cmd/compile: insert instrumentation more carefully in racewalk

Be more careful about inserting instrumentation in racewalk.
If the node being instrumented is an OAS, and it has a non-
empty Ninit, then append instrumentation to the Ninit list
rather than letting it be inserted before the OAS (and the
compilation of its init list).  This deals with the case that
the Ninit list defines a variable used in the RHS of the OAS.

Fixes #15091.

Change-Id: Iac91696d9104d07f0bf1bd3499bbf56b2e1ef073
Reviewed-on: https://go-review.googlesource.com/21771
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: David Chase <drchase@google.com>

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

@@ -737,6 +737,9 @@ func typefmt(t *Type, flag FmtFlag) string {
 			Fatalf("cannot use TDDDFIELD with old exporter")
 		}
 		return fmt.Sprintf("%v <%v> %v", Econv(t.Etype), t.Sym, t.DDDField())
+
+	case Txxx:
+		return "Txxx"
 	}
 
 	if fmtmode == FExp {

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

@@ -164,7 +164,13 @@ func instrumentnode(np **Node, init *Nodes, wr int, skip int) {
 				var outn Nodes
 				outn.Set(out)
 				instrumentnode(&ls[i], &outn, 0, 0)
-				out = append(outn.Slice(), ls[i])
+				if ls[i].Op != OAS || ls[i].Ninit.Len() == 0 {
+					out = append(outn.Slice(), ls[i])
+				} else {
+					// Splice outn onto end of ls[i].Ninit
+					ls[i].Ninit.AppendNodes(&outn)
+					out = append(out, ls[i])
+				}
 			}
 		}
 		n.List.Set(out)

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

@@ -3699,6 +3699,10 @@ func (s *state) resolveFwdRef(v *ssa.Value) {
 	if b == s.f.Entry {
 		// Live variable at start of function.
 		if s.canSSA(name) {
+			if strings.HasPrefix(name.Sym.Name, "autotmp_") {
+				// It's likely that this is an uninitialized variable in the entry block.
+				s.Fatalf("Treating auto as if it were arg, func %s, node %v, value %v", b.Func.Name, name, v)
+			}
 			v.Op = ssa.OpArg
 			v.Aux = name
 			return

src/cmd/compile/internal/ssa/regalloc.go

@@ -417,7 +417,7 @@ func (s *regAllocState) allocValToReg(v *Value, mask regMask, nospill bool, line
 		// Load v from its spill location.
 		case vi.spill != nil:
 			if s.f.pass.debug > logSpills {
-				s.f.Config.Warnl(vi.spill.Line, "load spill")
+				s.f.Config.Warnl(vi.spill.Line, "load spill for %v from %v", v, vi.spill)
 			}
 			c = s.curBlock.NewValue1(line, OpLoadReg, v.Type, vi.spill)
 			vi.spillUsed = true
@@ -1078,7 +1078,7 @@ func (s *regAllocState) regalloc(f *Func) {
 		vi := s.values[i]
 		if vi.spillUsed {
 			if s.f.pass.debug > logSpills {
-				s.f.Config.Warnl(vi.spill.Line, "spilled value")
+				s.f.Config.Warnl(vi.spill.Line, "spilled value at %v remains", vi.spill)
 			}
 			continue
 		}

test/fixedbugs/issue15091.go

@@ -0,0 +1,25 @@
+// errorcheck -0 -race
+
+// Copyright 2016 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 sample
+
+type Html struct {
+	headerIDs map[string]int
+}
+
+// We don't want to see:
+//    internal error: (*Html).xyzzy autotmp_3 (type *int) recorded as live on entry, p.Pc=0
+// or (now, with the error caught earlier)
+//    Treating auto as if it were arg, func (*Html).xyzzy, node ...
+// caused by racewalker inserting instrumentation before an OAS where the Ninit
+// of the OAS defines part of its right-hand-side. (I.e., the race instrumentation
+// references a variable before it is defined.)
+func (options *Html) xyzzy(id string) string {
+	for count, found := options.headerIDs[id]; found; count, found = options.headerIDs[id] {
+		_ = count
+	}
+	return ""
+}