runtime: handle and test large map values

This is from CL 5451105 but was dropped from that CL.
See also CL 6137051.

The only change compared to 5451105 is to check for
h != nil in reflect·mapiterinit; allowing use of nil maps
must have happened after that original CL.

Fixes #3573.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/6215078

test/bigmap.go

@@ -4,7 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Test behavior of maps with large elements.
+// Internally a map holds elements in up to 255 bytes of key+value.
+// When key or value or both are too large, it uses pointers to key+value
+// instead.  Test all the combinations.
 
 package main
 
@@ -33,4 +35,105 @@ func main() {
 	cmp(m[1], seq(11, 13))
 	cmp(m[2], seq(2, 9))
 	cmp(m[3], seq(3, 17))
+	
+
+	{
+		type T [1]byte
+		type V [1]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+  	}
+	{
+		type T [100]byte
+		type V [1]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [1]byte
+		type V [100]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [1000]byte
+		type V [1]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [1]byte
+		type V [1000]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [1000]byte
+		type V [1000]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [200]byte
+		type V [1]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [1]byte
+		type V [200]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+		}
+	}
+	{
+		type T [200]byte
+		type V [200]byte
+		m := make(map[T]V)
+		m[T{}] = V{1}
+		m[T{1}] = V{2}
+		if x, y := m[T{}][0], m[T{1}][0]; x != 1 || y != 2 {
+			println(x, y)
+			panic("bad map")
+  		}
+  	}
 }