[parser] Use n-ary addition for template strings

When closing untagged template string literals, create a single n-ary
addition operation, instead of a tree of binary operations.

As a clean-up, this also entirely removes the "second" field from n-ary
operations. This was proving to be too confusing an API when building
an n-ary operation incrementally from a single expression (rather than
converting a binary operation).

Bug: v8:6964
Change-Id: I8f2a395d413cf345bab0a1a347b47f412cde83b1
Reviewed-on: https://chromium-review.googlesource.com/739821Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49054}

src/ast/ast.h

@@ -1822,18 +1822,15 @@ class NaryOperation final : public Expression {
   Expression* first() const { return first_; }
   void set_first(Expression* e) { first_ = e; }
   Expression* subsequent(size_t index) const {
-    if (index == 0) return second_;
-    return subsequent_[index - 1].expression;
+    return subsequent_[index].expression;
   }
   Expression* set_subsequent(size_t index, Expression* e) {
-    if (index == 0) return second_ = e;
-    return subsequent_[index - 1].expression = e;
+    return subsequent_[index].expression = e;
   }
 
-  size_t subsequent_length() const { return 1 + subsequent_.size(); }
+  size_t subsequent_length() const { return subsequent_.size(); }
   int subsequent_op_position(size_t index) const {
-    if (index == 0) return position();
-    return subsequent_[index - 1].op_position;
+    return subsequent_[index].op_position;
   }
 
   void AddSubsequent(Expression* expr, int pos) {
@@ -1844,33 +1841,32 @@ class NaryOperation final : public Expression {
   friend class AstNodeFactory;
 
   NaryOperation(Zone* zone, Token::Value op, Expression* first,
-                Expression* second, int pos)
-      : Expression(pos, kNaryOperation),
+                size_t initial_subsequent_size)
+      : Expression(kNoSourcePosition, kNaryOperation),
         first_(first),
-        second_(second),
         subsequent_(zone) {
     bit_field_ |= OperatorField::encode(op);
     DCHECK(Token::IsBinaryOp(op));
     DCHECK_NE(op, Token::EXP);
+    subsequent_.reserve(initial_subsequent_size);
   }
 
-  // Nary operations store the first operation (and so first two child
-  // expressions) inline, where the position of the first operation is the
-  // position of this expression. Subsequent child expressions are stored
-  // out-of-line, along with with their operation's position and feedback slot.
+  // Nary operations store the first (lhs) child expression inline, and the
+  // child expressions (rhs of each op) are stored out-of-line, along with
+  // their operation's position. Note that the Nary operation expression's
+  // position has no meaning.
   //
   // So an nary add:
   //
-  //    expr + expr + expr + expr + ...
+  //    expr + expr + expr + ...
   //
   // is stored as:
   //
-  //    (expr + expr) [(+ expr), (+ expr), ...]
-  //    '-----.-----' '-----------.-----------'
-  //        this        subsequent entry list
+  //    (expr) [(+ expr), (+ expr), ...]
+  //    '-.--' '-----------.-----------'
+  //    first    subsequent entry list
 
   Expression* first_;
-  Expression* second_;
 
   struct NaryOperationEntry {
     Expression* expression;
@@ -3148,8 +3144,8 @@ class AstNodeFactory final BASE_EMBEDDED {
   }
 
   NaryOperation* NewNaryOperation(Token::Value op, Expression* first,
-                                  Expression* second, int pos) {
-    return new (zone_) NaryOperation(zone_, op, first, second, pos);
+                                  size_t initial_subsequent_size) {
+    return new (zone_) NaryOperation(zone_, op, first, initial_subsequent_size);
   }
 
   CountOperation* NewCountOperation(Token::Value op,

src/parsing/parser.cc

@@ -319,8 +319,8 @@ bool Parser::CollapseNaryExpression(Expression** x, Expression* y,
     BinaryOperation* binop = (*x)->AsBinaryOperation();
     if (binop->op() != op) return false;
 
-    nary = factory()->NewNaryOperation(op, binop->left(), binop->right(),
-                                       binop->position());
+    nary = factory()->NewNaryOperation(op, binop->left(), 2);
+    nary->AddSubsequent(binop->right(), binop->position());
     *x = nary;
   } else if ((*x)->IsNaryOperation()) {
     nary = (*x)->AsNaryOperation();
@@ -3488,9 +3488,16 @@ Expression* Parser::CloseTemplateLiteral(TemplateLiteralState* state, int start,
   DCHECK_EQ(cooked_strings->length(), expressions->length() + 1);
 
   if (!tag) {
-    // Build tree of BinaryOps to simplify code-generation
-    Expression* expr =
+    Expression* first_string =
         factory()->NewStringLiteral(cooked_strings->at(0), kNoSourcePosition);
+    if (expressions->length() == 0) return first_string;
+
+    // Build N-ary addition op to simplify code-generation.
+    // TODO(leszeks): Could we just store this expression in the
+    // TemplateLiteralState and build it as we go?
+    NaryOperation* expr = factory()->NewNaryOperation(
+        Token::ADD, first_string, 2 * expressions->length());
+
     int i = 0;
     while (i < expressions->length()) {
       Expression* sub = expressions->at(i++);
@@ -3501,13 +3508,11 @@ Expression* Parser::CloseTemplateLiteral(TemplateLiteralState* state, int start,
       ZoneList<Expression*>* args =
           new (zone()) ZoneList<Expression*>(1, zone());
       args->Add(sub, zone());
-      Expression* middle = factory()->NewCallRuntime(Runtime::kInlineToString,
-                                                     args, sub->position());
+      Expression* sub_to_string = factory()->NewCallRuntime(
+          Runtime::kInlineToString, args, sub->position());
 
-      expr = factory()->NewBinaryOperation(
-          Token::ADD,
-          factory()->NewBinaryOperation(Token::ADD, expr, middle,
-                                        expr->position()),
+      expr->AddSubsequent(sub_to_string, sub->position());
+      expr->AddSubsequent(
           factory()->NewStringLiteral(cooked_str, kNoSourcePosition),
           sub->position());
     }

test/js-perf-test/ExpressionDepth/run.js

@@ -24,6 +24,7 @@ AddTest('Add', '+');
 AddTest('Sub', '-');
 AddTest('BitwiseOr', '|');
 AddTestCustomPrologue('StringConcat', '+', '"string" +');
+AddTestCustomPrologue('TemplateString', '} ${', '`${', '}`');
 
 function TestExpressionDepth(depth, expression, prologue, epilogue) {
   var func = '(function f(a) {\n' + prologue;
@@ -84,6 +85,6 @@ function AddTest(name, expression, in_test) {
   RunTest(name, expression, prologue, epilogue);
 }
 
-function AddTestCustomPrologue(name, expression, prologue) {
-  RunTest(name, expression, prologue, '');
+function AddTestCustomPrologue(name, expression, prologue, epilogue='') {
+  RunTest(name, expression, prologue, epilogue);
 }