cmd/compile: add type-based alias analysis

Make ssa.disjoint call ssa.disjointTypes to disambiguate Values based on
their types. Only one type-based rule is employed: a Type can't alias
with a pointer (https://pkg.go.dev/unsafe#Pointer).

Fixes #70488

Change-Id: I5a7e75292c2b6b5a01fb9048e3e2360e31dbcdd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/632176
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: andreybokhanko

src/cmd/compile/internal/rttype/rttype.go

@@ -50,26 +50,26 @@ func Init() {
 	// Note: this has to be called explicitly instead of being
 	// an init function so it runs after the types package has
 	// been properly initialized.
-	Type = fromReflect(reflect.TypeOf(abi.Type{}))
-	ArrayType = fromReflect(reflect.TypeOf(abi.ArrayType{}))
-	ChanType = fromReflect(reflect.TypeOf(abi.ChanType{}))
-	FuncType = fromReflect(reflect.TypeOf(abi.FuncType{}))
-	InterfaceType = fromReflect(reflect.TypeOf(abi.InterfaceType{}))
-	OldMapType = fromReflect(reflect.TypeOf(abi.OldMapType{}))
-	SwissMapType = fromReflect(reflect.TypeOf(abi.SwissMapType{}))
-	PtrType = fromReflect(reflect.TypeOf(abi.PtrType{}))
-	SliceType = fromReflect(reflect.TypeOf(abi.SliceType{}))
-	StructType = fromReflect(reflect.TypeOf(abi.StructType{}))
+	Type = FromReflect(reflect.TypeOf(abi.Type{}))
+	ArrayType = FromReflect(reflect.TypeOf(abi.ArrayType{}))
+	ChanType = FromReflect(reflect.TypeOf(abi.ChanType{}))
+	FuncType = FromReflect(reflect.TypeOf(abi.FuncType{}))
+	InterfaceType = FromReflect(reflect.TypeOf(abi.InterfaceType{}))
+	OldMapType = FromReflect(reflect.TypeOf(abi.OldMapType{}))
+	SwissMapType = FromReflect(reflect.TypeOf(abi.SwissMapType{}))
+	PtrType = FromReflect(reflect.TypeOf(abi.PtrType{}))
+	SliceType = FromReflect(reflect.TypeOf(abi.SliceType{}))
+	StructType = FromReflect(reflect.TypeOf(abi.StructType{}))
 
-	IMethod = fromReflect(reflect.TypeOf(abi.Imethod{}))
-	Method = fromReflect(reflect.TypeOf(abi.Method{}))
-	StructField = fromReflect(reflect.TypeOf(abi.StructField{}))
-	UncommonType = fromReflect(reflect.TypeOf(abi.UncommonType{}))
+	IMethod = FromReflect(reflect.TypeOf(abi.Imethod{}))
+	Method = FromReflect(reflect.TypeOf(abi.Method{}))
+	StructField = FromReflect(reflect.TypeOf(abi.StructField{}))
+	UncommonType = FromReflect(reflect.TypeOf(abi.UncommonType{}))
 
-	InterfaceSwitch = fromReflect(reflect.TypeOf(abi.InterfaceSwitch{}))
-	TypeAssert = fromReflect(reflect.TypeOf(abi.TypeAssert{}))
+	InterfaceSwitch = FromReflect(reflect.TypeOf(abi.InterfaceSwitch{}))
+	TypeAssert = FromReflect(reflect.TypeOf(abi.TypeAssert{}))
 
-	ITab = fromReflect(reflect.TypeOf(abi.ITab{}))
+	ITab = FromReflect(reflect.TypeOf(abi.ITab{}))
 
 	// Make sure abi functions are correct. These functions are used
 	// by the linker which doesn't have the ability to do type layout,
@@ -92,8 +92,8 @@ func Init() {
 	}
 }
 
-// fromReflect translates from a host type to the equivalent target type.
-func fromReflect(rt reflect.Type) *types.Type {
+// FromReflect translates from a host type to the equivalent target type.
+func FromReflect(rt reflect.Type) *types.Type {
 	t := reflectToType(rt)
 	types.CalcSize(t)
 	return t
@@ -108,6 +108,10 @@ func reflectToType(rt reflect.Type) *types.Type {
 		return types.Types[types.TBOOL]
 	case reflect.Int:
 		return types.Types[types.TINT]
+	case reflect.Int8:
+		return types.Types[types.TINT8]
+	case reflect.Int16:
+		return types.Types[types.TINT16]
 	case reflect.Int32:
 		return types.Types[types.TINT32]
 	case reflect.Uint8:
@@ -116,9 +120,15 @@ func reflectToType(rt reflect.Type) *types.Type {
 		return types.Types[types.TUINT16]
 	case reflect.Uint32:
 		return types.Types[types.TUINT32]
+	case reflect.Float32:
+		return types.Types[types.TFLOAT32]
+	case reflect.Float64:
+		return types.Types[types.TFLOAT64]
 	case reflect.Uintptr:
 		return types.Types[types.TUINTPTR]
-	case reflect.Ptr, reflect.Func, reflect.UnsafePointer:
+	case reflect.Ptr:
+		return types.NewPtr(reflectToType(rt.Elem()))
+	case reflect.Func, reflect.UnsafePointer:
 		// TODO: there's no mechanism to distinguish different pointer types,
 		// so we treat them all as unsafe.Pointer.
 		return types.Types[types.TUNSAFEPTR]
@@ -134,6 +144,12 @@ func reflectToType(rt reflect.Type) *types.Type {
 			fields[i] = &types.Field{Sym: &types.Sym{Name: f.Name}, Type: ft}
 		}
 		return types.NewStruct(fields)
+	case reflect.Chan:
+		return types.NewChan(reflectToType(rt.Elem()), types.ChanDir(rt.ChanDir()))
+	case reflect.String:
+		return types.Types[types.TSTRING]
+	case reflect.Complex128:
+		return types.Types[types.TCOMPLEX128]
 	default:
 		base.Fatalf("unhandled kind %s", rt.Kind())
 		return nil
@@ -155,7 +171,7 @@ func NewCursor(lsym *obj.LSym, off int64, t *types.Type) Cursor {
 
 // WritePtr writes a pointer "target" to the component at the location specified by c.
 func (c Cursor) WritePtr(target *obj.LSym) {
-	if c.typ.Kind() != types.TUNSAFEPTR {
+	if c.typ.Kind() != types.TUNSAFEPTR && c.typ.Kind() != types.TPTR {
 		base.Fatalf("can't write ptr, it has kind %s", c.typ.Kind())
 	}
 	if target == nil {

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

@@ -863,6 +863,12 @@ func disjoint(p1 *Value, n1 int64, p2 *Value, n2 int64) bool {
 		}
 		return base, offset
 	}
+
+	// Run types-based analysis
+	if disjointTypes(p1.Type, p2.Type) {
+		return true
+	}
+
 	p1, off1 := baseAndOffset(p1)
 	p2, off2 := baseAndOffset(p2)
 	if isSamePtr(p1, p2) {
@@ -888,6 +894,39 @@ func disjoint(p1 *Value, n1 int64, p2 *Value, n2 int64) bool {
 	return false
 }
 
+// disjointTypes reports whether a memory region pointed to by a pointer of type
+// t1 does not overlap with a memory region pointed to by a pointer of type t2 --
+// based on type aliasing rules.
+func disjointTypes(t1 *types.Type, t2 *types.Type) bool {
+	// Unsafe pointer can alias with anything.
+	if t1.IsUnsafePtr() || t2.IsUnsafePtr() {
+		return false
+	}
+
+	if !t1.IsPtr() || !t2.IsPtr() {
+		panic("disjointTypes: one of arguments is not a pointer")
+	}
+
+	t1 = t1.Elem()
+	t2 = t2.Elem()
+
+	// Not-in-heap types are not supported -- they are rare and non-important; also,
+	// type.HasPointers check doesn't work for them correctly.
+	if t1.NotInHeap() || t2.NotInHeap() {
+		return false
+	}
+
+	isPtrShaped := func(t *types.Type) bool { return int(t.Size()) == types.PtrSize && t.HasPointers() }
+
+	// Pointers and non-pointers are disjoint (https://pkg.go.dev/unsafe#Pointer).
+	if (isPtrShaped(t1) && !t2.HasPointers()) ||
+		(isPtrShaped(t2) && !t1.HasPointers()) {
+		return true
+	}
+
+	return false
+}
+
 // moveSize returns the number of bytes an aligned MOV instruction moves.
 func moveSize(align int64, c *Config) int64 {
 	switch {

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

@@ -4,7 +4,12 @@
 
 package ssa
 
-import "testing"
+import (
+	"cmd/compile/internal/rttype"
+	"reflect"
+	"testing"
+	"unsafe"
+)
 
 // We generate memmove for copy(x[1:], x[:]), however we may change it to OpMove,
 // because size is known. Check that OpMove is alias-safe, or we did call memmove.
@@ -218,3 +223,65 @@ func TestMergePPC64AndSrwi(t *testing.T) {
 		}
 	}
 }
+
+func TestDisjointTypes(t *testing.T) {
+	tests := []struct {
+		v1, v2   any // two pointers to some types
+		expected bool
+	}{
+		{new(int8), new(int8), false},
+		{new(int8), new(float32), false},
+		{new(int8), new(*int8), true},
+		{new(*int8), new(*float32), false},
+		{new(*int8), new(chan<- int8), false},
+		{new(**int8), new(*int8), false},
+		{new(***int8), new(**int8), false},
+		{new(int8), new(chan<- int8), true},
+		{new(int), unsafe.Pointer(nil), false},
+		{new(byte), new(string), false},
+		{new(int), new(string), false},
+		{new(*int8), new(struct{ a, b int }), true},
+		{new(*int8), new(struct {
+			a *int
+			b int
+		}), false},
+		{new(*int8), new(struct {
+			a int
+			b *int
+		}), false}, // with more precise analysis it should be true
+		{new(*byte), new(string), false},
+		{new(int), new(struct {
+			a int
+			b *int
+		}), false},
+		{new(float64), new(complex128), false},
+		{new(*byte), new([]byte), false},
+		{new(int), new([]byte), false},
+		{new(int), new([2]*byte), false}, // with more recise analysis it should be true
+		{new([2]int), new(*byte), true},
+	}
+	for _, tst := range tests {
+		t1 := rttype.FromReflect(reflect.TypeOf(tst.v1))
+		t2 := rttype.FromReflect(reflect.TypeOf(tst.v2))
+		result := disjointTypes(t1, t2)
+		if result != tst.expected {
+			t.Errorf("disjointTypes(%s, %s) got %t expected %t", t1.String(), t2.String(), result, tst.expected)
+		}
+	}
+}
+
+//go:noinline
+func foo(p1 *int64, p2 *float64) int64 {
+	*p1 = 10
+	*p2 = 0 // disjointTypes shouldn't consider this and preceding stores as non-aliasing
+	return *p1
+}
+
+func TestDisjointTypesRun(t *testing.T) {
+	f := float64(0)
+	i := (*int64)(unsafe.Pointer(&f))
+	r := foo(i, &f)
+	if r != 0 {
+		t.Errorf("disjointTypes gives an incorrect answer that leads to an incorrect optimization.")
+	}
+}