.98%

Compared to Go 1:
benchmark                  old ns/op    new ns/op    delta
BenchmarkSearchWrappers         3208          354  -88.97%

Fixes #3520.

R=daniel.morsing, bradfitz, minux.ma, dave, rsc
CC=golang-dev
https://golang.org/cl/7397056

src/cmd/gc/closure.c

@@ -238,6 +238,7 @@ walkclosure(Node *func, NodeList **init)
 	}
 
 	clos = nod(OCOMPLIT, N, nod(OIND, typ, N));
+	clos->esc = func->esc;
 	clos->right->implicit = 1;
 	clos->list = concat(list1(nod(OCFUNC, func->closure->nname, N)), func->enter);
 
@@ -246,6 +247,9 @@ walkclosure(Node *func, NodeList **init)
 	clos->type = func->type;
 
 	typecheck(&clos, Erv);
+	// typecheck will insert a PTRLIT node under CONVNOP,
+	// tag it with escape analysis result.
+	clos->left->esc = func->esc;
 	walkexpr(&clos, init);
 
 	return clos;

src/pkg/sort/search_test.go

@@ -117,6 +117,28 @@ func TestSearchWrappers(t *testing.T) {
 	}
 }
 
+func runSearchWrappers() {
+	SearchInts(data, 11)
+	SearchFloat64s(fdata, 2.1)
+	SearchStrings(sdata, "")
+	IntSlice(data).Search(0)
+	Float64Slice(fdata).Search(2.0)
+	StringSlice(sdata).Search("x")
+}
+
+func TestSearchWrappersDontAlloc(t *testing.T) {
+	allocs := testing.AllocsPerRun(100, runSearchWrappers)
+	if allocs != 0 {
+		t.Errorf("expected no allocs for runSearchWrappers, got %v", allocs)
+	}
+}
+
+func BenchmarkSearchWrappers(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		runSearchWrappers()
+	}
+}
+
 // Abstract exhaustive test: all sizes up to 100,
 // all possible return values.  If there are any small
 // corner cases, this test exercises them.