cmd/compile: fix bug in DWARF inl handling of unused autos

The DWARF inline info generation hooks weren't properly
handling unused auto vars in certain cases, triggering an assert (now
fixed). Also with this change, introduce a new autom "flavor" to
use for autom entries that are added to insure that a specific
auto type makes it into the linker (this is a follow-on to the fix
for 22941).

Fixes #22962.

Change-Id: I7a2d8caf47f6ca897b12acb6a6de0eb25f5cac8f
Reviewed-on: https://go-review.googlesource.com/81557
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -322,7 +322,11 @@ func debuginfo(fnsym *obj.LSym, curfn interface{}) ([]dwarf.Scope, dwarf.InlCall
 		switch n.Class() {
 		case PAUTO:
 			if !n.Name.Used() {
-				Fatalf("debuginfo unused node (AllocFrame should truncate fn.Func.Dcl)")
+				// Text == nil -> generating abstract function
+				if fnsym.Func.Text != nil {
+					Fatalf("debuginfo unused node (AllocFrame should truncate fn.Func.Dcl)")
+				}
+				continue
 			}
 			name = obj.NAME_AUTO
 		case PPARAM, PPARAMOUT:
@@ -558,15 +562,14 @@ func createDwarfVars(fnsym *obj.LSym, debugInfo *ssa.FuncDebug, automDecls []*No
 			InlIndex:      int32(inlIndex),
 			ChildIndex:    -1,
 		})
-		// Note: the auto that we're appending here is simply to insure
-		// that the DWARF type in question is picked up by the linker --
-		// there isn't a real auto variable with this name. This is
-		// to fix issue 22941.
+		// Append a "deleted auto" entry to the autom list so as to
+		// insure that the type in question is picked up by the linker.
+		// See issue 22941.
 		gotype := ngotype(n).Linksym()
 		fnsym.Func.Autom = append(fnsym.Func.Autom, &obj.Auto{
 			Asym:    Ctxt.Lookup(n.Sym.Name),
 			Aoffset: int32(-1),
-			Name:    obj.NAME_AUTO,
+			Name:    obj.NAME_DELETED_AUTO,
 			Gotype:  gotype,
 		})
 

src/cmd/internal/obj/link.go

@@ -208,6 +208,9 @@ const (
 	// A reference to name@GOT(SB) is a reference to the entry in the global offset
 	// table for 'name'.
 	NAME_GOTREF
+	// Indicates auto that was optimized away, but whose type
+	// we want to preserve in the DWARF debug info.
+	NAME_DELETED_AUTO
 )
 
 type AddrType uint8

src/cmd/internal/obj/objfile.go

@@ -351,6 +351,8 @@ func (w *objWriter) writeSym(s *LSym) {
 			w.writeInt(objabi.A_AUTO)
 		} else if a.Name == NAME_PARAM {
 			w.writeInt(objabi.A_PARAM)
+		} else if a.Name == NAME_DELETED_AUTO {
+			w.writeInt(objabi.A_DELETED_AUTO)
 		} else {
 			log.Fatalf("%s: invalid local variable type %d", s.Name, a.Name)
 		}

src/cmd/internal/objabi/autotype.go

@@ -34,4 +34,5 @@ package objabi
 const (
 	A_AUTO = 1 + iota
 	A_PARAM
+	A_DELETED_AUTO
 )

src/cmd/link/internal/ld/dwarf.go

@@ -857,7 +857,7 @@ func defdwsymb(ctxt *Link, s *sym.Symbol, str string, t SymbolType, v int64, got
 		}
 		fallthrough
 
-	case AutoSym, ParamSym:
+	case AutoSym, ParamSym, DeletedAutoSym:
 		dt = defgotype(ctxt, gotype)
 	}
 

src/cmd/link/internal/ld/lib.go

@@ -1964,6 +1964,7 @@ func doversion() {
 type SymbolType int8
 
 const (
+	// see also http://9p.io/magic/man2html/1/nm
 	TextSym      SymbolType = 'T'
 	DataSym                 = 'D'
 	BSSSym                  = 'B'
@@ -1972,6 +1973,9 @@ const (
 	FrameSym                = 'm'
 	ParamSym                = 'p'
 	AutoSym                 = 'a'
+
+	// Deleted auto (not a real sym, just placeholder for type)
+	DeletedAutoSym = 'x'
 )
 
 func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int64, *sym.Symbol)) {
@@ -2096,6 +2100,11 @@ func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int6
 			continue
 		}
 		for _, a := range s.FuncInfo.Autom {
+			if a.Name == objabi.A_DELETED_AUTO {
+				put(ctxt, nil, "", DeletedAutoSym, 0, a.Gotype)
+				continue
+			}
+
 			// Emit a or p according to actual offset, even if label is wrong.
 			// This avoids negative offsets, which cannot be encoded.
 			if a.Name != objabi.A_AUTO && a.Name != objabi.A_PARAM {

test/fixedbugs/issue22962.dir/a.go

@@ -0,0 +1,11 @@
+// Copyright 2017 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 a
+
+func F() {
+	if x := 0; false {
+		_ = x
+	}
+}

test/fixedbugs/issue22962.dir/b.go

@@ -0,0 +1,9 @@
+// Copyright 2017 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 b
+
+import "a"
+
+var V = func() { a.F() }

test/fixedbugs/issue22962.go

@@ -0,0 +1,7 @@
+// compiledir
+
+// Copyright 2017 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 ignored