runtime: remove hmap field from maptypes

The hmap field in the maptype is only used by the runtime to check the sizes of
the hmap structure created by the compiler and runtime agree.

Comments are already present about the hmap structure definitions in the
compiler and runtime needing to be in sync.

Add a test that checks the runtimes hmap size is as expected to detect
when the compilers and runtimes hmap sizes diverge instead of checking
this at runtime when a map is created.

Change-Id: I974945ebfdb66883a896386a17bbcae62a18cf2a
Reviewed-on: https://go-review.googlesource.com/91796
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/gc/reflect.go

@@ -1247,12 +1247,10 @@ func dtypesym(t *types.Type) *obj.LSym {
 		s1 := dtypesym(t.Key())
 		s2 := dtypesym(t.Elem())
 		s3 := dtypesym(bmap(t))
-		s4 := dtypesym(hmap(t))
 		ot = dcommontype(lsym, t)
 		ot = dsymptr(lsym, ot, s1, 0)
 		ot = dsymptr(lsym, ot, s2, 0)
 		ot = dsymptr(lsym, ot, s3, 0)
-		ot = dsymptr(lsym, ot, s4, 0)
 		if t.Key().Width > MAXKEYSIZE {
 			ot = duint8(lsym, ot, uint8(Widthptr))
 			ot = duint8(lsym, ot, 1) // indirect

src/cmd/link/internal/ld/decodesym.go

@@ -359,7 +359,7 @@ func decodetypeMethods(arch *sys.Arch, s *sym.Symbol) []methodsig {
 	case kindChan: // reflect.chanType
 		off += 2 * arch.PtrSize
 	case kindMap: // reflect.mapType
-		off += 4*arch.PtrSize + 8
+		off += 3*arch.PtrSize + 8
 	case kindInterface: // reflect.interfaceType
 		off += 3 * arch.PtrSize
 	default:

src/reflect/type.go

@@ -398,7 +398,6 @@ type mapType struct {
 	key           *rtype // map key type
 	elem          *rtype // map element (value) type
 	bucket        *rtype // internal bucket structure
-	hmap          *rtype // internal map header
 	keysize       uint8  // size of key slot
 	indirectkey   uint8  // store ptr to key instead of key itself
 	valuesize     uint8  // size of value slot

src/runtime/export_test.go

@@ -376,6 +376,8 @@ func (rw *RWMutex) Unlock() {
 	rw.rw.unlock()
 }
 
+const RuntimeHmapSize = unsafe.Sizeof(hmap{})
+
 func MapBucketsCount(m map[int]int) int {
 	h := *(**hmap)(unsafe.Pointer(&m))
 	return 1 << h.B

src/runtime/map.go

@@ -296,13 +296,6 @@ func makemap_small() *hmap {
 // If h != nil, the map can be created directly in h.
 // If h.buckets != nil, bucket pointed to can be used as the first bucket.
 func makemap(t *maptype, hint int, h *hmap) *hmap {
-	// The size of hmap should be 48 bytes on 64 bit
-	// and 28 bytes on 32 bit platforms.
-	if sz := unsafe.Sizeof(hmap{}); sz != 8+5*sys.PtrSize {
-		println("runtime: sizeof(hmap) =", sz, ", t.hmap.size =", t.hmap.size)
-		throw("bad hmap size")
-	}
-
 	if hint < 0 || hint > int(maxSliceCap(t.bucket.size)) {
 		hint = 0
 	}
@@ -1150,10 +1143,6 @@ func ismapkey(t *_type) bool {
 //go:linkname reflect_makemap reflect.makemap
 func reflect_makemap(t *maptype, cap int) *hmap {
 	// Check invariants and reflects math.
-	if sz := unsafe.Sizeof(hmap{}); sz != t.hmap.size {
-		println("runtime: sizeof(hmap) =", sz, ", t.hmap.size =", t.hmap.size)
-		throw("bad hmap size")
-	}
 	if !ismapkey(t.key) {
 		throw("runtime.reflect_makemap: unsupported map key type")
 	}

src/runtime/map_test.go

@@ -9,6 +9,7 @@ import (
 	"math"
 	"reflect"
 	"runtime"
+	"runtime/internal/sys"
 	"sort"
 	"strconv"
 	"strings"
@@ -16,6 +17,17 @@ import (
 	"testing"
 )
 
+func TestHmapSize(t *testing.T) {
+	// The structure of hmap is defined in runtime/map.go
+	// and in cmd/compile/internal/gc/reflect.go and must be in sync.
+	// The size of hmap should be 48 bytes on 64 bit and 28 bytes on 32 bit platforms.
+	var hmapSize = uintptr(8 + 5*sys.PtrSize)
+	if runtime.RuntimeHmapSize != hmapSize {
+		t.Errorf("sizeof(runtime.hmap{})==%d, want %d", runtime.RuntimeHmapSize, hmapSize)
+	}
+
+}
+
 // negative zero is a good test because:
 //  1) 0 and -0 are equal, yet have distinct representations.
 //  2) 0 is represented as all zeros, -0 isn't.

src/runtime/type.go

@@ -350,7 +350,6 @@ type maptype struct {
 	key           *_type
 	elem          *_type
 	bucket        *_type // internal type representing a hash bucket
-	hmap          *_type // internal type representing a hmap
 	keysize       uint8  // size of key slot
 	indirectkey   bool   // store ptr to key instead of key itself
 	valuesize     uint8  // size of value slot