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https://github.com/golang/go
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a51e4cc9ce
Move the zeroing of results earlier. In particular, they need to
come before any move-to-heap operations, as those require allocation.
Those allocations are points at which the GC can see the uninitialized
result slots.
For the function:
func f() (x, y, z *int) {
defer(){}()
escape(&y)
return
}
We used to generate code like this:
x = nil
y = nil
&y = new(int)
z = nil
Now we will generate:
x = nil
y = nil
z = nil
&y = new(int)
Since the fix for #18860, the return slots are always live if there
is a defer, so the former ordering allowed the GC to see junk
in the z slot.
Fixes #19078
Change-Id: I71554ae437549725bb79e13b2c100b2911d47ed4
Reviewed-on: https://go-review.googlesource.com/38133
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
43 lines
1.2 KiB
Go
43 lines
1.2 KiB
Go
// run
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// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Issue 19078: liveness & zero-initialization of results
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// when there is a defer.
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package main
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import "unsafe"
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func main() {
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// Construct an invalid pointer. We do this by
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// making a pointer which points to the unused space
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// between the last 48-byte object in a span and the
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// end of the span (there are 32 unused bytes there).
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p := new([48]byte) // make a 48-byte object
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sink = &p // escape it, so it allocates for real
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u := uintptr(unsafe.Pointer(p)) // get its address
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u = u >> 13 << 13 // round down to page size
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u += 1<<13 - 1 // add almost a page
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for i := 0; i < 1000000; i++ {
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_ = identity(u) // installs u at return slot
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_ = liveReturnSlot(nil) // incorrectly marks return slot as live
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}
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}
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//go:noinline
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func liveReturnSlot(x *int) *int {
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defer func() {}() // causes return slot to be marked live
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sink = &x // causes x to be moved to the heap, triggering allocation
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return x
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}
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//go:noinline
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func identity(x uintptr) uintptr {
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return x
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}
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var sink interface{}
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