[scanner] Simplify special cases of ScanString

AdvanceUntil always starts at the character after c0_. This means that we
needed to handle first, causing some duplication. Instead of doing that, we can
also just not Advance() before AdvanceUntil, making c0_ a character that's
already handled.

Change-Id: I3e23f62a617b5d607579641fc19b12de4f179e8e
Reviewed-on: https://chromium-review.googlesource.com/c/1491512
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59897}

src/parsing/scanner.cc

@@ -501,48 +501,42 @@ uc32 Scanner::ScanOctalEscape(uc32 c, int length) {
 
 Token::Value Scanner::ScanString() {
   uc32 quote = c0_;
-  Advance();  // consume quote
 
   next().literal_chars.Start();
   while (true) {
-    if (V8_UNLIKELY(c0_ == kEndOfInput)) return Token::ILLEGAL;
-    if ((V8_UNLIKELY(static_cast<uint32_t>(c0_) >= kMaxAscii) &&
-         !unibrow::IsStringLiteralLineTerminator(c0_)) ||
-        !MayTerminateString(character_scan_flags[c0_])) {
-      AddLiteralChar(c0_);
-      AdvanceUntil([this](uc32 c0) {
-        if (V8_UNLIKELY(static_cast<uint32_t>(c0) > kMaxAscii)) {
-          if (V8_UNLIKELY(unibrow::IsStringLiteralLineTerminator(c0))) {
-            return true;
-          }
-          AddLiteralChar(c0);
-          return false;
+    AdvanceUntil([this](uc32 c0) {
+      if (V8_UNLIKELY(static_cast<uint32_t>(c0) > kMaxAscii)) {
+        if (V8_UNLIKELY(unibrow::IsStringLiteralLineTerminator(c0))) {
+          return true;
         }
-        uint8_t char_flags = character_scan_flags[c0];
-        if (MayTerminateString(char_flags)) return true;
         AddLiteralChar(c0);
         return false;
-      });
-    }
-    if (c0_ == quote) {
-      Advance();
-      return Token::STRING;
-    }
-    if (c0_ == '\\') {
+      }
+      uint8_t char_flags = character_scan_flags[c0];
+      if (MayTerminateString(char_flags)) return true;
+      AddLiteralChar(c0);
+      return false;
+    });
+
+    while (c0_ == '\\') {
       Advance();
       // TODO(verwaest): Check whether we can remove the additional check.
       if (V8_UNLIKELY(c0_ == kEndOfInput || !ScanEscape<false>())) {
         return Token::ILLEGAL;
       }
-      continue;
     }
+
+    if (c0_ == quote) {
+      Advance();
+      return Token::STRING;
+    }
+
     if (V8_UNLIKELY(c0_ == kEndOfInput ||
                     unibrow::IsStringLiteralLineTerminator(c0_))) {
       return Token::ILLEGAL;
     }
-    DCHECK_NE(quote, c0_);
-    DCHECK((c0_ == '\'' || c0_ == '"'));
-    AddLiteralCharAdvance();
+
+    AddLiteralChar(c0_);
   }
 }