[dev.typeparams] cmd/compile, runtime: remove _defer.siz field

As deferred function now always has zero arguments, _defer.siz is
always 0 and can be removed.

Change-Id: Ibb89f65b2f9d2ba4aeabe50438cc3d4b6a88320b
Reviewed-on: https://go-review.googlesource.com/c/go/+/325921
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

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

@@ -4940,24 +4940,20 @@ func (s *state) call(n *ir.CallExpr, k callKind, returnResultAddr bool) *ssa.Val
 		addr := s.addr(d)
 
 		// Must match deferstruct() below and src/runtime/runtime2.go:_defer.
-		// 0: siz
+		// 0: started, set in deferprocStack
-		s.store(types.Types[types.TUINT32],
+		// 1: heap, set in deferprocStack
-			s.newValue1I(ssa.OpOffPtr, types.Types[types.TUINT32].PtrTo(), t.FieldOff(0), addr),
+		// 2: openDefer
-			s.constInt32(types.Types[types.TUINT32], 0))
+		// 3: sp, set in deferprocStack
-		// 1: started, set in deferprocStack
+		// 4: pc, set in deferprocStack
-		// 2: heap, set in deferprocStack
+		// 5: fn
-		// 3: openDefer
-		// 4: sp, set in deferprocStack
-		// 5: pc, set in deferprocStack
-		// 6: fn
 		s.store(closure.Type,
-			s.newValue1I(ssa.OpOffPtr, closure.Type.PtrTo(), t.FieldOff(6), addr),
+			s.newValue1I(ssa.OpOffPtr, closure.Type.PtrTo(), t.FieldOff(5), addr),
 			closure)
-		// 7: panic, set in deferprocStack
+		// 6: panic, set in deferprocStack
-		// 8: link, set in deferprocStack
+		// 7: link, set in deferprocStack
-		// 9: framepc
+		// 8: fd
-		// 10: varp
+		// 9: varp
-		// 11: fd
+		// 10: framepc
 
 		// Call runtime.deferprocStack with pointer to _defer record.
 		ACArgs = append(ACArgs, types.Types[types.TUINTPTR])
@@ -7583,7 +7579,6 @@ func deferstruct() *types.Type {
 	// These fields must match the ones in runtime/runtime2.go:_defer and
 	// (*state).call above.
 	fields := []*types.Field{
-		makefield("siz", types.Types[types.TUINT32]),
 		makefield("started", types.Types[types.TBOOL]),
 		makefield("heap", types.Types[types.TBOOL]),
 		makefield("openDefer", types.Types[types.TBOOL]),
@@ -7595,9 +7590,9 @@ func deferstruct() *types.Type {
 		makefield("fn", types.Types[types.TUINTPTR]),
 		makefield("_panic", types.Types[types.TUINTPTR]),
 		makefield("link", types.Types[types.TUINTPTR]),
-		makefield("framepc", types.Types[types.TUINTPTR]),
-		makefield("varp", types.Types[types.TUINTPTR]),
 		makefield("fd", types.Types[types.TUINTPTR]),
+		makefield("varp", types.Types[types.TUINTPTR]),
+		makefield("framepc", types.Types[types.TUINTPTR]),
 	}
 
 	// build struct holding the above fields

src/runtime/panic.go

@@ -258,7 +258,7 @@ func deferproc(fn func()) {
 }
 
 // deferprocStack queues a new deferred function with a defer record on the stack.
-// The defer record must have its siz and fn fields initialized.
+// The defer record must have its fn field initialized.
 // All other fields can contain junk.
 // The defer record must be immediately followed in memory by
 // the arguments of the defer.
@@ -271,10 +271,7 @@ func deferprocStack(d *_defer) {
 		// go code on the system stack can't defer
 		throw("defer on system stack")
 	}
-	if d.siz != 0 {
+	// fn is already set.
-		throw("defer with non-empty frame")
-	}
-	// siz and fn are already set.
 	// The other fields are junk on entry to deferprocStack and
 	// are initialized here.
 	d.started = false
@@ -406,7 +403,6 @@ func newdefer(siz int32) *_defer {
 			d = (*_defer)(mallocgc(total, deferType, true))
 		})
 	}
-	d.siz = siz
 	d.heap = true
 	return d
 }
@@ -428,7 +424,7 @@ func freedefer(d *_defer) {
 	if !d.heap {
 		return
 	}
-	sc := deferclass(uintptr(d.siz))
+	sc := deferclass(0)
 	if sc >= uintptr(len(p{}.deferpool)) {
 		return
 	}
@@ -461,7 +457,6 @@ func freedefer(d *_defer) {
 
 	// These lines used to be simply `*d = _defer{}` but that
 	// started causing a nosplit stack overflow via typedmemmove.
-	d.siz = 0
 	d.started = false
 	d.openDefer = false
 	d.sp = 0

src/runtime/runtime2.go

@@ -940,14 +940,13 @@ func extendRandom(r []byte, n int) {
 
 // A _defer holds an entry on the list of deferred calls.
 // If you add a field here, add code to clear it in freedefer and deferProcStack
-// This struct must match the code in cmd/compile/internal/gc/reflect.go:deferstruct
+// This struct must match the code in cmd/compile/internal/ssagen/ssa.go:deferstruct
-// and cmd/compile/internal/gc/ssa.go:(*state).call.
+// and cmd/compile/internal/ssagen/ssa.go:(*state).call.
 // Some defers will be allocated on the stack and some on the heap.
 // All defers are logically part of the stack, so write barriers to
 // initialize them are not required. All defers must be manually scanned,
 // and for heap defers, marked.
 type _defer struct {
-	siz     int32 // includes both arguments and results
 	started bool
 	heap    bool
 	// openDefer indicates that this _defer is for a frame with open-coded