Index: lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
===================================================================
--- lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
+++ lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -458,12 +458,11 @@
 /// the function, because it's never reached. However, in WebAssembly, blocks
 /// that end at the function end need to have a return type signature that
 /// matches the function signature, even though it's unreachable. This function
-/// checks for such cases and fixes up the signatures.
+/// checks for such cases and inserts an explicit unreachable.
 static void FixEndsAtEndOfFunction(
     MachineFunction &MF,
     const WebAssemblyFunctionInfo &MFI,
-    DenseMap<const MachineInstr *, MachineInstr *> &BlockTops,
-    DenseMap<const MachineInstr *, MachineInstr *> &LoopTops) {
+    const WebAssemblyInstrInfo &TII) {
   assert(MFI.getResults().size() <= 1);
 
   if (MFI.getResults().empty())
@@ -488,15 +487,14 @@
     for (MachineInstr &MI : reverse(MBB)) {
       if (MI.isPosition() || MI.isDebugValue())
         continue;
-      if (MI.getOpcode() == WebAssembly::END_BLOCK) {
-        BlockTops[&MI]->getOperand(0).setImm(int32_t(retType));
-        continue;
-      }
-      if (MI.getOpcode() == WebAssembly::END_LOOP) {
-        LoopTops[&MI]->getOperand(0).setImm(int32_t(retType));
-        continue;
+
+      if (MI.getOpcode() == WebAssembly::END_BLOCK ||
+          MI.getOpcode() == WebAssembly::END_LOOP) {
+        const MCInstrDesc &Desc = TII.get(WebAssembly::UNREACHABLE);
+        BuildMI(&MBB, MI.getDebugLoc(), Desc);
       }
-      // Something other than an `end`. We're done.
+
+      // Only need to consider the first non-debug machineinstr
       return;
     }
   }
@@ -568,7 +566,7 @@
 
   // Fix up block/loop signatures at the end of the function to conform to
   // WebAssembly's rules.
-  FixEndsAtEndOfFunction(MF, MFI, BlockTops, LoopTops);
+  FixEndsAtEndOfFunction(MF, MFI, TII);
 }
 
 bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
Index: test/CodeGen/WebAssembly/cfg-stackify.ll
===================================================================
--- test/CodeGen/WebAssembly/cfg-stackify.ll
+++ test/CodeGen/WebAssembly/cfg-stackify.ll
@@ -275,17 +275,19 @@
 ; CHECK-LABEL: minimal_loop:
 ; CHECK-NOT: br
 ; CHECK: .LBB7_1:
-; CHECK: loop i32
+; CHECK: loop
 ; CHECK: i32.store 0($0), $pop{{[0-9]+}}{{$}}
 ; CHECK: br 0{{$}}
 ; CHECK: .LBB7_2:
+; CHECK: unreachable{{$}}
 ; OPT-LABEL: minimal_loop:
 ; OPT-NOT: br
 ; OPT: .LBB7_1:
-; OPT: loop i32
+; OPT: loop
 ; OPT: i32.store 0($0), $pop{{[0-9]+}}{{$}}
 ; OPT: br 0{{$}}
 ; OPT: .LBB7_2:
+; OPT: unreachable{{$}}
 define i32 @minimal_loop(i32* %p) {
 entry:
   store volatile i32 0, i32* %p
@@ -402,7 +404,7 @@
 
 ; CHECK-LABEL: doublediamond_in_a_loop:
 ; CHECK: .LBB11_1:
-; CHECK: loop i32{{$}}
+; CHECK: loop   {{$}}
 ; CHECK: block   {{$}}
 ; CHECK: br_if           0, $0{{$}}
 ; CHECK: br              1{{$}}
@@ -415,9 +417,10 @@
 ; CHECK: br              0{{$}}
 ; CHECK: .LBB11_6:
 ; CHECK-NEXT: end_loop{{$}}
+; CHECK-NEXT: unreachable{{$}}
 ; OPT-LABEL: doublediamond_in_a_loop:
 ; OPT:      .LBB11_1:
-; OPT:      loop i32{{$}}
+; OPT:      loop   {{$}}
 ; OPT:      block   {{$}}
 ; OPT:      br_if           0, {{[^,]+}}{{$}}
 ; OPT:      block   {{$}}
@@ -431,6 +434,7 @@
 ; OPT:      br              0{{$}}
 ; OPT:      .LBB11_6:
 ; OPT-NEXT: end_loop{{$}}
+; OPT-NEXT: unreachable{{$}}
 define i32 @doublediamond_in_a_loop(i32 %a, i32 %b, i32* %p) {
 entry:
   br label %header
@@ -762,20 +766,22 @@
 
 ; CHECK-LABEL: test8:
 ; CHECK:       .LBB17_1:
-; CHECK-NEXT:  loop i32{{$}}
+; CHECK-NEXT:  loop   {{$}}
 ; CHECK-NEXT:  i32.const $push{{[^,]+}}, 0{{$}}
 ; CHECK-NEXT:  br_if    0, {{[^,]+}}{{$}}
 ; CHECK-NEXT:  br       0{{$}}
 ; CHECK-NEXT:  .LBB17_2:
 ; CHECK-NEXT:  end_loop{{$}}
+; CHECK-NEXT:  unreachable{{$}}
 ; OPT-LABEL: test8:
 ; OPT:       .LBB17_1:
-; OPT-NEXT:  loop i32{{$}}
+; OPT-NEXT:  loop   {{$}}
 ; OPT-NEXT:  i32.const $push{{[^,]+}}, 0{{$}}
 ; OPT-NEXT:  br_if    0, {{[^,]+}}{{$}}
 ; OPT-NEXT:  br       0{{$}}
 ; OPT-NEXT:  .LBB17_2:
 ; OPT-NEXT:  end_loop{{$}}
+; OPT-NEXT:  unreachable{{$}}
 define i32 @test8() {
 bb:
   br label %bb1
@@ -1334,3 +1340,44 @@
 bb15:
   ret void
 }
+
+; Test that nested loops and blocks typecheck correctly.
+
+; CHECK-LABEL: test16
+; CHECK:      loop
+; CHECK:      loop
+; CHECK-NEXT: block
+; CHECK:      end_block{{$}}
+; CHECK:      end_loop{{$}}
+; CHECK-NEXT: end_loop{{$}}
+; CHECK-NEXT: unreachable{{$}}
+; CHECK-NEXT: .endfunc{{$}}
+; OPT-LABEL: test16
+; OPT:      loop
+; OPT:      loop
+; OPT-NEXT: block
+; OPT:      end_block{{$}}
+; OPT:      end_loop{{$}}
+; OPT-NEXT: end_loop{{$}}
+; OPT-NEXT: unreachable{{$}}
+; OPT-NEXT: .endfunc{{$}}
+@test16global.ptr = global i32 0
+define hidden i32 @test16() {
+entry:
+  br label %L1
+
+L1:
+  %x = load i32, i32* @test16global.ptr
+  %y = add i32 %x, 1
+  %cmp = icmp ne i32 %x, 2
+  br i1 %cmp, label %L2, label %L3
+
+L2:
+  store i32 %y, i32* @test16global.ptr
+  br i1 %cmp, label %L1, label %L3
+
+L3:
+  %z = add i32 %y, 3
+  store i32 %z, i32* @test16global.ptr
+  br label %L2
+}