path/filepath: avoid allocation in Clean of cleaned path

Alternative to https://golang.org/cl/6330044.

Fixes #3681.

R=golang-dev, r, hanwen, iant
CC=golang-dev
https://golang.org/cl/6335056

1 files changed, 18 insertions, 0 deletions

diff --git a/src/pkg/path/filepath/path_test.go b/src/pkg/path/filepath/path_test.go
index cb84d98b47..ec6af4db7e 100644
--- a/src/pkg/path/filepath/path_test.go
+++ b/src/pkg/path/filepath/path_test.go
@@ -99,6 +99,24 @@ func TestClean(t *testing.T) {
 		if s := filepath.Clean(test.path); s != test.result {
 			t.Errorf("Clean(%q) = %q, want %q", test.path, s, test.result)
 		}
+		if s := filepath.Clean(test.result); s != test.result {
+			t.Errorf("Clean(%q) = %q, want %q", test.result, s, test.result)
+		}
+	}
+
+	var ms runtime.MemStats
+	runtime.ReadMemStats(&ms)
+	allocs := -ms.Mallocs
+	const rounds = 100
+	for i := 0; i < rounds; i++ {
+		for _, test := range tests {
+			filepath.Clean(test.result)
+		}
+	}
+	runtime.ReadMemStats(&ms)
+	allocs += ms.Mallocs
+	if allocs >= rounds {
+		t.Errorf("Clean cleaned paths: %d allocations per test round, want zero", allocs/rounds)
 	}
 }