[parser] Use SmallVector(1) for DeclarationParsingResult::declarations

Typically we'll parse a single declaration when parsing variable declarations.
Using on-stack storage rather than std::vector that requires malloc is much
more efficient.

Change-Id: Id99515bb4ce7ea2dae46498f8f9f9d49c33c7353
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2418393
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69995}

src/parsing/parser-base.h

@@ -6,6 +6,7 @@
 #define V8_PARSING_PARSER_BASE_H_
 
 #include <stdint.h>
+
 #include <utility>
 #include <vector>
 
@@ -14,6 +15,7 @@
 #include "src/ast/scopes.h"
 #include "src/base/flags.h"
 #include "src/base/hashmap.h"
+#include "src/base/small-vector.h"
 #include "src/base/v8-fallthrough.h"
 #include "src/codegen/bailout-reason.h"
 #include "src/common/globals.h"
@@ -539,14 +541,17 @@ class ParserBase {
 
   struct DeclarationParsingResult {
     struct Declaration {
-      Declaration(ExpressionT pattern, ExpressionT initializer)
-          : pattern(pattern), initializer(initializer) {
+      Declaration(ExpressionT pattern, ExpressionT initializer,
+                  int value_beg_pos = kNoSourcePosition)
+          : pattern(pattern),
+            initializer(initializer),
+            value_beg_pos(value_beg_pos) {
         DCHECK_IMPLIES(Impl::IsNull(pattern), Impl::IsNull(initializer));
       }
 
       ExpressionT pattern;
       ExpressionT initializer;
-      int value_beg_pos = kNoSourcePosition;
+      int value_beg_pos;
     };
 
     DeclarationParsingResult()
@@ -554,7 +559,7 @@ class ParserBase {
           bindings_loc(Scanner::Location::invalid()) {}
 
     DeclarationDescriptor descriptor;
-    std::vector<Declaration> declarations;
+    base::SmallVector<Declaration, 1> declarations;
     Scanner::Location first_initializer_loc;
     Scanner::Location bindings_loc;
   };
@@ -3929,10 +3934,7 @@ void ParserBase<Impl>::ParseVariableDeclarations(
                    impl()->IsNull(value) ||
                        (var_context == kForStatement && PeekInOrOf()));
 
-    typename DeclarationParsingResult::Declaration decl(pattern, value);
-    decl.value_beg_pos = value_beg_pos;
-
-    parsing_result->declarations.push_back(decl);
+    parsing_result->declarations.emplace_back(pattern, value, value_beg_pos);
   } while (Check(Token::COMMA));
 
   parsing_result->bindings_loc =