cmd/gc: don't copy []byte during string comparison

Currently we allocate a new string during []byte->string conversion
in string comparison expressions. String allocation is unnecessary in
this case, because comparison does memorize the strings for later use.
This change uses slicebytetostringtmp to construct temp string directly
from []byte buffer and passes it to runtime.eqstring.

Change-Id: If00f1faaee2076baa6f6724d245d5b5e0f59b563
Reviewed-on: https://go-review.googlesource.com/3410
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 18 insertions, 0 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 8aa0dd076d..6f7de4de1e 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -76,6 +76,7 @@ func slicebytetostringtmp(b []byte) string {
 	// First such case is a m[string(k)] lookup where
 	// m is a string-keyed map and k is a []byte.
 	// Second such case is "<"+string(b)+">" concatenation where b is []byte.
+	// Third such case is string(b)=="foo" comparison where b is []byte.
 
 	if raceenabled && len(b) > 0 {
 		racereadrangepc(unsafe.Pointer(&b[0]),
diff --git a/src/runtime/string_test.go b/src/runtime/string_test.go
index 1551ecc82b..03c8948467 100644
--- a/src/runtime/string_test.go
+++ b/src/runtime/string_test.go
@@ -158,3 +158,20 @@ func TestGostringnocopy(t *testing.T) {
 		t.Errorf("want %d, got %d", max+9, newmax)
 	}
 }
+
+func TestCompareTempString(t *testing.T) {
+	s := "foo"
+	b := []byte(s)
+	n := testing.AllocsPerRun(1000, func() {
+		if string(b) != s {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
+		if string(b) == s {
+		} else {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
+	})
+	if n != 0 {
+		t.Fatalf("want 0 allocs, got %v", n)
+	}
+}