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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>
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Original file line number | Diff line number | Diff line change |
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// run | ||
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// 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. | ||
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package main | ||
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import ( | ||
"unsafe" | ||
) | ||
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//go:notinheap | ||
type S struct{ x int } | ||
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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") | ||
} | ||
} |