[release-branch.go1.15] cmd/compile: fix checkptr handling of &^

checkptr has code to recognize &^ expressions, but it didn't take into
account that "p &^ x" gets rewritten to "p & ^x" during walk, which
resulted in false positive diagnostics.

This CL changes walkexpr to mark OANDNOT expressions with Implicit
when they're rewritten to OAND, so that walkCheckPtrArithmetic can
still recognize them later.

It would be slightly more idiomatic to instead mark the OBITNOT
expression as Implicit (as it's a compiler-generated Node), but the
OBITNOT expression might get constant folded. It's not worth the extra
complexity/subtlety of relying on n.Right.Orig, so we set Implicit on
the OAND node instead.

To atone for this transgression, I add documentation for nodeImplicit.

Updates #40917.
Fixes #40934.

Change-Id: I386304171ad299c530e151e5924f179e9a5fd5b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/249477
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
(cherry picked from commit e94544cf012535da6b3c9e735bc4026e2db1c99c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/249879
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

5 files changed, 40 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/gc/syntax.go b/src/cmd/compile/internal/gc/syntax.go
index b658410c53..47e5e59156 100644
--- a/src/cmd/compile/internal/gc/syntax.go
+++ b/src/cmd/compile/internal/gc/syntax.go
@@ -141,8 +141,8 @@ const (
 	nodeInitorder, _                   // tracks state during init1; two bits
 	_, _                               // second nodeInitorder bit
 	_, nodeHasBreak
-	_, nodeNoInline // used internally by inliner to indicate that a function call should not be inlined; set for OCALLFUNC and OCALLMETH only
-	_, nodeImplicit
+	_, nodeNoInline  // used internally by inliner to indicate that a function call should not be inlined; set for OCALLFUNC and OCALLMETH only
+	_, nodeImplicit  // implicit OADDR or ODEREF; ++/-- statement represented as OASOP; or ANDNOT lowered to OAND
 	_, nodeIsDDD     // is the argument variadic
 	_, nodeDiag      // already printed error about this
 	_, nodeColas     // OAS resulting from :=
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 1e6d913ae6..b7cf313938 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -968,6 +968,7 @@ opswitch:
 	case OANDNOT:
 		n.Left = walkexpr(n.Left, init)
 		n.Op = OAND
+		n.SetImplicit(true) // for walkCheckPtrArithmetic
 		n.Right = nod(OBITNOT, n.Right, nil)
 		n.Right = typecheck(n.Right, ctxExpr)
 		n.Right = walkexpr(n.Right, init)
@@ -3993,8 +3994,12 @@ func walkCheckPtrArithmetic(n *Node, init *Nodes) *Node {
 		case OADD:
 			walk(n.Left)
 			walk(n.Right)
-		case OSUB, OANDNOT:
+		case OSUB:
 			walk(n.Left)
+		case OAND:
+			if n.Implicit() { // was OANDNOT
+				walk(n.Left)
+			}
 		case OCONVNOP:
 			if n.Left.Type.Etype == TUNSAFEPTR {
 				n.Left = cheapexpr(n.Left, init)
diff --git a/src/runtime/checkptr_test.go b/src/runtime/checkptr_test.go
index 8ab8a4937c..194cc1243a 100644
--- a/src/runtime/checkptr_test.go
+++ b/src/runtime/checkptr_test.go
@@ -27,6 +27,7 @@ func TestCheckPtr(t *testing.T) {
 		{"CheckPtrAlignmentPtr", "fatal error: checkptr: misaligned pointer conversion\n"},
 		{"CheckPtrAlignmentNoPtr", ""},
 		{"CheckPtrArithmetic", "fatal error: checkptr: pointer arithmetic result points to invalid allocation\n"},
+		{"CheckPtrArithmetic2", "fatal error: checkptr: pointer arithmetic result points to invalid allocation\n"},
 		{"CheckPtrSize", "fatal error: checkptr: converted pointer straddles multiple allocations\n"},
 		{"CheckPtrSmall", "fatal error: checkptr: pointer arithmetic computed bad pointer value\n"},
 	}
diff --git a/src/runtime/testdata/testprog/checkptr.go b/src/runtime/testdata/testprog/checkptr.go
index 45e6fb1aa5..e0a2794f4c 100644
--- a/src/runtime/testdata/testprog/checkptr.go
+++ b/src/runtime/testdata/testprog/checkptr.go
@@ -10,6 +10,7 @@ func init() {
 	register("CheckPtrAlignmentNoPtr", CheckPtrAlignmentNoPtr)
 	register("CheckPtrAlignmentPtr", CheckPtrAlignmentPtr)
 	register("CheckPtrArithmetic", CheckPtrArithmetic)
+	register("CheckPtrArithmetic2", CheckPtrArithmetic2)
 	register("CheckPtrSize", CheckPtrSize)
 	register("CheckPtrSmall", CheckPtrSmall)
 }
@@ -32,6 +33,13 @@ func CheckPtrArithmetic() {
 	sink2 = (*int)(unsafe.Pointer(i))
 }
 
+func CheckPtrArithmetic2() {
+	var x [2]int64
+	p := unsafe.Pointer(&x[1])
+	var one uintptr = 1
+	sink2 = unsafe.Pointer(uintptr(p) & ^one)
+}
+
 func CheckPtrSize() {
 	p := new(int64)
 	sink2 = p
diff --git a/test/fixedbugs/issue40917.go b/test/fixedbugs/issue40917.go
new file mode 100644
index 0000000000..2128be5eca
--- /dev/null
+++ b/test/fixedbugs/issue40917.go
@@ -0,0 +1,23 @@
+// run -gcflags=-d=checkptr
+
+// Copyright 2020 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 "unsafe"
+
+func main() {
+	var x [2]uint64
+	a := unsafe.Pointer(&x[1])
+
+	b := a
+	b = unsafe.Pointer(uintptr(b) + 2)
+	b = unsafe.Pointer(uintptr(b) - 1)
+	b = unsafe.Pointer(uintptr(b) &^ 1)
+
+	if a != b {
+		panic("pointer arithmetic failed")
+	}
+}