From 4421e7c33eb255aaa4fdbd442a4e1f17434bfb55 Mon Sep 17 00:00:00 2001
From: Nicolas Geoffray <ngeoffray@google.com>
Date: Thu, 23 Jan 2025 20:20:22 +0000
Subject: Optimize field lookup.

Try indexing directly in the array, or close to the given index.

Test: test.py

Change-Id: I3260edf7a0c54de241f71dbbb31d2efb1c4103c8
---
 runtime/mirror/class.cc | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

(limited to 'runtime/mirror/class.cc')

diff --git a/runtime/mirror/class.cc b/runtime/mirror/class.cc
index fe923cbaa6..0bf1b1cc0a 100644
--- a/runtime/mirror/class.cc
+++ b/runtime/mirror/class.cc
@@ -1173,10 +1173,31 @@ ArtField* Class::FindDeclaredStaticField(std::string_view name, std::string_view
 }
 
 ArtField* Class::FindDeclaredField(ObjPtr<DexCache> dex_cache, uint32_t dex_field_idx) {
-  if (dex_cache == GetDexCache()) {
-    for (ArtField& field : GetFields()) {
-      if (field.GetDexFieldIndex() == dex_field_idx) {
-        return &field;
+  size_t num_fields = NumFields();
+  if (dex_cache == GetDexCache() && num_fields > 0) {
+    // The field array is an ordered list of fields where there may be missing
+    // indices. For example, it could be [40, 42], but in 90% of cases cases we have
+    // [40, 41, 42]. The latter is the case we are optimizing for, where for
+    // example `dex_field_idx` is 41, and we can just substract it with the
+    // first field index (40) and directly access the array with that index (1).
+    uint32_t index = dex_field_idx - GetField(0)->GetDexFieldIndex();
+    if (index < num_fields) {
+      ArtField* field = GetField(index);
+      if (field->GetDexFieldIndex() == dex_field_idx) {
+        return field;
+      }
+    } else {
+      index = num_fields;
+    }
+    // If there is a field, it's down the array. The array is ordered by field
+    // index, so we know we can stop the search if `dex_field_idx` is greater
+    // than the current field's index.
+    for (; index > 0; --index) {
+      ArtField* field = GetField(index - 1);
+      if (field->GetDexFieldIndex() == dex_field_idx) {
+        return field;
+      } else if (field->GetDexFieldIndex() < dex_field_idx) {
+        break;
       }
     }
   }
-- 
cgit v1.2.3-59-g8ed1b