cmd/compile: split exported/non-exported methods for interface type

Currently, mhdr/methods is emitted with the same len/cap. There's no way
to distinguish between exported and non-exported methods statically.

This CL splits mhdr/methods into two parts, use "len" for number of
exported methods, and "cap" for all methods. This fixes the bug in
issue #22075, which intends to return the number of exported methods but
currently return all methods.

Note that with this encoding, we still can access either
all/exported-only/non-exported-only methods:

	mhdr[:cap(mhdr)]          // all methods
	mhdr                      // exported methods
	mhdr[len(mhdr):cap(mhdr)] // non-exported methods

Thank to Matthew Dempsky (@mdempsky) for suggesting this encoding.

Fixes #22075

Change-Id: If662adb03ccff27407d55a5578a0ed05a15e7cdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/259237
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

1 files changed, 20 insertions, 6 deletions

diff --git a/src/runtime/type.go b/src/runtime/type.go
index 81455f3532..36492619e1 100644
--- a/src/runtime/type.go
+++ b/src/runtime/type.go
@@ -366,7 +366,19 @@ type imethod struct {
 type interfacetype struct {
 	typ     _type
 	pkgpath name
-	mhdr    []imethod
+	// expMethods contains all interface methods.
+	//
+	// - len(expMethods) returns number of exported methods.
+	// - cap(expMethods) returns all interface methods, including both exported/non-exported methods.
+	expMethods []imethod
+}
+
+func (it *interfacetype) methods() []imethod {
+	return it.expMethods[:cap(it.expMethods)]
+}
+
+func (it *interfacetype) isEmpty() bool {
+	return cap(it.expMethods) == 0
 }
 
 type maptype struct {
@@ -664,13 +676,15 @@ func typesEqual(t, v *_type, seen map[_typePair]struct{}) bool {
 		if it.pkgpath.name() != iv.pkgpath.name() {
 			return false
 		}
-		if len(it.mhdr) != len(iv.mhdr) {
+		itmethods := it.methods()
+		ivmethods := iv.methods()
+		if len(itmethods) != len(ivmethods) {
 			return false
 		}
-		for i := range it.mhdr {
-			tm := &it.mhdr[i]
-			vm := &iv.mhdr[i]
-			// Note the mhdr array can be relocated from
+		for i := range itmethods {
+			tm := &itmethods[i]
+			vm := &ivmethods[i]
+			// Note the expMethods array can be relocated from
 			// another module. See #17724.
 			tname := resolveNameOff(unsafe.Pointer(tm), tm.name)
 			vname := resolveNameOff(unsafe.Pointer(vm), vm.name)