[WebAssembly] Make assembler check for proper nesting of control flow.

Summary:
It does so using a simple nesting stack, and gives clear errors upon
violation. This is unique to wasm, since most CPUs do not have
any nested constructs.

Had to add an end of file check to the general assembler for this.

Note: if/else/end instructions are not currently supported in our
tablegen defs, so these tests will be enabled in a follow-up.
They already pass the nesting check.

Reviewers: dschuff, aheejin

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D55797

llvm-svn: 350078

llvm/include/llvm/MC/MCParser/MCTargetAsmParser.h

@@ -490,6 +490,9 @@ class MCTargetAsmParser : public MCAsmParserExtension {
                                               MCContext &Ctx) {
     return nullptr;
   }
+
+  // For any checks or cleanups at the end of parsing.
+  virtual void onEndOfFile() {}
 };
 
 } // end namespace llvm

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -899,6 +899,9 @@ bool AsmParser::Run(bool NoInitialTextSection, bool NoFinalize) {
       eatToEndOfStatement();
   }
 
+  getTargetParser().onEndOfFile();
+  printPendingErrors();
+
   // All errors should have been emitted.
   assert(!hasPendingError() && "unexpected error from parseStatement");
 

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

@@ -172,6 +172,18 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
     Instructions,
   } CurrentState = FileStart;
 
+  // For ensuring blocks are properly nested.
+  enum NestingType {
+    Function,
+    Block,
+    Loop,
+    Try,
+    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 +196,6 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
     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 +205,60 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
     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));
+  }
+
+  std::pair<StringRef, StringRef> nestingString(NestingType NT) {
+    switch (NT) {
+    case Function:
+      return {"function", "end_function"};
+    case Block:
+      return {"block", "end_block"};
+    case Loop:
+      return {"loop", "end_loop"};
+    case Try:
+      return {"try", "end_try"};
+    case If:
+      return {"if", "end_if"};
+    case Else:
+      return {"else", "end_if"};
+    default:
+      llvm_unreachable("unknown NestingType");
+    }
+  }
+
+  void push(NestingType NT) { NestingStack.push_back(NT); }
+
+  bool pop(StringRef Ins, NestingType NT1, NestingType NT2 = Undefined) {
+    if (NestingStack.empty())
+      return error(Twine("End of block construct with no start: ") + Ins);
+    auto Top = NestingStack.back();
+    if (Top != NT1 && Top != NT2)
+      return error(Twine("Block construct type mismatch, expected: ") +
+                   nestingString(Top).second + ", instead got: " + Ins);
+    NestingStack.pop_back();
+    return false;
+  }
+
+  bool ensureEmptyNestingStack() {
+    auto err = !NestingStack.empty();
+    while (!NestingStack.empty()) {
+      error(Twine("Unmatched block construct(s) at function end: ") +
+            nestingString(NestingStack.back()).first);
+      NestingStack.pop_back();
+    }
+    return err;
+  }
+
   bool isNext(AsmToken::TokenKind Kind) {
     auto Ok = Lexer.is(Kind);
     if (Ok)
@@ -327,6 +385,45 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
     // 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 == "try") {
+      push(Try);
+    } else if (BaseName == "if") {
+      push(If);
+    } else if (BaseName == "else") {
+      if (pop(BaseName, If))
+        return true;
+      push(Else);
+    } else if (BaseName == "catch") {
+      if (pop(BaseName, Try))
+        return true;
+      push(Try);
+    } else if (BaseName == "catch_all") {
+      if (pop(BaseName, Try))
+        return true;
+      push(Try);
+    } else if (BaseName == "end_if") {
+      if (pop(BaseName, If, Else))
+        return true;
+    } else if (BaseName == "end_try") {
+      if (pop(BaseName, Try))
+        return true;
+    } else if (BaseName == "end_loop") {
+      if (pop(BaseName, Loop))
+        return true;
+    } else if (BaseName == "end_block") {
+      if (pop(BaseName, Block))
+        return true;
+    } else if (BaseName == "end_function") {
+      if (pop(BaseName, Function) || ensureEmptyNestingStack())
+        return true;
+    }
+
     while (Lexer.isNot(AsmToken::EndOfStatement)) {
       auto &Tok = Lexer.getTok();
       switch (Tok.getKind()) {
@@ -476,7 +573,10 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
           TOut.getStreamer().getContext().getOrCreateSymbol(SymName));
       if (CurrentState == Label && WasmSym == LastLabel) {
         // This .functype indicates a start of a function.
+        if (ensureEmptyNestingStack())
+          return true;
         CurrentState = FunctionStart;
+        push(Function);
       }
       auto Signature = make_unique<wasm::WasmSignature>();
       if (parseSignature(Signature.get()))
@@ -565,6 +665,8 @@ class WebAssemblyAsmParser final : public MCTargetAsmParser {
     }
     llvm_unreachable("Implement any new match types added!");
   }
+
+  void onEndOfFile() override { ensureEmptyNestingStack(); }
 };
 } // end anonymous namespace
 

llvm/test/MC/WebAssembly/basic-assembly-errors.s

@@ -0,0 +1,25 @@
+# RUN: not llvm-mc -triple=wasm32-unknown-unknown -mattr=+simd128,+nontrapping-fptoint,+exception-handling < %s 2>&1 | FileCheck %s
+
+    .text
+    .section    .text.main,"",@
+    .type       test0,@function
+# CHECK: End of block construct with no start: end_try
+    end_try
+test0:
+    .functype   test0 () -> ()
+# CHECK: Block construct type mismatch, expected: end_function, instead got: end_loop
+    end_loop
+    block
+# CHECK: Block construct type mismatch, expected: end_block, instead got: end_if
+    end_if
+    try
+    loop
+# CHECK: Block construct type mismatch, expected: end_loop, instead got: end_function
+# CHECK: error: Unmatched block construct(s) at function end: loop
+# CHECK: error: Unmatched block construct(s) at function end: try
+# CHECK: error: Unmatched block construct(s) at function end: block
+# CHECK: error: Unmatched block construct(s) at function end: function
+    end_function
+.Lfunc_end0:
+    .size       test0, .Lfunc_end0-test0
+

llvm/test/MC/WebAssembly/basic-assembly.s

@@ -59,6 +59,11 @@ test0:
     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 @@ test0:
 # 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

llvm/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