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

diff --git a/src/cmd/compile/internal/gc/fmt.go b/src/cmd/compile/internal/gc/fmt.go
index 5c55036..19f1090 100644
--- a/src/cmd/compile/internal/gc/fmt.go
+++ b/src/cmd/compile/internal/gc/fmt.go
@@ -737,6 +737,9 @@
 			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

diff --git a/src/cmd/compile/internal/gc/racewalk.go b/src/cmd/compile/internal/gc/racewalk.go
index 09889a4..f6e6514 100644
--- a/src/cmd/compile/internal/gc/racewalk.go
+++ b/src/cmd/compile/internal/gc/racewalk.go
@@ -164,7 +164,13 @@
 				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

diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index 90c4d4e..7c5f906 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -3699,6 +3699,10 @@
 	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

diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index 22b9d12..aec23a1 100644
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -417,7 +417,7 @@
 		// 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 @@
 		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
 		}