From 38b9a25e83d2c71139e9cb07659dd9205da98fb4 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Mon, 26 Jun 2023 23:56:45 +0900
Subject: [PATCH] gc.c (mrb_obj_alloc): remove free_next,free_prev from heap
 pages

It used to be maintain heap pages with allocatable objects (free_heaps)
but we can save two words (16 bytes) per page by scanning live heap pages.
---
 include/mruby/gc.h |  3 ---
 src/gc.c           | 48 ++++++++++------------------------------------
 2 files changed, 10 insertions(+), 41 deletions(-)

diff --git a/include/mruby/gc.h b/include/mruby/gc.h
index f7876e9a0..17280a19f 100644
--- a/include/mruby/gc.h
+++ b/include/mruby/gc.h
@@ -45,8 +45,6 @@ typedef struct mrb_heap_page {
   struct RBasic *freelist;
   struct mrb_heap_page *prev;
   struct mrb_heap_page *next;
-  struct mrb_heap_page *free_next;
-  struct mrb_heap_page *free_prev;
   mrb_bool old:1;
   /* Flexible array members area a C99 feature, not C++ compatible */
   /* void* objects[]; */
@@ -59,7 +57,6 @@ typedef struct mrb_heap_page {
 typedef struct mrb_gc {
   mrb_heap_page *heaps;            /* heaps for GC */
   mrb_heap_page *sweeps;           /* page where sweep starts */
-  mrb_heap_page *free_heaps;
   struct RBasic *gray_list;        /* list of gray objects to be traversed incrementally */
   struct RBasic *atomic_gray_list; /* list of objects to be traversed atomically */
   size_t live;                     /* count of live objects */
diff --git a/src/gc.c b/src/gc.c
index 191f2f0ea..f84ddb641 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -309,29 +309,6 @@ unlink_heap_page(mrb_gc *gc, mrb_heap_page *page)
   page->next = NULL;
 }
 
-static void
-link_free_heap_page(mrb_gc *gc, mrb_heap_page *page)
-{
-  page->free_next = gc->free_heaps;
-  if (gc->free_heaps) {
-    gc->free_heaps->free_prev = page;
-  }
-  gc->free_heaps = page;
-}
-
-static void
-unlink_free_heap_page(mrb_gc *gc, mrb_heap_page *page)
-{
-  if (page->free_prev)
-    page->free_prev->free_next = page->free_next;
-  if (page->free_next)
-    page->free_next->free_prev = page->free_prev;
-  if (gc->free_heaps == page)
-    gc->free_heaps = page->free_next;
-  page->free_prev = NULL;
-  page->free_next = NULL;
-}
-
 static void
 add_heap(mrb_state *mrb, mrb_gc *gc)
 {
@@ -347,7 +324,6 @@ add_heap(mrb_state *mrb, mrb_gc *gc)
   page->freelist = prev;
 
   link_heap_page(gc, page);
-  link_free_heap_page(gc, page);
 }
 
 #define DEFAULT_GC_INTERVAL_RATIO 200
@@ -368,7 +344,6 @@ mrb_gc_init(mrb_state *mrb, mrb_gc *gc)
 
   gc->current_white_part = GC_WHITE_A;
   gc->heaps = NULL;
-  gc->free_heaps = NULL;
   add_heap(mrb, gc);
   gc->interval_ratio = DEFAULT_GC_INTERVAL_RATIO;
   gc->step_ratio = DEFAULT_GC_STEP_RATIO;
@@ -497,7 +472,6 @@ mrb_gc_unregister(mrb_state *mrb, mrb_value obj)
 MRB_API struct RBasic*
 mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
 {
-  struct RBasic *p;
   static const RVALUE RVALUE_zero = { { { NULL, NULL, MRB_TT_FALSE } } };
   mrb_gc *gc = &mrb->gc;
 
@@ -534,15 +508,18 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
   if (gc->threshold < gc->live) {
     mrb_incremental_gc(mrb);
   }
-  if (gc->free_heaps == NULL) {
-    add_heap(mrb, gc);
-  }
 
-  p = gc->free_heaps->freelist;
-  gc->free_heaps->freelist = ((struct free_obj*)p)->next;
-  if (gc->free_heaps->freelist == NULL) {
-    unlink_free_heap_page(gc, gc->free_heaps);
+  mrb_heap_page *page = gc->heaps;
+  struct RBasic *p = NULL;
+  while (page->freelist == NULL) {
+    page = page->next;
+    if (page == NULL) {
+      add_heap(mrb, gc);
+      page = gc->heaps;
+    }
   }
+  p = page->freelist;
+  page->freelist = ((struct free_obj*)p)->next;
 
   gc->live++;
   gc_protect(mrb, gc, p);
@@ -1109,7 +1086,6 @@ incremental_sweep_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
     RVALUE *e = p + MRB_HEAP_PAGE_SIZE;
     size_t freed = 0;
     mrb_bool dead_slot = TRUE;
-    mrb_bool full = (page->freelist == NULL);
 
     if (is_minor_gc(gc) && page->old) {
       /* skip a slot which doesn't contain any young object */
@@ -1143,14 +1119,10 @@ incremental_sweep_phase(mrb_state *mrb, mrb_gc *gc, size_t limit)
       mrb_heap_page *next = page->next;
 
       unlink_heap_page(gc, page);
-      unlink_free_heap_page(gc, page);
       mrb_free(mrb, page);
       page = next;
     }
     else {
-      if (full && freed > 0) {
-        link_free_heap_page(gc, page);
-      }
       if (page->freelist == NULL && is_minor_gc(gc))
         page->old = TRUE;
       else