[Torque] Implement Flatten() and EmitCCValue without std::tuple_cat

The implementation of Flatten() Torque generates for CSA struct types
and the current implementation of EmitCCValue make heavy use of
std::make_tuple and std::tuple_cat, which had a noticeable impact on
build times of generated files including these outputs.
This CL eliminates all uses of std::tuple_cat and most uses of
std::make_tuple.

Bug: v8:9861
Change-Id: Ib950b2c87983171b8c32c388fc115d2407087717
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2784563
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#73660}

src/torque/cc-generator.cc

@@ -472,33 +472,38 @@ void CCGenerator::EmitInstruction(const StoreBitFieldInstruction& instruction,
   ReportError("Not supported in C++ output: StoreBitField");
 }
 
-// static
-void CCGenerator::EmitCCValue(VisitResult result,
-                              const Stack<std::string>& values,
-                              std::ostream& out) {
+namespace {
+
+void CollectAllFields(const VisitResult& result,
+                      const Stack<std::string>& values,
+                      std::vector<std::string>& all_fields) {
   if (!result.IsOnStack()) {
-    out << result.constexpr_value();
+    all_fields.push_back(result.constexpr_value());
   } else if (auto struct_type = result.type()->StructSupertype()) {
-    out << "std::tuple_cat(";
-    bool first = true;
-    for (auto& field : (*struct_type)->fields()) {
-      if (!first) {
-        out << ", ";
-      }
-      first = false;
-      if (!field.name_and_type.type->IsStructType()) {
-        out << "std::make_tuple(";
-      }
-      EmitCCValue(ProjectStructField(result, field.name_and_type.name), values,
-                  out);
-      if (!field.name_and_type.type->IsStructType()) {
-        out << ")";
-      }
+    for (const Field& field : (*struct_type)->fields()) {
+      CollectAllFields(ProjectStructField(result, field.name_and_type.name),
+                       values, all_fields);
     }
-    out << ")";
   } else {
     DCHECK_EQ(1, result.stack_range().Size());
-    out << values.Peek(result.stack_range().begin());
+    all_fields.push_back(values.Peek(result.stack_range().begin()));
+  }
+}
+
+}  // namespace
+
+// static
+void CCGenerator::EmitCCValue(VisitResult result,
+                              const Stack<std::string>& values,
+                              std::ostream& out) {
+  std::vector<std::string> all_fields;
+  CollectAllFields(result, values, all_fields);
+  if (all_fields.size() == 1) {
+    out << all_fields[0];
+  } else {
+    out << "std::make_tuple(";
+    PrintCommaSeparatedList(out, all_fields);
+    out << ")";
   }
 }
 

src/torque/implementation-visitor.cc

@@ -20,6 +20,7 @@
 #include "src/torque/type-inference.h"
 #include "src/torque/type-visitor.h"
 #include "src/torque/types.h"
+#include "src/torque/utils.h"
 
 namespace v8 {
 namespace internal {
@@ -5158,6 +5159,24 @@ void ImplementationVisitor::GenerateExportedMacrosAssembler(
   WriteFile(output_directory + "/" + file_name + ".cc", cc_contents.str());
 }
 
+namespace {
+
+void CollectAllFields(const std::string& path, const Field& field,
+                      std::vector<std::string>& result) {
+  if (field.name_and_type.type->StructSupertype()) {
+    std::string next_path = path + field.name_and_type.name + ".";
+    const StructType* struct_type =
+        StructType::DynamicCast(field.name_and_type.type);
+    for (const auto& inner_field : struct_type->fields()) {
+      CollectAllFields(next_path, inner_field, result);
+    }
+  } else {
+    result.push_back(path + field.name_and_type.name);
+  }
+}
+
+}  // namespace
+
 void ImplementationVisitor::GenerateCSATypes(
     const std::string& output_directory) {
   std::string file_name = "csa-types";
@@ -5188,20 +5207,13 @@ void ImplementationVisitor::GenerateCSATypes(
         first = false;
         h_contents << type->GetGeneratedTypeName();
       }
-      h_contents << "> Flatten() const {\n"
-                 << "    return std::tuple_cat(";
-      first = true;
+      std::vector<std::string> all_fields;
       for (auto& field : struct_type->fields()) {
-        if (!first) {
-          h_contents << ", ";
-        }
-        first = false;
-        if (field.name_and_type.type->StructSupertype()) {
-          h_contents << field.name_and_type.name << ".Flatten()";
-        } else {
-          h_contents << "std::make_tuple(" << field.name_and_type.name << ")";
-        }
+        CollectAllFields("", field, all_fields);
       }
+      h_contents << "> Flatten() const {\n"
+                    "    return std::make_tuple(";
+      PrintCommaSeparatedList(h_contents, all_fields);
       h_contents << ");\n";
       h_contents << "  }\n";
       h_contents << "};\n";