From dc2977b3d668479b65ae076fa4442362ba726f3a Mon Sep 17 00:00:00 2001
From: "Wei Zhang (Devinfra)" <zwei@meta.com>
Date: Tue, 23 Apr 2024 21:54:24 -0700
Subject: [PATCH] Mark merged field final if all input fields are final

Summary: If all the input fields are `final`, we keep them as final in the merged class. There seems to be a growing trend that Kotlin code tends to mark fields with `final` by default.

Reviewed By: NTillmann

Differential Revision: D56374098

fbshipit-source-id: 11309a1ed8b3bfac93e99d638a9cccc0beaf9b67
---
 .../class-merging/ClassAssemblingUtils.cpp    | 24 ++++++++++++++++---
 service/class-merging/ClassAssemblingUtils.h  |  4 ++--
 service/class-merging/ModelMerger.cpp         |  7 +++---
 3 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/service/class-merging/ClassAssemblingUtils.cpp b/service/class-merging/ClassAssemblingUtils.cpp
index ce72487091..70e49aaca8 100644
--- a/service/class-merging/ClassAssemblingUtils.cpp
+++ b/service/class-merging/ClassAssemblingUtils.cpp
@@ -104,11 +104,12 @@ DexClass* create_class(const DexType* type,
   return cls;
 }
 
-std::vector<DexField*> create_merger_fields(
-    const DexType* owner, const std::vector<DexField*>& mergeable_fields) {
+std::vector<DexField*> create_merger_fields(const DexType* owner,
+                                            const FieldsMap& fields_map) {
   std::vector<DexField*> res;
+  const auto& mergeable_fields = fields_map.begin()->second;
   size_t cnt = 0;
-  for (const auto f : mergeable_fields) {
+  for (const auto* f : mergeable_fields) {
     auto type = f->get_type();
     std::string name;
     if (type == type::_byte() || type == type::_char() ||
@@ -147,6 +148,23 @@ std::vector<DexField*> create_merger_fields(
     cnt++;
   }
 
+  std::vector<bool> acc_final_map(mergeable_fields.size(), true);
+  for (const auto& fmap : fields_map) {
+    const auto& fields = fmap.second;
+    for (size_t i = 0; i < fields.size(); i++) {
+      redex_assert(acc_final_map.size() > i);
+      acc_final_map[i] = acc_final_map.at(i) && is_final(fields[i]);
+    }
+  }
+
+  for (size_t i = 0; i < res.size(); i++) {
+    redex_assert(acc_final_map.size() > i);
+    if (acc_final_map[i]) {
+      set_final(res.at(i));
+      TRACE(CLMG, 5, "marking merger field final %s", SHOW(res.at(i)));
+    }
+  }
+
   TRACE(CLMG, 8, "  created merger fields %zu ", res.size());
   return res;
 }
diff --git a/service/class-merging/ClassAssemblingUtils.h b/service/class-merging/ClassAssemblingUtils.h
index 3e5faa0392..6a25267bf0 100644
--- a/service/class-merging/ClassAssemblingUtils.h
+++ b/service/class-merging/ClassAssemblingUtils.h
@@ -21,8 +21,8 @@ struct ModelSpec;
 constexpr const char* INTERNAL_TYPE_TAG_FIELD_NAME = "$t";
 constexpr const char* EXTERNAL_TYPE_TAG_FIELD_NAME = "mTypeTag";
 
-std::vector<DexField*> create_merger_fields(
-    const DexType* owner, const std::vector<DexField*>& mergeable_fields);
+std::vector<DexField*> create_merger_fields(const DexType* owner,
+                                            const FieldsMap& fields_map);
 
 void cook_merger_fields_lookup(
     const std::vector<DexField*>& new_fields,
diff --git a/service/class-merging/ModelMerger.cpp b/service/class-merging/ModelMerger.cpp
index 80c8288987..7a75a33611 100644
--- a/service/class-merging/ModelMerger.cpp
+++ b/service/class-merging/ModelMerger.cpp
@@ -504,10 +504,9 @@ const std::vector<DexField*> ModelMerger::empty_fields =
     std::vector<DexField*>();
 
 void ModelMerger::update_merger_fields(const MergerType& merger) {
-  auto merger_fields =
-      merger.has_fields()
-          ? create_merger_fields(merger.type, merger.field_map.begin()->second)
-          : empty_fields;
+  auto merger_fields = merger.has_fields()
+                           ? create_merger_fields(merger.type, merger.field_map)
+                           : empty_fields;
   m_merger_fields[merger.type] = merger_fields;
 }