cmd/compile: fix panic with dead hidden closures

Currently, for hidden closures, we always push them to compile queue
during typechecking. If the hidden closure is discarded from the outer
function body during deadcode, any desugaring phase after deadcode won't
be applied to the closure. Thus, some un-expected OPs are passed to
downstream passes, which they can't handle, the compiler goes boom!

To fix this, we keep track of discarded hidden closures during deadcode
pass, and won't compile them then.

Fixes #47712

Change-Id: I078717d5d1f4f2fa39cbaf610cfffbb042e70ceb
Reviewed-on: https://go-review.googlesource.com/c/go/+/342350
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/deadcode/deadcode.go

@@ -38,6 +38,7 @@ func Func(fn *ir.Func) {
 		}
 	}
 
+	ir.VisitList(fn.Body, markHiddenClosureDead)
 	fn.Body = []ir.Node{ir.NewBlockStmt(base.Pos, nil)}
 }
 
@@ -62,9 +63,11 @@ func stmts(nn *ir.Nodes) {
 			if ir.IsConst(n.Cond, constant.Bool) {
 				var body ir.Nodes
 				if ir.BoolVal(n.Cond) {
+					ir.VisitList(n.Else, markHiddenClosureDead)
 					n.Else = ir.Nodes{}
 					body = n.Body
 				} else {
+					ir.VisitList(n.Body, markHiddenClosureDead)
 					n.Body = ir.Nodes{}
 					body = n.Else
 				}
@@ -150,3 +153,13 @@ func expr(n ir.Node) ir.Node {
 	}
 	return n
 }
+
+func markHiddenClosureDead(n ir.Node) {
+	if n.Op() != ir.OCLOSURE {
+		return
+	}
+	clo := n.(*ir.ClosureExpr)
+	if clo.Func.IsHiddenClosure() {
+		clo.Func.SetIsDeadcodeClosure(true)
+	}
+}

src/cmd/compile/internal/gc/main.go

@@ -289,6 +289,10 @@ func Main(archInit func(*ssagen.ArchInfo)) {
 	fcount := int64(0)
 	for i := 0; i < len(typecheck.Target.Decls); i++ {
 		if fn, ok := typecheck.Target.Decls[i].(*ir.Func); ok {
+			// Don't try compiling dead hidden closure.
+			if fn.IsDeadcodeClosure() {
+				continue
+			}
 			enqueueFunc(fn)
 			fcount++
 		}

src/cmd/compile/internal/ir/func.go

@@ -196,6 +196,7 @@ const (
 	// true if closure inside a function; false if a simple function or a
 	// closure in a global variable initialization
 	funcIsHiddenClosure
+	funcIsDeadcodeClosure        // true if closure is deadcode
 	funcHasDefer                 // contains a defer statement
 	funcNilCheckDisabled         // disable nil checks when compiling this function
 	funcInlinabilityChecked      // inliner has already determined whether the function is inlinable
@@ -216,6 +217,7 @@ func (f *Func) ABIWrapper() bool               { return f.flags&funcABIWrapper !
 func (f *Func) Needctxt() bool                 { return f.flags&funcNeedctxt != 0 }
 func (f *Func) ReflectMethod() bool            { return f.flags&funcReflectMethod != 0 }
 func (f *Func) IsHiddenClosure() bool          { return f.flags&funcIsHiddenClosure != 0 }
+func (f *Func) IsDeadcodeClosure() bool        { return f.flags&funcIsDeadcodeClosure != 0 }
 func (f *Func) HasDefer() bool                 { return f.flags&funcHasDefer != 0 }
 func (f *Func) NilCheckDisabled() bool         { return f.flags&funcNilCheckDisabled != 0 }
 func (f *Func) InlinabilityChecked() bool      { return f.flags&funcInlinabilityChecked != 0 }
@@ -230,6 +232,7 @@ func (f *Func) SetABIWrapper(b bool)               { f.flags.set(funcABIWrapper,
 func (f *Func) SetNeedctxt(b bool)                 { f.flags.set(funcNeedctxt, b) }
 func (f *Func) SetReflectMethod(b bool)            { f.flags.set(funcReflectMethod, b) }
 func (f *Func) SetIsHiddenClosure(b bool)          { f.flags.set(funcIsHiddenClosure, b) }
+func (f *Func) SetIsDeadcodeClosure(b bool)        { f.flags.set(funcIsDeadcodeClosure, b) }
 func (f *Func) SetHasDefer(b bool)                 { f.flags.set(funcHasDefer, b) }
 func (f *Func) SetNilCheckDisabled(b bool)         { f.flags.set(funcNilCheckDisabled, b) }
 func (f *Func) SetInlinabilityChecked(b bool)      { f.flags.set(funcInlinabilityChecked, b) }

test/fixedbugs/issue47712.go

@@ -0,0 +1,23 @@
+// compile
+
+// Copyright 2021 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 p
+
+func f() {
+	if false {
+		defer func() {
+			_ = recover()
+		}()
+	}
+}
+
+func g() {
+	for false {
+		defer func() {
+			_ = recover()
+		}()
+	}
+}