[parser] Pass next_ value from Next to Scan

For some reason the C++ compiler fails to realise that next_ cannot
change on entry into Scan from Next, and re-loads it, creating what
looks like a data dependency that stalls the next instruction.

Passing through a cached next_ value cleans up the generated code.

Change-Id: Iab62ed1890a3a720e5fa90a90e802305e3d55a82
Reviewed-on: https://chromium-review.googlesource.com/c/1331551
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57464}

src/parsing/scanner-inl.h

@@ -588,16 +588,17 @@ V8_INLINE Token::Value Scanner::ScanSingleToken() {
   return token;
 }
 
-void Scanner::Scan() {
-  TokenDesc& next_desc = next();
-  next_desc.literal_chars.Drop();
-  next_desc.raw_literal_chars.Drop();
-  next_desc.contextual_token = Token::UNINITIALIZED;
-  next_desc.invalid_template_escape_message = MessageTemplate::kNone;
+void Scanner::Scan(TokenDesc* next_desc) {
+  DCHECK_EQ(next_desc, &next());
 
-  next_desc.token = ScanSingleToken();
-  DCHECK_IMPLIES(has_parser_error(), next_desc.token == Token::ILLEGAL);
-  next_desc.location.end_pos = source_pos();
+  next_desc->literal_chars.Drop();
+  next_desc->raw_literal_chars.Drop();
+  next_desc->contextual_token = Token::UNINITIALIZED;
+  next_desc->invalid_template_escape_message = MessageTemplate::kNone;
+
+  next_desc->token = ScanSingleToken();
+  DCHECK_IMPLIES(has_parser_error(), next_desc->token == Token::ILLEGAL);
+  next_desc->location.end_pos = source_pos();
 
 #ifdef DEBUG
   SanityCheckTokenDesc(current());
@@ -606,6 +607,8 @@ void Scanner::Scan() {
 #endif
 }
 
+void Scanner::Scan() { Scan(next_); }
+
 }  // namespace internal
 }  // namespace v8
 

src/parsing/scanner.cc

@@ -245,8 +245,10 @@ Token::Value Scanner::Next() {
   // current_ as next_ and scan into it, leaving next_next_ uninitialized.
   if (V8_LIKELY(next_next().token == Token::UNINITIALIZED)) {
     next_ = previous;
-    next().after_line_terminator = false;
-    Scan();
+    // User 'previous' instead of 'next_' because for some reason the compiler
+    // thinks 'next_' could be modified before the entry into Scan.
+    previous->after_line_terminator = false;
+    Scan(previous);
   } else {
     next_ = next_next_;
     next_next_ = previous;

src/parsing/scanner.h

@@ -762,6 +762,11 @@ class Scanner {
   // Scans a single JavaScript token.
   V8_INLINE Token::Value ScanSingleToken();
   V8_INLINE void Scan();
+  // Performance hack: pass through a pre-calculated "next()" value to avoid
+  // having to re-calculate it in Scan. You'd think the compiler would be able
+  // to hoist the next() calculation out of the inlined Scan method, but seems
+  // that pointer aliasing analysis fails show that this is safe.
+  V8_INLINE void Scan(TokenDesc* next_desc);
 
   V8_INLINE Token::Value SkipWhiteSpace();
   Token::Value SkipSingleHTMLComment();