cmd/compile,reflect: allow longer type names

Encode the length of type names and tags in a varint encoding
instead of a fixed 2-byte encoding. This allows lengths longer
than 65535 (which can happen for large unnamed structs).

Removed the alignment check for #14962, it isn't relevant any more
since we're no longer reading pointers directly out of this data
(it is encoded as an offset which is copied out bytewise).

Fixes #44155
Update #14962

Change-Id: I6084f6027e5955dc16777c87b0dd5ea2baa49629
Reviewed-on: https://go-review.googlesource.com/c/go/+/318249
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 29 insertions, 36 deletions

diff --git a/src/internal/reflectlite/all_test.go b/src/internal/reflectlite/all_test.go
index e2c4f30487..e15f364fcd 100644
--- a/src/internal/reflectlite/all_test.go
+++ b/src/internal/reflectlite/all_test.go
@@ -982,19 +982,6 @@ func TestNames(t *testing.T) {
 	}
 }
 
-type embed struct {
-	EmbedWithUnexpMeth
-}
-
-func TestNameBytesAreAligned(t *testing.T) {
-	typ := TypeOf(embed{})
-	b := FirstMethodNameBytes(typ)
-	v := uintptr(unsafe.Pointer(b))
-	if v%unsafe.Alignof((*byte)(nil)) != 0 {
-		t.Errorf("reflect.name.bytes pointer is not aligned: %x", v)
-	}
-}
-
 // TestUnaddressableField tests that the reflect package will not allow
 // a type from another package to be used as a named type with an
 // unexported field.
diff --git a/src/internal/reflectlite/type.go b/src/internal/reflectlite/type.go
index 15ba30da36..f529f7c5fc 100644
--- a/src/internal/reflectlite/type.go
+++ b/src/internal/reflectlite/type.go
@@ -321,49 +321,55 @@ func (n name) isExported() bool {
 	return (*n.bytes)&(1<<0) != 0
 }
 
-func (n name) nameLen() int {
-	return int(uint16(*n.data(1, "name len field"))<<8 | uint16(*n.data(2, "name len field")))
-}
-
-func (n name) tagLen() int {
-	if *n.data(0, "name flag field")&(1<<1) == 0 {
-		return 0
+func (n name) hasTag() bool {
+	return (*n.bytes)&(1<<1) != 0
+}
+
+// readVarint parses a varint as encoded by encoding/binary.
+// It returns the number of encoded bytes and the encoded value.
+func (n name) readVarint(off int) (int, int) {
+	v := 0
+	for i := 0; ; i++ {
+		x := *n.data(off+i, "read varint")
+		v += int(x&0x7f) << (7 * i)
+		if x&0x80 == 0 {
+			return i + 1, v
+		}
 	}
-	off := 3 + n.nameLen()
-	return int(uint16(*n.data(off, "name taglen field"))<<8 | uint16(*n.data(off+1, "name taglen field")))
 }
 
 func (n name) name() (s string) {
 	if n.bytes == nil {
 		return
 	}
-	b := (*[4]byte)(unsafe.Pointer(n.bytes))
-
+	i, l := n.readVarint(1)
 	hdr := (*unsafeheader.String)(unsafe.Pointer(&s))
-	hdr.Data = unsafe.Pointer(&b[3])
-	hdr.Len = int(b[1])<<8 | int(b[2])
-	return s
+	hdr.Data = unsafe.Pointer(n.data(1+i, "non-empty string"))
+	hdr.Len = l
+	return
 }
 
 func (n name) tag() (s string) {
-	tl := n.tagLen()
-	if tl == 0 {
+	if !n.hasTag() {
 		return ""
 	}
-	nl := n.nameLen()
+	i, l := n.readVarint(1)
+	i2, l2 := n.readVarint(1 + i + l)
 	hdr := (*unsafeheader.String)(unsafe.Pointer(&s))
-	hdr.Data = unsafe.Pointer(n.data(3+nl+2, "non-empty string"))
-	hdr.Len = tl
-	return s
+	hdr.Data = unsafe.Pointer(n.data(1+i+l+i2, "non-empty string"))
+	hdr.Len = l2
+	return
 }
 
 func (n name) pkgPath() string {
 	if n.bytes == nil || *n.data(0, "name flag field")&(1<<2) == 0 {
 		return ""
 	}
-	off := 3 + n.nameLen()
-	if tl := n.tagLen(); tl > 0 {
-		off += 2 + tl
+	i, l := n.readVarint(1)
+	off := 1 + i + l
+	if n.hasTag() {
+		i2, l2 := n.readVarint(off)
+		off += i2 + l2
 	}
 	var nameOff int32
 	// Note that this field may not be aligned in memory,