test: stress test for stack objects

Allocate a long linked list on the stack. This tests both
lots of live stack objects, and lots of intra-stack pointers
to those objects.

Change-Id: I169e067416455737774851633b1e5367e10e1cf2
Reviewed-on: https://go-review.googlesource.com/c/135296
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/stackobj2.go

+// run
+
+// Copyright 2018 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"
+	"runtime"
+)
+
+// linked list up the stack, to test lots of stack objects.
+
+type T struct {
+	// points to a heap object. Test will make sure it isn't freed.
+	data *int64
+	// next pointer for a linked list of stack objects
+	next *T
+	// duplicate of next, to stress test the pointer buffers
+	// used during stack tracing.
+	next2 *T
+}
+
+func main() {
+	makelist(nil, 10000)
+}
+
+func makelist(x *T, n int64) {
+	if n%2 != 0 {
+		panic("must be multiple of 2")
+	}
+	if n == 0 {
+		runtime.GC()
+		i := int64(1)
+		for ; x != nil; x, i = x.next, i+1 {
+			// Make sure x.data hasn't been collected.
+			if got := *x.data; got != i {
+				panic(fmt.Sprintf("bad data want %d, got %d", i, got))
+			}
+		}
+		return
+	}
+	// Put 2 objects in each frame, to test intra-frame pointers.
+	// Use both orderings to ensure the linked list isn't always in address order.
+	var a, b T
+	if n%3 == 0 {
+		a.data = newInt(n)
+		a.next = x
+		a.next2 = x
+		b.data = newInt(n - 1)
+		b.next = &a
+		b.next2 = &a
+		x = &b
+	} else {
+		b.data = newInt(n)
+		b.next = x
+		b.next2 = x
+		a.data = newInt(n - 1)
+		a.next = &b
+		a.next2 = &b
+		x = &a
+	}
+
+	makelist(x, n-2)
+}
+
+// big enough and pointer-y enough to not be tinyalloc'd
+type NotTiny struct {
+	n int64
+	p *byte
+}
+
+// newInt allocates n on the heap and returns a pointer to it.
+func newInt(n int64) *int64 {
+	h := &NotTiny{n: n}
+	p := &h.n
+	escape = p
+	return p
+}
+
+var escape *int64