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cmd/compile: eliminate dead code in if statements after typechecking
This is a more thorough and cleaner fix than doing dead code elimination separately during inlining, escape analysis, and export. Unfortunately, it does add another full walk of the AST. The performance impact is very small, but not non-zero. If a label or goto is present in the dead code, it is not eliminated. This restriction can be removed once label/goto checking occurs much earlier in the compiler. In practice, it probably doesn't matter much. Updates #19699 Fixes #19705 name old alloc/op new alloc/op delta Template 39.2MB ± 0% 39.3MB ± 0% +0.28% (p=0.008 n=5+5) Unicode 29.8MB ± 0% 29.8MB ± 0% ~ (p=1.000 n=5+5) GoTypes 113MB ± 0% 113MB ± 0% -0.55% (p=0.008 n=5+5) SSA 1.25GB ± 0% 1.25GB ± 0% +0.02% (p=0.008 n=5+5) Flate 25.3MB ± 0% 25.3MB ± 0% -0.24% (p=0.032 n=5+5) GoParser 31.7MB ± 0% 31.8MB ± 0% +0.31% (p=0.008 n=5+5) Reflect 78.2MB ± 0% 78.3MB ± 0% ~ (p=0.421 n=5+5) Tar 26.6MB ± 0% 26.7MB ± 0% +0.21% (p=0.008 n=5+5) XML 42.2MB ± 0% 42.2MB ± 0% ~ (p=0.056 n=5+5) name old allocs/op new allocs/op delta Template 385k ± 0% 387k ± 0% +0.51% (p=0.016 n=5+5) Unicode 321k ± 0% 321k ± 0% ~ (p=1.000 n=5+5) GoTypes 1.14M ± 0% 1.14M ± 0% ~ (p=1.000 n=5+5) SSA 9.71M ± 0% 9.72M ± 0% +0.10% (p=0.008 n=5+5) Flate 234k ± 1% 234k ± 1% ~ (p=0.690 n=5+5) GoParser 315k ± 0% 317k ± 0% +0.71% (p=0.008 n=5+5) Reflect 980k ± 0% 983k ± 0% +0.30% (p=0.032 n=5+5) Tar 251k ± 0% 252k ± 0% +0.55% (p=0.016 n=5+5) XML 392k ± 0% 393k ± 0% +0.30% (p=0.008 n=5+5) Change-Id: Ia10ff4bbf5c6eae782582cc9cbc9785494d4fb83 Reviewed-on: https://go-review.googlesource.com/38773 Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com>
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@ -1469,22 +1469,8 @@ func (p *exporter) stmt(n *Node) {
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p.pos(n)
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p.stmtList(n.Ninit)
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p.expr(n.Left)
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nbody := n.Nbody
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rlist := n.Rlist
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if Isconst(n.Left, CTBOOL) {
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// if false { ... } or if true { ... }
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// Only export the taken branch.
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// This is more efficient,
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// and avoids trying to export
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// un-exportable nodes.
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if n.Left.Bool() {
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rlist = Nodes{}
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} else {
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nbody = Nodes{}
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}
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}
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p.stmtList(nbody)
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p.stmtList(rlist)
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p.stmtList(n.Nbody)
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p.stmtList(n.Rlist)
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case OFOR:
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p.op(OFOR)
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@ -690,20 +690,11 @@ func (e *EscState) esc(n *Node, parent *Node) {
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e.escassignSinkWhy(n, n, "too large for stack") // TODO category: tooLarge
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}
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if n.Op == OIF && Isconst(n.Left, CTBOOL) {
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// Don't examine dead code.
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if n.Left.Bool() {
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e.esclist(n.Nbody, n)
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} else {
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e.esclist(n.Rlist, n)
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}
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} else {
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e.esc(n.Left, n)
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e.esc(n.Right, n)
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e.esclist(n.Nbody, n)
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e.esclist(n.List, n)
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e.esclist(n.Rlist, n)
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}
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e.esc(n.Left, n)
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e.esc(n.Right, n)
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e.esclist(n.Nbody, n)
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e.esclist(n.List, n)
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e.esclist(n.Rlist, n)
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if n.Op == OFOR || n.Op == OFORUNTIL || n.Op == ORANGE {
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e.loopdepth--
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@ -279,16 +279,6 @@ func ishairy(n *Node, budget *int32, reason *string) bool {
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return true
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}
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if n.Op == OIF && Isconst(n.Left, CTBOOL) {
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var taken Nodes // statements for the branch that is always taken
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if n.Left.Bool() {
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taken = n.Nbody // then case
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} else {
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taken = n.Rlist // else case
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}
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return ishairylist(n.Ninit, budget, reason) || ishairylist(taken, budget, reason)
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}
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return ishairy(n.Left, budget, reason) || ishairy(n.Right, budget, reason) ||
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ishairylist(n.List, budget, reason) || ishairylist(n.Rlist, budget, reason) ||
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ishairylist(n.Ninit, budget, reason) || ishairylist(n.Nbody, budget, reason)
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@ -443,6 +443,9 @@ func Main(archInit func(*Arch)) {
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if nerrors != 0 {
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Curfn.Nbody.Set(nil) // type errors; do not compile
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}
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// Now that we've checked whether n terminates,
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// we can eliminate some obviously dead code.
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deadcode(Curfn)
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fcount++
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}
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}
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@ -3842,13 +3842,7 @@ func markbreak(n *Node, implicit *Node) {
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lab.SetHasBreak(true)
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}
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}
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case OFOR,
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OFORUNTIL,
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OSWITCH,
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OTYPESW,
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OSELECT,
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ORANGE:
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case OFOR, OFORUNTIL, OSWITCH, OTYPESW, OSELECT, ORANGE:
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implicit = n
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fallthrough
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default:
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@ -3883,8 +3877,7 @@ func markbreaklist(l Nodes, implicit *Node) {
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}
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}
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// Isterminating whether the Nodes list ends with a terminating
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// statement.
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// isterminating reports whether the Nodes list ends with a terminating statement.
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func (l Nodes) isterminating() bool {
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s := l.Slice()
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c := len(s)
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@ -3894,7 +3887,7 @@ func (l Nodes) isterminating() bool {
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return s[c-1].isterminating()
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}
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// Isterminating returns whether the node n, the last one in a
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// Isterminating reports whether the node n, the last one in a
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// statement list, is a terminating statement.
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func (n *Node) isterminating() bool {
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switch n.Op {
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@ -3906,11 +3899,7 @@ func (n *Node) isterminating() bool {
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case OBLOCK:
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return n.List.isterminating()
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case OGOTO,
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ORETURN,
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ORETJMP,
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OPANIC,
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OXFALL:
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case OGOTO, ORETURN, ORETJMP, OPANIC, OXFALL:
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return true
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case OFOR, OFORUNTIL:
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@ -3948,6 +3937,7 @@ func (n *Node) isterminating() bool {
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return false
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}
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// checkreturn makes sure that fn terminates appropriately.
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func checkreturn(fn *Node) {
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if fn.Type.Results().NumFields() != 0 && fn.Nbody.Len() != 0 {
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markbreaklist(fn.Nbody, nil)
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@ -3956,3 +3946,48 @@ func checkreturn(fn *Node) {
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}
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}
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}
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func deadcode(fn *Node) {
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deadcodeslice(fn.Nbody)
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}
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func deadcodeslice(nn Nodes) {
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for _, n := range nn.Slice() {
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if n == nil {
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continue
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}
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if n.Op == OIF && Isconst(n.Left, CTBOOL) {
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var dead *Nodes
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if n.Left.Bool() {
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dead = &n.Rlist
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} else {
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dead = &n.Nbody
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}
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// TODO(mdempsky/josharian): eliminate need for haslabelgoto
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// by checking labels and gotos earlier. See issue 19699.
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if !(*dead).haslabelgoto() {
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*dead = Nodes{}
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}
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}
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deadcodeslice(n.Ninit)
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deadcodeslice(n.Nbody)
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deadcodeslice(n.List)
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deadcodeslice(n.Rlist)
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}
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}
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// haslabelgoto reports whether the Nodes list contains any label or goto statements.
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func (l Nodes) haslabelgoto() bool {
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for _, n := range l.Slice() {
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if n == nil {
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continue
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}
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if n.Op == OLABEL || n.Op == OGOTO {
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return true
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}
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if n.Ninit.haslabelgoto() || n.Nbody.haslabelgoto() || n.List.haslabelgoto() || n.Rlist.haslabelgoto() {
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return true
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}
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}
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return false
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}
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12
test/fixedbugs/issue19699.dir/a.go
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12
test/fixedbugs/issue19699.dir/a.go
Normal file
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@ -0,0 +1,12 @@
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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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package a
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func F() {
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l1:
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if false {
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goto l1
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}
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}
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11
test/fixedbugs/issue19699.dir/b.go
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11
test/fixedbugs/issue19699.dir/b.go
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@ -0,0 +1,11 @@
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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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package main
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import "./a"
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func main() {
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a.F()
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}
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7
test/fixedbugs/issue19699.go
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7
test/fixedbugs/issue19699.go
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@ -0,0 +1,7 @@
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// compiledir
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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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package ignored
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14
test/fixedbugs/issue19699b.go
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14
test/fixedbugs/issue19699b.go
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@ -0,0 +1,14 @@
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// errorcheck
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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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package p
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func f() bool {
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if false {
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} else {
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return true
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}
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} // ERROR "missing return at end of function"
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17
test/fixedbugs/issue19705.go
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17
test/fixedbugs/issue19705.go
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@ -0,0 +1,17 @@
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// compile
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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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package p
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func f1() {
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f2()
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}
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func f2() {
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if false {
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_ = func() {}
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}
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}
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