diff --git a/llvm/test/tools/llvm-diff/phinode.ll b/llvm/test/tools/llvm-diff/phinode.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-diff/phinode.ll
@@ -0,0 +1,28 @@
+; RUN: rm -f %t.ll
+; RUN: cat %s | sed -e 's/ 0, %2 / 1, %2 /' > %t.ll
+; RUN: not llvm-diff %s %t.ll 2>&1 | FileCheck %s
+
+; CHECK:       in function foo:
+; CHECK-NEXT:   in block %6 / %6:
+; CHECK-NEXT:    >   %7 = phi i32 [ 1, %2 ], [ -1, %1 ]
+; CHECK-NEXT:    >   ret i32 %7
+; CHECK-NEXT:    <   %7 = phi i32 [ 0, %2 ], [ -1, %1 ]
+; CHECK-NEXT:    <   ret i32 %7
+define i32 @foo(i32 %0) #0 {
+  callbr void asm sideeffect "", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@foo, %6))
+          to label %2 [label %6]
+
+2:
+  %3 = icmp eq i32 %0, 0
+  br i1 %3, label %6, label %4
+
+4:
+  br label %5
+
+5:
+  br label %5
+
+6:
+  %7 = phi i32 [ 0, %2 ], [ -1, %1 ]
+  ret i32 %7
+}
diff --git a/llvm/tools/llvm-diff/lib/DifferenceEngine.cpp b/llvm/tools/llvm-diff/lib/DifferenceEngine.cpp
--- a/llvm/tools/llvm-diff/lib/DifferenceEngine.cpp
+++ b/llvm/tools/llvm-diff/lib/DifferenceEngine.cpp
@@ -269,15 +269,35 @@
     } else if (isa<CallInst>(L)) {
       return diffCallSites(cast<CallInst>(*L), cast<CallInst>(*R), Complain);
     } else if (isa<PHINode>(L)) {
-      // FIXME: implement.
+      const PHINode &LI = cast<PHINode>(*L);
+      const PHINode &RI = cast<PHINode>(*R);
 
       // This is really weird;  type uniquing is broken?
-      if (L->getType() != R->getType()) {
-        if (!L->getType()->isPointerTy() || !R->getType()->isPointerTy()) {
+      if (LI.getType() != RI.getType()) {
+        if (!LI.getType()->isPointerTy() || !RI.getType()->isPointerTy()) {
           if (Complain) Engine.log("different phi types");
           return true;
         }
       }
+
+      if (LI.getNumIncomingValues() != RI.getNumIncomingValues()) {
+        if (Complain)
+          Engine.log("PHI node # of incoming values differ");
+        return true;
+      }
+
+      for (unsigned I = 0; I < LI.getNumIncomingValues(); ++I) {
+        if (TryUnify)
+          tryUnify(LI.getIncomingBlock(I), RI.getIncomingBlock(I));
+
+        if (!equivalentAsOperands(LI.getIncomingValue(I),
+                                  RI.getIncomingValue(I))) {
+          if (Complain)
+            Engine.log("PHI node incoming values differ");
+          return true;
+        }
+      }
+
       return false;
 
     // Terminators.