Index: llvm/trunk/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
===================================================================
--- llvm/trunk/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
+++ llvm/trunk/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
@@ -85,16 +85,16 @@
       break;
     case WebAssembly::END_LOOP:
     case WebAssembly::END_LOOP_S:
-      // Have to guard against an empty stack, in case of mismatched pairs
-      // in assembly parsing.
-      if (!ControlFlowStack.empty())
-        ControlFlowStack.pop_back();
+      if (ControlFlowStack.empty())
+        report_fatal_error("End marker mismatch!");
+      ControlFlowStack.pop_back();
       break;
     case WebAssembly::END_BLOCK:
     case WebAssembly::END_BLOCK_S:
-      if (!ControlFlowStack.empty())
-        printAnnotation(
-            OS, "label" + utostr(ControlFlowStack.pop_back_val().first) + ':');
+      if (ControlFlowStack.empty())
+        report_fatal_error("END marker mismatch!");
+      printAnnotation(
+          OS, "label" + utostr(ControlFlowStack.pop_back_val().first) + ':');
       break;
     }
 
Index: llvm/trunk/test/CodeGen/WebAssembly/block-mismatch.mir
===================================================================
--- llvm/trunk/test/CodeGen/WebAssembly/block-mismatch.mir
+++ llvm/trunk/test/CodeGen/WebAssembly/block-mismatch.mir
@@ -0,0 +1,18 @@
+# RUN: not llc -mtriple=wasm32-unknown-unknown -start-after xray-instrumentation %s -o /dev/null 2>&1 | FileCheck %s
+
+# This tests if there are block/loop marker mismatches, the program crashes.
+
+---
+name: block_mismatch
+liveins:
+  - { reg: '$arguments', reg: '$value_stack' }
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $arguments, $value_stack
+    BLOCK 64, implicit-def $value_stack, implicit $value_stack
+    END_BLOCK implicit-def $value_stack, implicit $value_stack
+    ; CHECK: LLVM ERROR: END marker mismatch!
+    END_BLOCK implicit-def $value_stack, implicit $value_stack
+    RETURN_VOID implicit-def dead $arguments
+...