diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index 61626089a6..ad9222f3e2 100644
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -181,4 +181,45 @@ func checkFunc(f *Func) {
 			f.Fatalf("used value v%d in free list", id)
 		}
 	}
+
+	// Check to make sure all args dominate uses.
+	if f.RegAlloc == nil {
+		// Note: regalloc introduces non-dominating args.
+		// See TODO in regalloc.go.
+		idom := dominators(f)
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				for i, arg := range v.Args {
+					x := arg.Block
+					y := b
+					if v.Op == OpPhi {
+						y = b.Preds[i]
+					}
+					if !domCheck(f, idom, x, y) {
+						f.Fatalf("arg %d of value %s does not dominate", i, v.LongString())
+					}
+				}
+			}
+			if b.Control != nil && !domCheck(f, idom, b.Control.Block, b) {
+				f.Fatalf("control value %s for %s doesn't dominate", b.Control, b)
+			}
+		}
+	}
+}
+
+// domCheck reports whether x dominates y (including x==y).
+func domCheck(f *Func, idom []*Block, x, y *Block) bool {
+	if y != f.Entry && idom[y.ID] == nil {
+		// unreachable - ignore
+		return true
+	}
+	for {
+		if x == y {
+			return true
+		}
+		y = idom[y.ID]
+		if y == nil {
+			return false
+		}
+	}
 }
diff --git a/src/cmd/compile/internal/ssa/phielim.go b/src/cmd/compile/internal/ssa/phielim.go
index 19c0d077e5..be9503248b 100644
--- a/src/cmd/compile/internal/ssa/phielim.go
+++ b/src/cmd/compile/internal/ssa/phielim.go
@@ -11,33 +11,27 @@ package ssa
 //   v = phi(x,x,x)
 //   v = phi(x,v,x,v)
 func phielim(f *Func) {
-	args := newSparseSet(f.NumValues())
+	argSet := newSparseSet(f.NumValues())
+	var args []*Value
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if v.Op != OpPhi {
 				continue
 			}
-			args.clear()
+			argSet.clear()
+			args = args[:0]
 			for _, x := range v.Args {
 				for x.Op == OpCopy {
 					x = x.Args[0]
 				}
-				args.add(x.ID)
-			}
-			switch {
-			case args.size() == 1:
-				v.Op = OpCopy
-				v.SetArgs1(v.Args[0])
-			case args.size() == 2 && args.contains(v.ID):
-				var w *Value
-				for _, x := range v.Args {
-					if x.ID != v.ID {
-						w = x
-						break
-					}
+				if x != v && !argSet.contains(x.ID) {
+					argSet.add(x.ID)
+					args = append(args, x)
 				}
+			}
+			if len(args) == 1 {
 				v.Op = OpCopy
-				v.SetArgs1(w)
+				v.SetArgs1(args[0])
 			}
 		}
 	}