diff --git a/src/pkg/compress/flate/copy.go b/src/pkg/compress/flate/copy.go
index 06e5d2e66d9..a3200a8f49e 100644
--- a/src/pkg/compress/flate/copy.go
+++ b/src/pkg/compress/flate/copy.go
@@ -6,12 +6,27 @@ package flate
 
 // forwardCopy is like the built-in copy function except that it always goes
 // forward from the start, even if the dst and src overlap.
-func forwardCopy(dst, src []byte) int {
-	if len(src) > len(dst) {
-		src = src[:len(dst)]
+// It is equivalent to:
+//   for i := 0; i < n; i++ {
+//     mem[dst+i] = mem[src+i]
+//   }
+func forwardCopy(mem []byte, dst, src, n int) {
+	if dst <= src {
+		copy(mem[dst:dst+n], mem[src:src+n])
+		return
 	}
-	for i, x := range src {
-		dst[i] = x
+	for {
+		if dst >= src+n {
+			copy(mem[dst:dst+n], mem[src:src+n])
+			return
+		}
+		// There is some forward overlap.  The destination
+		// will be filled with a repeated pattern of mem[src:src+k].
+		// We copy one instance of the pattern here, then repeat.
+		// Each time around this loop k will double.
+		k := dst - src
+		copy(mem[dst:dst+k], mem[src:src+k])
+		n -= k
+		dst += k
 	}
-	return len(src)
 }
diff --git a/src/pkg/compress/flate/copy_test.go b/src/pkg/compress/flate/copy_test.go
index a9281d446e9..2011b1547c9 100644
--- a/src/pkg/compress/flate/copy_test.go
+++ b/src/pkg/compress/flate/copy_test.go
@@ -30,10 +30,12 @@ func TestForwardCopy(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		b := []byte("0123456789")
-		dst := b[tc.dst0:tc.dst1]
-		src := b[tc.src0:tc.src1]
-		n := forwardCopy(dst, src)
-		got := string(dst[:n])
+		n := tc.dst1 - tc.dst0
+		if tc.src1-tc.src0 < n {
+			n = tc.src1 - tc.src0
+		}
+		forwardCopy(b, tc.dst0, tc.src0, n)
+		got := string(b[tc.dst0 : tc.dst0+n])
 		if got != tc.want {
 			t.Errorf("dst=b[%d:%d], src=b[%d:%d]: got %q, want %q",
 				tc.dst0, tc.dst1, tc.src0, tc.src1, got, tc.want)
diff --git a/src/pkg/compress/flate/inflate.go b/src/pkg/compress/flate/inflate.go
index beca34b4d8c..f529c9e7c23 100644
--- a/src/pkg/compress/flate/inflate.go
+++ b/src/pkg/compress/flate/inflate.go
@@ -511,7 +511,7 @@ func (f *decompressor) copyHist() bool {
 		if x := len(f.hist) - p; n > x {
 			n = x
 		}
-		forwardCopy(f.hist[f.hp:f.hp+n], f.hist[p:p+n])
+		forwardCopy(f.hist[:], f.hp, p, n)
 		p += n
 		f.hp += n
 		f.copyLen -= n