diff --git a/src/cmd/compile/internal/ssa/lca_test.go b/src/cmd/compile/internal/ssa/lca_test.go
new file mode 100644
index 0000000..beb33e0
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/lca_test.go
@@ -0,0 +1,103 @@
+// 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 ssa
+
+import "testing"
+
+type lca interface {
+	find(a, b *Block) *Block
+}
+
+func lcaEqual(f *Func, lca1, lca2 lca) bool {
+	for _, b := range f.Blocks {
+		for _, c := range f.Blocks {
+			if lca1.find(b, c) != lca2.find(b, c) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+func testLCAgen(t *testing.T, bg blockGen, size int) {
+	c := NewConfig("amd64", DummyFrontend{t}, nil, true)
+	fun := Fun(c, "entry", bg(size)...)
+	CheckFunc(fun.f)
+	if size == 4 {
+		t.Logf(fun.f.String())
+	}
+	lca1 := makeLCArange(fun.f)
+	lca2 := makeLCAeasy(fun.f)
+	for _, b := range fun.f.Blocks {
+		for _, c := range fun.f.Blocks {
+			l1 := lca1.find(b, c)
+			l2 := lca2.find(b, c)
+			if l1 != l2 {
+				t.Errorf("lca(%s,%s)=%s, want %s", b, c, l1, l2)
+			}
+		}
+	}
+}
+
+func TestLCALinear(t *testing.T) {
+	testLCAgen(t, genLinear, 10)
+	testLCAgen(t, genLinear, 100)
+}
+
+func TestLCAFwdBack(t *testing.T) {
+	testLCAgen(t, genFwdBack, 10)
+	testLCAgen(t, genFwdBack, 100)
+}
+
+func TestLCAManyPred(t *testing.T) {
+	testLCAgen(t, genManyPred, 10)
+	testLCAgen(t, genManyPred, 100)
+}
+
+func TestLCAMaxPred(t *testing.T) {
+	testLCAgen(t, genMaxPred, 10)
+	testLCAgen(t, genMaxPred, 100)
+}
+
+func TestLCAMaxPredValue(t *testing.T) {
+	testLCAgen(t, genMaxPredValue, 10)
+	testLCAgen(t, genMaxPredValue, 100)
+}
+
+// Simple implementation of LCA to compare against.
+type lcaEasy struct {
+	parent []*Block
+}
+
+func makeLCAeasy(f *Func) *lcaEasy {
+	return &lcaEasy{parent: dominators(f)}
+}
+
+func (lca *lcaEasy) find(a, b *Block) *Block {
+	da := lca.depth(a)
+	db := lca.depth(b)
+	for da > db {
+		da--
+		a = lca.parent[a.ID]
+	}
+	for da < db {
+		db--
+		b = lca.parent[b.ID]
+	}
+	for a != b {
+		a = lca.parent[a.ID]
+		b = lca.parent[b.ID]
+	}
+	return a
+}
+
+func (lca *lcaEasy) depth(b *Block) int {
+	n := 0
+	for b != nil {
+		b = lca.parent[b.ID]
+		n++
+	}
+	return n
+}