[dev.ssa] cmd/compile: unify and check LoweredGetClosurePtr

The comments were mostly duplicated; unify them.
Add a check that the required invariant holds.

Change-Id: I42fe09dcd1fac76d3c4e191f7a58c591c5ce429b
Reviewed-on: https://go-review.googlesource.com/24719
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/amd64/ssa.go

@@ -720,11 +720,8 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 	case ssa.OpArg:
 		// input args need no code
 	case ssa.OpAMD64LoweredGetClosurePtr:
-		// Output is hardwired to DX only,
-		// and DX contains the closure pointer on
-		// closure entry, and this "instruction"
-		// is scheduled to the very beginning
-		// of the entry block.
+		// Closure pointer is DX.
+		gc.CheckLoweredGetClosurePtr(v)
 	case ssa.OpAMD64LoweredGetG:
 		r := gc.SSARegNum(v)
 		// See the comments in cmd/internal/obj/x86/obj6.go

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

@@ -708,12 +708,8 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		ssa.OpARMLoweredSelect1:
 		// nothing to do
 	case ssa.OpARMLoweredGetClosurePtr:
-		// Output is hardwired to R7 (arm.REGCTXT) only,
-		// and R7 contains the closure pointer on
-		// closure entry, and this "instruction"
-		// is scheduled to the very beginning
-		// of the entry block.
-		// nothing to do here.
+		// Closure pointer is R7 (arm.REGCTXT).
+		gc.CheckLoweredGetClosurePtr(v)
 	default:
 		v.Unimplementedf("genValue not implemented: %s", v.LongString())
 	}

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

@@ -4281,6 +4281,16 @@ func CheckLoweredPhi(v *ssa.Value) {
 	}
 }
 
+// CheckLoweredGetClosurePtr checks that v is the first instruction in the function's entry block.
+// The output of LoweredGetClosurePtr is generally hardwired to the correct register.
+// That register contains the closure pointer on closure entry.
+func CheckLoweredGetClosurePtr(v *ssa.Value) {
+	entry := v.Block.Func.Entry
+	if entry != v.Block || entry.Values[0] != v {
+		Fatalf("badly placed LoweredGetClosurePtr: %v %v", v.Block, v)
+	}
+}
+
 // AutoVar returns a *Node and int64 representing the auto variable and offset within it
 // where v should be spilled.
 func AutoVar(v *ssa.Value) (*Node, int64) {