cmd/compile: load map length with the right type

len(map) is lowered to loading the first field of the map
structure, which is the length. Currently it is a load of an int.
With the old map, the first field is indeed an int. With Swiss
map, however, it is a uint64. On big-endian 32-bit machine,
loading an (32-bit) int from a uint64 would load just the high
bits, which are (probably) all 0. Change to a load with the proper
type.

Fixes #70248.

Change-Id: I39cf2d1e6658dac5a8de25c858e1580e2a14b894
Reviewed-on: https://go-review.googlesource.com/c/go/+/638375
Run-TryBot: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssagen/ssa.go

@@ -5452,12 +5452,15 @@ func (s *state) referenceTypeBuiltin(n *ir.UnaryExpr, x *ssa.Value) *ssa.Value {
 	if n.X.Type().IsChan() && n.Op() == ir.OCAP {
 		s.Fatalf("cannot inline cap(chan)") // must use runtime.chancap now
 	}
+	if n.X.Type().IsMap() && n.Op() == ir.OCAP {
+		s.Fatalf("cannot inline cap(map)") // cap(map) does not exist
+	}
 	// if n == nil {
 	//   return 0
 	// } else {
-	//   // len
-	//   return *((*int)n)
-	//   // cap
+	//   // len, the actual loadType depends
+	//   return int(*((*loadType)n))
+	//   // cap (chan only, not used for now)
 	//   return *(((*int)n)+1)
 	// }
 	lenType := n.Type()
@@ -5485,7 +5488,9 @@ func (s *state) referenceTypeBuiltin(n *ir.UnaryExpr, x *ssa.Value) *ssa.Value {
 	case ir.OLEN:
 		if buildcfg.Experiment.SwissMap && n.X.Type().IsMap() {
 			// length is stored in the first word.
-			s.vars[n] = s.load(lenType, x)
+			loadType := reflectdata.SwissMapType().Field(0).Type // uint64
+			load := s.load(loadType, x)
+			s.vars[n] = s.conv(nil, load, loadType, lenType) // integer conversion doesn't need Node
 		} else {
 			// length is stored in the first word for map/chan
 			s.vars[n] = s.load(lenType, x)

src/internal/runtime/maps/map.go

@@ -194,6 +194,7 @@ func h2(h uintptr) uintptr {
 type Map struct {
 	// The number of filled slots (i.e. the number of elements in all
 	// tables). Excludes deleted slots.
+	// Must be first (known by the compiler, for len() builtin).
 	used uint64
 
 	// seed is the hash seed, computed as a unique random number per map.