From c38117e8a1f5a0d255c6680619f3bb8a553aaa99 Mon Sep 17 00:00:00 2001
From: Max Regan <mgregan2@gmail.com>
Date: Wed, 30 Nov 2022 20:42:44 -0500
Subject: [PATCH] Remove cache, rotate arena_get start scan index

This yields a ~23% overall performance improvment. It operates on the
idea that always iterating from the beginning will constantly get slower
as more objects are allocated- the free objects have higher and higher
indices. Instead, rotate around the arena so we
---
 src/arena.c | 58 ++++++++++++++++-------------------------------------
 src/types.h |  2 +-
 2 files changed, 18 insertions(+), 42 deletions(-)

diff --git a/src/arena.c b/src/arena.c
index b9fd61c..eed67bf 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -20,7 +20,7 @@ struct ucl_arena *ucl_arena_create(size_t object_size, size_t capacity) {
     arena->capacity = capacity;
     arena->objects = malloc(capacity * object_size);
     arena->used_map = malloc(used_map_size);
-    arena->next_free = 0;
+    arena->last_allocated_index = 0;
     arena->stats.used = 0;
     arena->stats.freed = 0;
 
@@ -50,45 +50,25 @@ void ucl_arena_map(struct ucl_arena *arena, void (*map_function)(struct ucl_aren
 
 
 void *ucl_arena_get(struct ucl_arena *arena) {
-    int used_map_ints = DIV_ROUND_UP(arena->capacity, INT_BITS);
-
-
-    if (arena->next_free != -1) {
-        // Cache hit
-        /* unsigned int int_index = arena->next_free / INT_BITS; */
-        /* unsigned int bit_index = arena->next_free % INT_BITS; */
-
-        unsigned int int_index = arena->next_free >> 5;
-        unsigned int bit_index = arena->next_free & 31;
-
-        arena->used_map[int_index] |= 1 << bit_index;
-
-        void *ret = (char *) arena->objects + (arena->next_free * arena->object_size);
-        arena->next_free = -1;
-
-        arena->stats.used += 1;
-        return ret;
-    }
-
-    // Cache miss
-    for (int i = 0; i < used_map_ints; i++ ) {
-        // TODO: Maybe keep a cache of available used_map_ints
-        int map = arena->used_map[i];
-        if (!(~map)) {
-            continue;
+    size_t index = arena->last_allocated_index;
+    do {
+        // TODO: Restructure for speed with ffs
+        index++;
+        if (index == arena->capacity) {
+            index = 0;
         }
 
-        unsigned int bit_index = __builtin_ffs(~map) - 1;
-        unsigned int index = bit_index + INT_BITS * i;
-        if (index >= arena->capacity) {
-            // This might happen in the last used_map_int when (capacity % int_bits != 0)
-            return NULL;
-        }
-        arena->used_map[i] |= 1 << bit_index;
-        arena->stats.used += 1;
+        size_t int_index = index / INT_BITS;
+        size_t bit_index = index % INT_BITS;
 
-        return (char *) arena->objects + (index * arena->object_size);
-    }
+        if (!(arena->used_map[int_index] & 1 << bit_index)) {
+            arena->used_map[int_index] |= 1 << bit_index;
+            arena->last_allocated_index = index;
+            arena->stats.used += 1;
+            return (char *) arena->objects + (index * arena->object_size);
+        }
+
+    } while (index != arena->last_allocated_index);
 
     return NULL;
 }
@@ -113,10 +93,6 @@ void ucl_arena_put(struct ucl_arena *arena, void *object) {
 
     arena->used_map[int_index] &= ~(1 << bit_index);
 
-    /* if (arena->next_free == -1) { */
-    arena->next_free = index;
-    /* } */
-
     arena->stats.used -= 1;
     arena->stats.freed += 1;
 }
diff --git a/src/types.h b/src/types.h
index 631e990..729d788 100644
--- a/src/types.h
+++ b/src/types.h
@@ -56,7 +56,7 @@ struct ucl_arena {
     void *objects;
     int *used_map;
     /** Cache of size 1 **/
-    int next_free;
+    size_t last_allocated_index;
 
     struct {
         size_t used;