cmd/compile, syscall: add //go:uintptrescapes comment, and use it

This new comment can be used to declare that the uintptr arguments to a
function may be converted from pointers, and that those pointers should
be considered to escape. This is used for the Call methods in
dll_windows.go that take uintptr arguments, because they call Syscall.

We can't treat these functions as we do syscall.Syscall, because unlike
Syscall they may cause the stack to grow. For Syscall we can assume that
stack arguments can remain on the stack, but for these functions we need
them to escape.

Fixes golang#16035.

Change-Id: Ia0e5b4068c04f8d303d95ab9ea394939f1f57454
Reviewed-on: https://go-review.googlesource.com/24551
Reviewed-by: David Chase <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

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

@@ -1551,10 +1551,12 @@ func esccall(e *EscState, n *Node, up *Node) {
 	}
 
 	var src *Node
+	note := ""
 	i := 0
 	lls := ll.Slice()
 	for t, it := IterFields(fntype.Params()); i < len(lls); i++ {
 		src = lls[i]
+		note = t.Note
 		if t.Isddd && !n.Isddd {
 			// Introduce ODDDARG node to represent ... allocation.
 			src = Nod(ODDDARG, nil, nil)
@@ -1566,7 +1568,7 @@ func esccall(e *EscState, n *Node, up *Node) {
 		}
 
 		if haspointers(t.Type) {
-			if escassignfromtag(e, t.Note, nE.Escretval, src) == EscNone && up.Op != ODEFER && up.Op != OPROC {
+			if escassignfromtag(e, note, nE.Escretval, src) == EscNone && up.Op != ODEFER && up.Op != OPROC {
 				a := src
 				for a.Op == OCONVNOP {
 					a = a.Left
@@ -1596,14 +1598,24 @@ func esccall(e *EscState, n *Node, up *Node) {
 			// This occurs when function parameter type Isddd and n not Isddd
 			break
 		}
+
+		if note == uintptrEscapesTag {
+			escassignSinkNilWhy(e, src, src, "escaping uintptr")
+		}
+
 		t = it.Next()
 	}
 
+	// Store arguments into slice for ... arg.
 	for ; i < len(lls); i++ {
 		if Debug['m'] > 3 {
 			fmt.Printf("%v::esccall:: ... <- %v\n", linestr(lineno), Nconv(lls[i], FmtShort))
 		}
-		escassignNilWhy(e, src, lls[i], "arg to ...") // args to slice
+		if note == uintptrEscapesTag {
+			escassignSinkNilWhy(e, src, lls[i], "arg to uintptrescapes ...")
+		} else {
+			escassignNilWhy(e, src, lls[i], "arg to ...")
+		}
 	}
 }
 
@@ -1963,9 +1975,20 @@ recurse:
 // lets us take the address below to get a *string.
 var unsafeUintptrTag = "unsafe-uintptr"
 
+// This special tag is applied to uintptr parameters of functions
+// marked go:uintptrescapes.
+const uintptrEscapesTag = "uintptr-escapes"
+
 func esctag(e *EscState, func_ *Node) {
 	func_.Esc = EscFuncTagged
 
+	name := func(s *Sym, narg int) string {
+		if s != nil {
+			return s.Name
+		}
+		return fmt.Sprintf("arg#%d", narg)
+	}
+
 	// External functions are assumed unsafe,
 	// unless //go:noescape is given before the declaration.
 	if func_.Nbody.Len() == 0 {
@@ -1988,13 +2011,7 @@ func esctag(e *EscState, func_ *Node) {
 			narg++
 			if t.Type.Etype == TUINTPTR {
 				if Debug['m'] != 0 {
-					var name string
-					if t.Sym != nil {
-						name = t.Sym.Name
-					} else {
-						name = fmt.Sprintf("arg#%d", narg)
-					}
-					Warnl(func_.Lineno, "%v assuming %v is unsafe uintptr", funcSym(func_), name)
+					Warnl(func_.Lineno, "%v assuming %v is unsafe uintptr", funcSym(func_), name(t.Sym, narg))
 				}
 				t.Note = unsafeUintptrTag
 			}
@@ -2003,6 +2020,27 @@ func esctag(e *EscState, func_ *Node) {
 		return
 	}
 
+	if func_.Func.Pragma&UintptrEscapes != 0 {
+		narg := 0
+		for _, t := range func_.Type.Params().Fields().Slice() {
+			narg++
+			if t.Type.Etype == TUINTPTR {
+				if Debug['m'] != 0 {
+					Warnl(func_.Lineno, "%v marking %v as escaping uintptr", funcSym(func_), name(t.Sym, narg))
+				}
+				t.Note = uintptrEscapesTag
+			}
+
+			if t.Isddd && t.Type.Elem().Etype == TUINTPTR {
+				// final argument is ...uintptr.
+				if Debug['m'] != 0 {
+					Warnl(func_.Lineno, "%v marking %v as escaping ...uintptr", funcSym(func_), name(t.Sym, narg))
+				}
+				t.Note = uintptrEscapesTag
+			}
+		}
+	}
+
 	savefn := Curfn
 	Curfn = func_
 
@@ -2015,7 +2053,9 @@ func esctag(e *EscState, func_ *Node) {
 		case EscNone, // not touched by escflood
 			EscReturn:
 			if haspointers(ln.Type) { // don't bother tagging for scalars
-				ln.Name.Param.Field.Note = mktag(int(ln.Esc))
+				if ln.Name.Param.Field.Note != uintptrEscapesTag {
+					ln.Name.Param.Field.Note = mktag(int(ln.Esc))
+				}
 			}
 
 		case EscHeap, // touched by escflood, moved to heap

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

@@ -72,6 +72,7 @@ const (
 	Nowritebarrier           // emit compiler error instead of write barrier
 	Nowritebarrierrec        // error on write barrier in this or recursive callees
 	CgoUnsafeArgs            // treat a pointer to one arg as a pointer to them all
+	UintptrEscapes           // pointers converted to uintptr escape
 )
 
 type lexer struct {
@@ -930,6 +931,19 @@ func (l *lexer) getlinepragma() rune {
 			l.pragma |= Nowritebarrierrec | Nowritebarrier // implies Nowritebarrier
 		case "go:cgo_unsafe_args":
 			l.pragma |= CgoUnsafeArgs
+		case "go:uintptrescapes":
+			// For the next function declared in the file
+			// any uintptr arguments may be pointer values
+			// converted to uintptr. This directive
+			// ensures that the referenced allocated
+			// object, if any, is retained and not moved
+			// until the call completes, even though from
+			// the types alone it would appear that the
+			// object is no longer needed during the
+			// call. The conversion to uintptr must appear
+			// in the argument list.
+			// Used in syscall/dll_windows.go.
+			l.pragma |= UintptrEscapes
 		}
 		return c
 	}

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

@@ -373,7 +373,7 @@ func ordercall(n *Node, order *Order) {
 			if t == nil {
 				break
 			}
-			if t.Note == unsafeUintptrTag {
+			if t.Note == unsafeUintptrTag || t.Note == uintptrEscapesTag {
 				xp := n.List.Addr(i)
 				for (*xp).Op == OCONVNOP && !(*xp).Type.IsPtr() {
 					xp = &(*xp).Left
@@ -385,7 +385,11 @@ func ordercall(n *Node, order *Order) {
 					*xp = x
 				}
 			}
-			t = it.Next()
+			next := it.Next()
+			if next == nil && t.Isddd && t.Note == uintptrEscapesTag {
+				next = t
+			}
+			t = next
 		}
 	}
 }

src/syscall/dll_windows.go

@@ -130,6 +130,8 @@ func (p *Proc) Addr() uintptr {
 	return p.addr
 }
 
+//go:uintptrescapes
+
 // Call executes procedure p with arguments a. It will panic, if more than 15 arguments
 // are supplied.
 //
@@ -288,6 +290,8 @@ func (p *LazyProc) Addr() uintptr {
 	return p.proc.Addr()
 }
 
+//go:uintptrescapes
+
 // Call executes procedure p with arguments a. It will panic, if more than 15 arguments
 // are supplied.
 //

test/uintptrescapes.dir/a.go

@@ -0,0 +1,54 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+import (
+	"unsafe"
+)
+
+func recurse(i int, s []byte) byte {
+	s[0] = byte(i)
+	if i == 0 {
+		return s[i]
+	} else {
+		var a [1024]byte
+		r := recurse(i-1, a[:])
+		return r + a[0]
+	}
+}
+
+//go:uintptrescapes
+func F1(a uintptr) {
+	var s [16]byte
+	recurse(4096, s[:])
+	*(*int)(unsafe.Pointer(a)) = 42
+}
+
+//go:uintptrescapes
+func F2(a ...uintptr) {
+	var s [16]byte
+	recurse(4096, s[:])
+	*(*int)(unsafe.Pointer(a[0])) = 42
+}
+
+type t struct{}
+
+func GetT() *t {
+	return &t{}
+}
+
+//go:uintptrescapes
+func (*t) M1(a uintptr) {
+	var s [16]byte
+	recurse(4096, s[:])
+	*(*int)(unsafe.Pointer(a)) = 42
+}
+
+//go:uintptrescapes
+func (*t) M2(a ...uintptr) {
+	var s [16]byte
+	recurse(4096, s[:])
+	*(*int)(unsafe.Pointer(a[0])) = 42
+}

test/uintptrescapes.dir/main.go

@@ -0,0 +1,91 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"sync"
+	"unsafe"
+
+	"./a"
+)
+
+func F1() int {
+	var buf [1024]int
+	a.F1(uintptr(unsafe.Pointer(&buf[0])))
+	return buf[0]
+}
+
+func F2() int {
+	var buf [1024]int
+	a.F2(uintptr(unsafe.Pointer(&buf[0])))
+	return buf[0]
+}
+
+var t = a.GetT()
+
+func M1() int {
+	var buf [1024]int
+	t.M1(uintptr(unsafe.Pointer(&buf[0])))
+	return buf[0]
+}
+
+func M2() int {
+	var buf [1024]int
+	t.M2(uintptr(unsafe.Pointer(&buf[0])))
+	return buf[0]
+}
+
+func main() {
+	// Use different goroutines to force stack growth.
+	var wg sync.WaitGroup
+	wg.Add(4)
+	c := make(chan bool, 4)
+
+	go func() {
+		defer wg.Done()
+		b := F1()
+		if b != 42 {
+			fmt.Printf("F1: got %d, expected 42\n", b)
+			c <- false
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		b := F2()
+		if b != 42 {
+			fmt.Printf("F2: got %d, expected 42\n", b)
+			c <- false
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		b := M1()
+		if b != 42 {
+			fmt.Printf("M1: got %d, expected 42\n", b)
+			c <- false
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		b := M2()
+		if b != 42 {
+			fmt.Printf("M2: got %d, expected 42\n", b)
+			c <- false
+		}
+	}()
+
+	wg.Wait()
+
+	select {
+	case <-c:
+		os.Exit(1)
+	default:
+	}
+}

test/uintptrescapes.go

@@ -0,0 +1,9 @@
+// rundir
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that the go:uintptrescapes comment works as expected.
+
+package ignored

test/uintptrescapes2.go

@@ -0,0 +1,31 @@
+// errorcheck -0 -m -live
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test escape analysis and liveness inferred for uintptrescapes functions.
+
+package p
+
+import (
+	"unsafe"
+)
+
+//go:uintptrescapes
+//go:noinline
+func F1(a uintptr) {} // ERROR "escaping uintptr"
+
+//go:uintptrescapes
+//go:noinline
+func F2(a ...uintptr) {} // ERROR "escaping ...uintptr" "live at entry" "a does not escape"
+
+func G() {
+	var t int // ERROR "moved to heap"
+	F1(uintptr(unsafe.Pointer(&t))) // ERROR "live at call to F1: autotmp" "&t escapes to heap"
+}
+
+func H() {
+	var v int // ERROR "moved to heap"
+	F2(0, 1, uintptr(unsafe.Pointer(&v)), 2) // ERROR "live at call to newobject: autotmp" "live at call to F2: autotmp" "escapes to heap"
+}