reflect: remove short-circuits for zero-sized types in ABI algorithm

This change removes two short-circuits for zero-sized types (zero-sized
structs and zero-sized struct fields) in the recursive cases of the ABI
algorithm, because this does not match the spec's algorithm, nor the
compiler's algorithm.

The failing case here is a struct with a field that is an array of
non-zero length but whose element type is zero-sized. This struct must
be stack-assigned because of the array, according to the algorithm.
The reflect package was register-assigning it.

Because there were two short-circuits, this can also appear if a struct
has a field that is a zero-sized struct but contains such an array,
also.

This change adds regression tests for both of these cases.

For #44816.
For #40724.

Change-Id: I956804170962448197a1c9853826e3436fc8b1ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/306929
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/reflect/abi.go

@@ -233,19 +233,9 @@ func (a *abiSeq) regAssign(t *rtype, offset uintptr) bool {
 			return false
 		}
 	case Struct:
-		if t.size == 0 {
-			// There's nothing to assign, so don't modify
-			// a.steps but succeed so the caller doesn't
-			// try to stack-assign this value.
-			return true
-		}
 		st := (*structType)(unsafe.Pointer(t))
 		for i := range st.fields {
 			f := &st.fields[i]
-			if f.typ.Size() == 0 {
-				// Ignore zero-sized fields.
-				continue
-			}
 			if !a.regAssign(f.typ, offset+f.offset()) {
 				return false
 			}

src/reflect/abi_test.go

@@ -300,6 +300,8 @@ var abiCallTestCases = []interface{}{
 	passStruct11,
 	passStruct12,
 	passStruct13,
+	passStruct14,
+	passStruct15,
 	pass2Struct1,
 	passEmptyStruct,
 	passStruct10AndSmall,
@@ -521,6 +523,18 @@ func passStruct13(a Struct13) Struct13 {
 	return a
 }
 
+//go:registerparams
+//go:noinline
+func passStruct14(a Struct14) Struct14 {
+	return a
+}
+
+//go:registerparams
+//go:noinline
+func passStruct15(a Struct15) Struct15 {
+	return a
+}
+
 //go:registerparams
 //go:noinline
 func pass2Struct1(a, b Struct1) (x, y Struct1) {
@@ -581,6 +595,8 @@ var abiMakeFuncTestCases = []interface{}{
 	callArgsStruct11,
 	callArgsStruct12,
 	callArgsStruct13,
+	callArgsStruct14,
+	callArgsStruct15,
 	callArgs2Struct1,
 	callArgsEmptyStruct,
 }
@@ -801,6 +817,18 @@ func callArgsStruct13(f func(Struct13, MagicLastTypeNameForTestingRegisterABI) S
 	return f(a0, MagicLastTypeNameForTestingRegisterABI{})
 }
 
+//go:registerparams
+//go:noinline
+func callArgsStruct14(f func(Struct14, MagicLastTypeNameForTestingRegisterABI) Struct14, a0 Struct14) Struct14 {
+	return f(a0, MagicLastTypeNameForTestingRegisterABI{})
+}
+
+//go:registerparams
+//go:noinline
+func callArgsStruct15(f func(Struct15, MagicLastTypeNameForTestingRegisterABI) Struct15, a0 Struct15) Struct15 {
+	return f(a0, MagicLastTypeNameForTestingRegisterABI{})
+}
+
 //go:registerparams
 //go:noinline
 func callArgs2Struct1(f func(Struct1, Struct1, MagicLastTypeNameForTestingRegisterABI) (Struct1, Struct1), a0, a1 Struct1) (r0, r1 Struct1) {
@@ -904,6 +932,25 @@ type Struct13 struct {
 	B int
 }
 
+// Struct14 tests a non-zero-sized (and otherwise register-assignable)
+// struct with a field that is a non-zero length array with zero-sized members.
+type Struct14 struct {
+	A uintptr
+	X [3]struct{}
+	B float64
+}
+
+// Struct15 tests a non-zero-sized (and otherwise register-assignable)
+// struct with a struct field that is zero-sized but contains a
+// non-zero length array with zero-sized members.
+type Struct15 struct {
+	A uintptr
+	X struct {
+		Y [3]struct{}
+	}
+	B float64
+}
+
 const genValueRandSeed = 0
 
 // genValue generates a pseudorandom reflect.Value with type t.