diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index 220877242cd..796d899f7cf 100644
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -253,6 +253,7 @@ func checkFunc(f *Func) {
 		// Note: regalloc introduces non-dominating args.
 		// See TODO in regalloc.go.
 		idom := dominators(f)
+		sdom := newSparseTree(f, idom)
 		for _, b := range f.Blocks {
 			for _, v := range b.Values {
 				for i, arg := range v.Args {
@@ -261,12 +262,12 @@ func checkFunc(f *Func) {
 					if v.Op == OpPhi {
 						y = b.Preds[i]
 					}
-					if !domCheck(f, idom, x, y) {
+					if !domCheck(f, sdom, x, y) {
 						f.Fatalf("arg %d of value %s does not dominate, arg=%s", i, v.LongString(), arg.LongString())
 					}
 				}
 			}
-			if b.Control != nil && !domCheck(f, idom, b.Control.Block, b) {
+			if b.Control != nil && !domCheck(f, sdom, b.Control.Block, b) {
 				f.Fatalf("control value %s for %s doesn't dominate", b.Control, b)
 			}
 		}
@@ -274,18 +275,10 @@ func checkFunc(f *Func) {
 }
 
 // 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 {
+func domCheck(f *Func, sdom sparseTree, x, y *Block) bool {
+	if !sdom.isAncestorEq(y, f.Entry) {
 		// unreachable - ignore
 		return true
 	}
-	for {
-		if x == y {
-			return true
-		}
-		y = idom[y.ID]
-		if y == nil {
-			return false
-		}
-	}
+	return sdom.isAncestorEq(x, y)
 }