cmd/compile: mark ... argument to checkptrArithmetic as not escaping

Fixes #36516

Change-Id: Ibf4f86fb3a25fa30e0cd54e2dd2e12c60ee75ddb
Reviewed-on: https://go-review.googlesource.com/c/go/+/214679
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

2 files changed, 37 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index ef88db4742..b8b954c4fc 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -1705,7 +1705,6 @@ func mkdotargslice(typ *types.Type, args []*Node, init *Nodes, ddd *Node) *Node
 	if ddd != nil {
 		esc = ddd.Esc
 	}
-
 	if len(args) == 0 {
 		n := nodnil()
 		n.Type = typ
@@ -1740,6 +1739,9 @@ func walkCall(n *Node, init *Nodes) {
 	// then assign the remaining arguments as a slice.
 	if nf := params.NumFields(); nf > 0 {
 		if last := params.Field(nf - 1); last.IsDDD() && !n.IsDDD() {
+			// The callsite does not use a ..., but the called function is declared
+			// with a final argument that has a ... . Build the slice that we will
+			// pass as the ... argument.
 			tail := args[nf-1:]
 			slice := mkdotargslice(last.Type, tail, init, n.Right)
 			// Allow immediate GC.
@@ -4067,11 +4069,15 @@ func walkCheckPtrArithmetic(n *Node, init *Nodes) *Node {
 
 	n = cheapexpr(n, init)
 
-	slice := mkdotargslice(types.NewSlice(types.Types[TUNSAFEPTR]), originals, init, nil)
-	slice.Esc = EscNone
-	slice.SetTransient(true)
+	ddd := nodl(n.Pos, ODDDARG, nil, nil)
+	ddd.Type = types.NewPtr(types.NewArray(types.Types[TUNSAFEPTR], int64(len(originals))))
+	ddd.Esc = EscNone
+	slice := mkdotargslice(types.NewSlice(types.Types[TUNSAFEPTR]), originals, init, ddd)
 
 	init.Append(mkcall("checkptrArithmetic", nil, init, convnop(n, types.Types[TUNSAFEPTR]), slice))
+	// TODO(khr): Mark backing store of slice as dead. This will allow us to reuse
+	// the backing store for multiple calls to checkptrArithmetic.
+
 	return n
 }
 
diff --git a/test/fixedbugs/issue36516.go b/test/fixedbugs/issue36516.go
new file mode 100644
index 0000000000..1472d4c222
--- /dev/null
+++ b/test/fixedbugs/issue36516.go
@@ -0,0 +1,27 @@
+// +build linux,amd64
+// run -race
+
+// 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 (
+	"fmt"
+	"testing"
+	"unsafe"
+)
+
+var buf [2]byte
+var x unsafe.Pointer = unsafe.Pointer(&buf[0])
+
+func main() {
+	n := testing.AllocsPerRun(1000, func() {
+		x = unsafe.Pointer(uintptr(x) + 1)
+		x = unsafe.Pointer(uintptr(x) - 1)
+	})
+	if n > 0 {
+		panic(fmt.Sprintf("too many allocations; want 0 got %f", n))
+	}
+}