cmd/compile: recognize (*[Big]T)(ptr)[:n:m] pattern for -d=checkptr

A common idiom for turning an unsafe.Pointer into a slice is to write:

    s := (*[Big]T)(ptr)[:n:m]

This technically violates Go's unsafe pointer rules (rule #1 says T2
can't be bigger than T1), but it's fairly common and not too difficult
to recognize, so might as well allow it for now so we can make
progress on golang#34972.

This should be revisited if golang#19367 is accepted.

Updates golang#22218.
Updates golang#34972.

Change-Id: Id824e2461904e770910b6e728b4234041d2cc8bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/201839
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -235,7 +235,7 @@ func racewriterange(addr, size uintptr)
 func msanread(addr, size uintptr)
 func msanwrite(addr, size uintptr)
 
-func checkptrAlignment(unsafe.Pointer, *byte)
+func checkptrAlignment(unsafe.Pointer, *byte, uintptr)
 func checkptrArithmetic(unsafe.Pointer, []unsafe.Pointer)
 
 // architecture variants

src/cmd/compile/internal/gc/walk.go

@@ -953,7 +953,7 @@ opswitch:
 		n.Left = walkexpr(n.Left, init)
 		if n.Op == OCONVNOP && checkPtr(Curfn, 1) {
 			if n.Type.IsPtr() && n.Left.Type.Etype == TUNSAFEPTR { // unsafe.Pointer to *T
-				n = walkCheckPtrAlignment(n, init)
+				n = walkCheckPtrAlignment(n, init, nil)
 				break
 			}
 			if n.Type.Etype == TUNSAFEPTR && n.Left.Type.Etype == TUINTPTR { // uintptr to unsafe.Pointer
@@ -1120,7 +1120,12 @@ opswitch:
 		n.List.SetSecond(walkexpr(n.List.Second(), init))
 
 	case OSLICE, OSLICEARR, OSLICESTR, OSLICE3, OSLICE3ARR:
-		n.Left = walkexpr(n.Left, init)
+		checkSlice := checkPtr(Curfn, 1) && n.Op == OSLICE3ARR && n.Left.Op == OCONVNOP && n.Left.Left.Type.Etype == TUNSAFEPTR
+		if checkSlice {
+			n.Left.Left = walkexpr(n.Left.Left, init)
+		} else {
+			n.Left = walkexpr(n.Left, init)
+		}
 		low, high, max := n.SliceBounds()
 		low = walkexpr(low, init)
 		if low != nil && isZero(low) {
@@ -1130,6 +1135,9 @@ opswitch:
 		high = walkexpr(high, init)
 		max = walkexpr(max, init)
 		n.SetSliceBounds(low, high, max)
+		if checkSlice {
+			n.Left = walkCheckPtrAlignment(n.Left, init, max)
+		}
 		if n.Op.IsSlice3() {
 			if max != nil && max.Op == OCAP && samesafeexpr(n.Left, max.Left) {
 				// Reduce x[i:j:cap(x)] to x[i:j].
@@ -3912,13 +3920,29 @@ func isRuneCount(n *Node) bool {
 	return Debug['N'] == 0 && !instrumenting && n.Op == OLEN && n.Left.Op == OSTR2RUNES
 }
 
-func walkCheckPtrAlignment(n *Node, init *Nodes) *Node {
-	if n.Type.Elem().Alignment() == 1 && n.Type.Elem().Size() == 1 {
+func walkCheckPtrAlignment(n *Node, init *Nodes, count *Node) *Node {
+	if !n.Type.IsPtr() {
+		Fatalf("expected pointer type: %v", n.Type)
+	}
+	elem := n.Type.Elem()
+	if count != nil {
+		if !elem.IsArray() {
+			Fatalf("expected array type: %v", elem)
+		}
+		elem = elem.Elem()
+	}
+
+	size := elem.Size()
+	if elem.Alignment() == 1 && (size == 0 || size == 1 && count == nil) {
 		return n
 	}
 
+	if count == nil {
+		count = nodintconst(1)
+	}
+
 	n.Left = cheapexpr(n.Left, init)
-	init.Append(mkcall("checkptrAlignment", nil, init, convnop(n.Left, types.Types[TUNSAFEPTR]), typename(n.Type.Elem())))
+	init.Append(mkcall("checkptrAlignment", nil, init, convnop(n.Left, types.Types[TUNSAFEPTR]), typename(elem), conv(count, types.Types[TUINTPTR])))
 	return n
 }
 

src/runtime/checkptr.go

@@ -9,18 +9,19 @@ import "unsafe"
 type ptrAlign struct {
 	ptr  unsafe.Pointer
 	elem *_type
+	n    uintptr
 }
 
-func checkptrAlignment(p unsafe.Pointer, elem *_type) {
-	// Check that (*T)(p) is appropriately aligned.
+func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
+	// Check that (*[n]elem)(p) is appropriately aligned.
 	// TODO(mdempsky): What about fieldAlign?
 	if uintptr(p)&(uintptr(elem.align)-1) != 0 {
-		panic(ptrAlign{p, elem})
+		panic(ptrAlign{p, elem, n})
 	}
 
-	// Check that (*T)(p) doesn't straddle multiple heap objects.
-	if elem.size != 1 && checkptrBase(p) != checkptrBase(add(p, elem.size-1)) {
-		panic(ptrAlign{p, elem})
+	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
+	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
+		panic(ptrAlign{p, elem, n})
 	}
 }
 
@@ -34,6 +35,9 @@ func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
 		panic(ptrArith{p, originals})
 	}
 
+	// Check that if the computed pointer p points into a heap
+	// object, then one of the original pointers must have pointed
+	// into the same object.
 	base := checkptrBase(p)
 	if base == 0 {
 		return