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1. On entry to a function, only zero the ambiguously live stack variables. Before, we were zeroing all stack variables containing pointers. The zeroing is pretty inefficient right now (issue 7624), but there are also too many stack variables detected as ambiguously live (issue 7345), and that must be addressed before deciding how to improve the zeroing code. (Changes in 5g/ggen.c, 6g/ggen.c, 8g/ggen.c, gc/pgen.c) Fixes #7647. 2. Make the regopt word-based liveness analysis preserve the whole-variable liveness property expected by the garbage collection bitmap liveness analysis. That is, if the regopt liveness decides that one word in a struct needs to be preserved, make sure it preserves the entire struct. This is particularly important for multiword values such as strings, slices, and interfaces, in which all the words need to be present in order to understand the meaning. (Changes in 5g/reg.c, 6g/reg.c, 8g/reg.c.) Fixes #7591. 3. Make the regopt word-based liveness analysis treat a variable as having its address taken - which makes it preserved across all future calls - whenever n->addrtaken is set, for consistency with the gc bitmap liveness analysis, even if there is no machine instruction actually taking the address. In this case n->addrtaken is incorrect (a nicer way to put it is overconservative), and ideally there would be no such cases, but they can happen and the two analyses need to agree. (Changes in 5g/reg.c, 6g/reg.c, 8g/reg.c; test in bug484.go.) Fixes crashes found by turning off "zero everything" in step 1. 4. Remove spurious VARDEF annotations. As the comment in gc/pgen.c explains, the VARDEF must immediately precede the initialization. It cannot be too early, and it cannot be too late. In particular, if a function call sits between the VARDEF and the actual machine instructions doing the initialization, the variable will be treated as live during that function call even though it is uninitialized, leading to problems. (Changes in gc/gen.c; test in live.go.) Fixes crashes found by turning off "zero everything" in step 1. 5. Do not treat loading the address of a wide value as a signal that the value must be initialized. Instead depend on the existence of a VARDEF or the first actual read/write of a word in the value. If the load is in order to pass the address to a function that does the actual initialization, treating the load as an implicit VARDEF causes the same problems as described in step 4. The alternative is to arrange to zero every such value before passing it to the real initialization function, but this is a much easier and more efficient change. (Changes in gc/plive.c.) Fixes crashes found by turning off "zero everything" in step 1. 6. Treat wide input parameters with their address taken as initialized on entry to the function. Otherwise they look "ambiguously live" and we will try to emit code to zero them. (Changes in gc/plive.c.) Fixes crashes found by turning off "zero everything" in step 1. 7. An array of length 0 has no pointers, even if the element type does. Without this change, the zeroing code complains when asked to clear a 0-length array. (Changes in gc/reflect.c.) LGTM=khr R=khr CC=golang-codereviews https://golang.org/cl/80160044
91 lines
1.8 KiB
Go
91 lines
1.8 KiB
Go
// run
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// Copyright 2014 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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// The liveness code used to say that, in func g, s was live
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// starting at its declaration, because it appears to have its
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// address taken by the closure (different s, but the parser
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// gets slightly confused, a separate bug). The liveness analysis
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// saw s as having its address taken but the register optimizer
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// did not. This mismatch meant that s would be marked live
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// (and therefore initialized) at the call to f, but the register optimizer
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// would optimize away the initialization of s before f, causing the
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// garbage collector to use unused data.
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// The register optimizer has been changed to respect the
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// same "address taken" flag that the liveness analysis uses,
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// even if it cannot see any address being taken in the actual
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// machine code. This is conservative but keeps the two consistent,
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// which is the most important thing.
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package main
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import "runtime"
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var c bool
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func f() interface{} {
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if c { // disable inlining
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f()
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}
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runtime.GC()
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return nil
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}
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func g() {
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if c { // disable inlining
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g()
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}
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var s interface{}
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_ = func() {
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s := f()
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_ = s
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}
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s = f()
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useiface(s)
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useiface(s)
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}
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func useiface(x interface{}) {
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if c { // disable inlining
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useiface(x)
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}
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}
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func h() {
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if c { // disable inlining
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h()
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}
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var x [16]uintptr
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for i := range x {
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x[i] = 1
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}
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useint(x[0])
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useint(x[1])
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useint(x[2])
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useint(x[3])
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}
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func useint(x uintptr) {
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if c { // disable inlining
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useint(x)
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}
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}
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func main() {
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// scribble non-zero values on stack
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h()
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// call function that used to let the garbage collector
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// see uninitialized stack values; it will see the
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// nonzero values.
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g()
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}
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func big(x int) {
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if x >= 0 {
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big(x-1)
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}
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}
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