[release-branch.go1.15] cmd/cgo: use go:notinheap for anonymous structs

They can't reasonably be allocated on the heap. Not a huge deal, but
it has an interesting and useful side effect.

After CL 249917, the compiler and runtime treat pointers to
go:notinheap types as uintptrs instead of real pointers (no write
barrier, not processed during stack scanning, ...). That feature is
exactly what we want for cgo to fix #40954. All the cases we have of
pointers declared in C, but which might actually be filled with
non-pointer data, are of this form (JNI's jobject heirarch, Darwin's
CFType heirarchy, ...).

Fixes #40954

Change-Id: I44a3b9bc2513d4287107e39d0cbbd0efd46a3aae
Reviewed-on: https://go-review.googlesource.com/c/go/+/250940
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/255321

1 files changed, 35 insertions, 0 deletions

diff --git a/test/fixedbugs/issue40954.go b/test/fixedbugs/issue40954.go
new file mode 100644
index 0000000000..53e9ccf387
--- /dev/null
+++ b/test/fixedbugs/issue40954.go
@@ -0,0 +1,35 @@
+// run
+
+// 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"
+)
+
+//go:notinheap
+type S struct{ x int }
+
+func main() {
+	var i int
+	p := (*S)(unsafe.Pointer(uintptr(unsafe.Pointer(&i))))
+	v := uintptr(unsafe.Pointer(p))
+	// p is a pointer to a go:notinheap type. Like some C libraries,
+	// we stored an integer in that pointer. That integer just happens
+	// to be the address of i.
+	// v is also the address of i.
+	// p has a base type which is marked go:notinheap, so it
+	// should not be adjusted when the stack is copied.
+	recurse(100, p, v)
+}
+func recurse(n int, p *S, v uintptr) {
+	if n > 0 {
+		recurse(n-1, p, v)
+	}
+	if uintptr(unsafe.Pointer(p)) != v {
+		panic("adjusted notinheap pointer")
+	}
+}