Index: include/llvm/MC/MCParser/MCTargetAsmParser.h
===================================================================
--- include/llvm/MC/MCParser/MCTargetAsmParser.h
+++ include/llvm/MC/MCParser/MCTargetAsmParser.h
@@ -490,6 +490,9 @@
                                               MCContext &Ctx) {
     return nullptr;
   }
+
+  // For any checks or cleanups at the end of parsing.
+  virtual void onEndOfFile() {}
 };
 
 } // end namespace llvm
Index: lib/MC/MCParser/AsmParser.cpp
===================================================================
--- lib/MC/MCParser/AsmParser.cpp
+++ lib/MC/MCParser/AsmParser.cpp
@@ -898,6 +898,9 @@
       eatToEndOfStatement();
   }
 
+  getTargetParser().onEndOfFile();
+  printPendingErrors();
+
   // All errors should have been emitted.
   assert(!hasPendingError() && "unexpected error from parseStatement");
 
Index: lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
===================================================================
--- lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
+++ lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
@@ -172,6 +172,17 @@
     Instructions,
   } CurrentState = FileStart;
 
+  // For ensuring blocks are properly nested.
+  enum NestingType {
+    Function,
+    Block,
+    Loop,
+    If,
+    Else,
+    Undefined,
+  };
+  std::vector<NestingType> NestingStack;
+
   // We track this to see if a .functype following a label is the same,
   // as this is how we recognize the start of a function.
   MCSymbol *LastLabel = nullptr;
@@ -184,10 +195,6 @@
     setAvailableFeatures(ComputeAvailableFeatures(STI.getFeatureBits()));
   }
 
-  void addSignature(std::unique_ptr<wasm::WasmSignature> &&Sig) {
-    Signatures.push_back(std::move(Sig));
-  }
-
 #define GET_ASSEMBLER_HEADER
 #include "WebAssemblyGenAsmMatcher.inc"
 
@@ -197,10 +204,56 @@
     llvm_unreachable("ParseRegister is not implemented.");
   }
 
-  bool error(const StringRef &Msg, const AsmToken &Tok) {
+  bool error(const Twine &Msg, const AsmToken &Tok) {
     return Parser.Error(Tok.getLoc(), Msg + Tok.getString());
   }
 
+  bool error(const Twine &Msg) {
+    return Parser.Error(Lexer.getTok().getLoc(), Msg);
+  }
+
+  void addSignature(std::unique_ptr<wasm::WasmSignature> &&Sig) {
+    Signatures.push_back(std::move(Sig));
+  }
+
+  StringRef NestingString(NestingType NT) {
+    switch (NT) {
+    case Function:
+      return " function";
+    case Block:
+      return " block";
+    case Loop:
+      return " loop";
+    case If:
+      return " if";
+    case Else:
+      return " else";
+    default:
+      return "";
+    }
+  }
+
+  void Push(NestingType NT) { NestingStack.push_back(NT); }
+
+  bool Pop(NestingType NT1, NestingType NT2 = Undefined) {
+    if (NestingStack.empty())
+      return error(Twine("End of block construct with no start:") +
+                   NestingString(NT1) + NestingString(NT2));
+    auto Top = NestingStack.back();
+    NestingStack.pop_back();
+    if (Top != NT1 && Top != NT2)
+      return error(Twine("Block construct type mismatch, expected:") +
+                   NestingString(NT1) + NestingString(NT2) +
+                   ", instead got:" + NestingString(Top));
+    return false;
+  }
+
+  bool CheckFunctionEnd() {
+    return !NestingStack.empty() &&
+           error(Twine("Unmatched block construct(s) at function end:") +
+                 NestingString(NestingStack.back()));
+  }
+
   bool isNext(AsmToken::TokenKind Kind) {
     auto Ok = Lexer.is(Kind);
     if (Ok)
@@ -327,6 +380,32 @@
     // If no '.', there is no type prefix.
     auto BaseName = NamePair.second.empty() ? NamePair.first : NamePair.second;
 
+    // If this instruction is part of a control flow structure, ensure
+    // proper nesting.
+    if (BaseName == "block") {
+      Push(Block);
+    } else if (BaseName == "loop") {
+      Push(Loop);
+    } else if (BaseName == "if") {
+      Push(If);
+    } else if (BaseName == "else") {
+      if (Pop(If))
+        return true;
+      Push(Else);
+    } else if (BaseName == "end_if") {
+      if (Pop(If, Else))
+        return true;
+    } else if (BaseName == "end_loop") {
+      if (Pop(Loop))
+        return true;
+    } else if (BaseName == "end_block") {
+      if (Pop(Block))
+        return true;
+    } else if (BaseName == "end_function") {
+      if (Pop(Function) || CheckFunctionEnd())
+        return true;
+    }
+
     while (Lexer.isNot(AsmToken::EndOfStatement)) {
       auto &Tok = Lexer.getTok();
       switch (Tok.getKind()) {
@@ -476,7 +555,10 @@
           TOut.getStreamer().getContext().getOrCreateSymbol(SymName));
       if (CurrentState == Label && WasmSym == LastLabel) {
         // This .functype indicates a start of a function.
+        if (CheckFunctionEnd())
+          return true;
         CurrentState = FunctionStart;
+        Push(Function);
       }
       auto Signature = make_unique<wasm::WasmSignature>();
       if (parseSignature(Signature.get()))
@@ -565,6 +647,8 @@
     }
     llvm_unreachable("Implement any new match types added!");
   }
+
+  void onEndOfFile() override { CheckFunctionEnd(); }
 };
 } // end anonymous namespace
 
Index: test/MC/WebAssembly/basic-assembly.s
===================================================================
--- test/MC/WebAssembly/basic-assembly.s
+++ test/MC/WebAssembly/basic-assembly.s
@@ -59,6 +59,11 @@
     end_block            # default jumps here.
     i32.const 3
     end_block            # "switch" exit.
+    #if                  # These are not in tablegen defs yet..
+    #if
+    #end_if
+    #else
+    #end_if
     f32x4.add
     # Test correct parsing of instructions with / and : in them:
     # TODO: enable once instruction has been added.
@@ -129,6 +134,11 @@
 # CHECK-NEXT:      end_block           # label3:
 # CHECK-NEXT:      i32.const 3
 # CHECK-NEXT:      end_block           # label2:
+# DONT-CHECK-NEXT:      if
+# DONT-CHECK-NEXT:      if
+# DONT-CHECK-NEXT:      end_if
+# DONT-CHECK-NEXT:      else
+# DONT-CHECK-NEXT:      end_if
 # CHECK-NEXT:      f32x4.add
 # CHECK-NEXT:      i32.trunc_s/f32
 # CHECK-NEXT:      try
Index: test/MC/WebAssembly/simd-encodings.s
===================================================================
--- test/MC/WebAssembly/simd-encodings.s
+++ test/MC/WebAssembly/simd-encodings.s
@@ -1,5 +1,8 @@
 # RUN: llvm-mc -show-encoding -triple=wasm32-unkown-unknown -mattr=+sign-ext,+simd128 < %s | FileCheck %s
 
+main:
+    .functype main () -> ()
+
     # CHECK: v128.load 48:p2align=0 # encoding: [0xfd,0x00,0x00,0x30]
     v128.load 48