[asm.js] Use token position instead of stream position.

This switches the parser to use token positions (i.e. {Position})
instead of stream positions (i.e. {GetPosition}) everywhere. Access to
the latter is being removed as it is unsupported when the scanner is in
rewind state anyways. This prevents "skipping" a token when seeking.

R=bradnelson@chromium.org
BUG=v8:6127

Change-Id: I9c13dd20a981061a2bccc4fb57e5c57d2a64ac5c
Reviewed-on: https://chromium-review.googlesource.com/480300Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44699}

src/asmjs/asm-parser.cc

@@ -29,7 +29,7 @@ namespace wasm {
 #define FAIL_AND_RETURN(ret, msg)                                        \
   failed_ = true;                                                        \
   failure_message_ = msg;                                                \
-  failure_location_ = scanner_.GetPosition();                            \
+  failure_location_ = static_cast<int>(scanner_.Position());             \
   if (FLAG_trace_asm_parser) {                                           \
     PrintF("[asm.js failure: %s, token: '%s', see: %s:%d]\n", msg,       \
            scanner_.Name(scanner_.Token()).c_str(), __FILE__, __LINE__); \
@@ -39,7 +39,7 @@ namespace wasm {
 #define FAIL_AND_RETURN(ret, msg)                            \
   failed_ = true;                                            \
   failure_message_ = msg;                                    \
-  failure_location_ = scanner_.GetPosition(); \
+  failure_location_ = static_cast<int>(scanner_.Position()); \
   return ret;
 #endif
 
@@ -1302,7 +1302,8 @@ void AsmJsParser::SwitchStatement() {
   pending_label_ = 0;
   // TODO(bradnelson): Make less weird.
   std::vector<int32_t> cases;
-  GatherCases(&cases);  // Skips { implicitly.
+  GatherCases(&cases);
+  EXPECT_TOKEN('{');
   size_t count = cases.size() + 1;
   for (size_t i = 0; i < count; ++i) {
     BareBegin(BlockKind::kOther);
@@ -1958,17 +1959,17 @@ AsmType* AsmJsParser::BitwiseORExpression() {
     call_coercion_deferred_ = nullptr;
     // TODO(bradnelson): Make it prettier.
     bool zero = false;
-    int old_pos;
+    size_t old_pos;
     size_t old_code;
     if (a->IsA(AsmType::Intish()) && CheckForZero()) {
-      old_pos = scanner_.GetPosition();
+      old_pos = scanner_.Position();
       old_code = current_function_builder_->GetPosition();
       scanner_.Rewind();
       zero = true;
     }
     RECURSEn(b = BitwiseXORExpression());
     // Handle |0 specially.
-    if (zero && old_pos == scanner_.GetPosition()) {
+    if (zero && old_pos == scanner_.Position()) {
       current_function_builder_->StashCode(nullptr, old_code);
       a = AsmType::Signed();
       continue;
@@ -2440,7 +2441,7 @@ void AsmJsParser::ValidateFloatCoercion() {
 }
 
 void AsmJsParser::GatherCases(std::vector<int32_t>* cases) {
-  int start = scanner_.GetPosition();
+  size_t start = scanner_.Position();
   int depth = 0;
   for (;;) {
     if (Peek('{')) {

src/asmjs/asm-parser.h

@@ -34,7 +34,7 @@ class AsmJsParser {
   explicit AsmJsParser(Isolate* isolate, Zone* zone, Handle<Script> script,
                        int start, int end);
   bool Run();
-  const char* failure_message() const { return failure_message_.c_str(); }
+  const char* failure_message() const { return failure_message_; }
   int failure_location() const { return failure_location_; }
   WasmModuleBuilder* module_builder() { return module_builder_; }
   const AsmTyper::StdlibSet* stdlib_uses() const { return &stdlib_uses_; }
@@ -115,7 +115,7 @@ class AsmJsParser {
 
   // Error Handling related
   bool failed_;
-  std::string failure_message_;
+  const char* failure_message_;
   int failure_location_;
 
   // Module Related.

src/asmjs/asm-scanner.cc

@@ -210,12 +210,7 @@ std::string AsmJsScanner::Name(token_t token) const {
 }
 #endif
 
-int AsmJsScanner::GetPosition() const {
-  DCHECK(!rewind_);
-  return static_cast<int>(stream_->pos());
-}
-
-void AsmJsScanner::Seek(int pos) {
+void AsmJsScanner::Seek(size_t pos) {
   stream_->Seek(pos);
   preceding_token_ = kUninitialized;
   token_ = kUninitialized;

src/asmjs/asm-scanner.h

@@ -62,10 +62,9 @@ class V8_EXPORT_PRIVATE AsmJsScanner {
   std::string Name(token_t token) const;
 #endif
 
-  // Get current position (to use with Seek).
-  int GetPosition() const;
-  // Restores old position (token after that position).
-  void Seek(int pos);
+  // Restores old position (token after that position). Note that it is not
+  // allowed to rewind right after a seek, because previous tokens are unknown.
+  void Seek(size_t pos);
 
   // Select whether identifiers are resolved in global or local scope,
   // and which scope new identifiers are added to.

test/unittests/asmjs/asm-scanner-unittest.cc

@@ -254,7 +254,7 @@ TEST_F(AsmJsScannerTest, TrailingCComment) {
 TEST_F(AsmJsScannerTest, Seeking) {
   SetupSource("var eval do arguments function break\n");
   Skip(TOK(var));
-  int old_pos = scanner.GetPosition();
+  size_t old_pos = scanner.Position();
   Skip(TOK(eval));
   Skip(TOK(do));
   Skip(TOK(arguments));
@@ -262,6 +262,7 @@ TEST_F(AsmJsScannerTest, Seeking) {
   Skip(TOK(arguments));
   scanner.Rewind();
   scanner.Seek(old_pos);
+  Skip(TOK(eval));
   Skip(TOK(do));
   Skip(TOK(arguments));
   Skip(TOK(function));