From 37b5d3093c40c24aaad3662db23f883910eb7bd3 Mon Sep 17 00:00:00 2001
From: Taylan Kammer <taylan.kammer@gmail.com>
Date: Fri, 21 Aug 2026 15:14:37 +0200
Subject: Meta alloc improvement.

---
 src/zisp/gc/meta_alloc.zig | 55 +++++++++++++++++++++-------------------------
 1 file changed, 25 insertions(+), 30 deletions(-)

diff --git a/src/zisp/gc/meta_alloc.zig b/src/zisp/gc/meta_alloc.zig
index e8502df..1fbe1ab 100644
--- a/src/zisp/gc/meta_alloc.zig
+++ b/src/zisp/gc/meta_alloc.zig
@@ -397,9 +397,9 @@ fn free_into_fl(
 }
 
 /// For every size class: Flushes this thread's free slot cache into the global
-/// free-list, and splits remaining reserved memory into slots which are then
-/// also pushed into the global free-list.  It's crucial for threads to call
-/// this function before terminating, or else memory is leaked.
+/// free-list, and splits remaining reserved memory into slots which are also
+/// pushed onto the global free-list.  It's crucial for threads to call this
+/// function before terminating, or else memory is leaked.
 export fn flush_thread_reserves() void {
     var sci: u8 = 0;
     var slot_len: UnitIdx = SIZES[0] / @sizeOf(Unit);
@@ -411,38 +411,33 @@ export fn flush_thread_reserves() void {
         const tl = &tl_slab_infos[sci];
         const slab: *Slab = &slabs[sci];
         const fsc_lim = maxFscCountForSizeClassIndex(sci);
+        const flush_max = fsc_lim / 2;
 
-        const chunk_max = fsc_lim / 2;
+        // We could first flush the current fsc and then move on to splitting
+        // reserved memory, but this could result in an "undersized" chunk from
+        // the current fsc and then a second undersized chunk when we reach the
+        // end of reserved memory.  A simple improvement is to do it this way
+        // around instead:
 
-        // Up to two iterations to flush the entire fsc.
-        while (tl.fsc_count != 0) {
-            flush_fsc(info, tl, slab, @min(chunk_max, tl.fsc_count - 1));
-        }
-
-        // Now turn slots between wm_lo and wm_hi into free-list entries; most
-        // easily done by writing them into tl.fsc and reusing the function to
-        // flush the fsc entries every time chunk_max + 1 is hit.
+        // Add remaining reserved memory to fsc in a loop, flushing every time
+        // we hit the fsc limit.
         while (tl.wm_lo < tl.wm_hi) : (tl.wm_lo += slot_len) {
-            if (tl.fsc_count > chunk_max) {
-                flush_fsc(info, tl, slab, chunk_max);
+            const idx = tl.wm_lo;
+            const ptr = slab[idx..].ptr;
+            if (tl.fsc_count == fsc_lim) {
+                free_into_fl(info, tl, ptr, idx, flush_max);
+            } else {
+                tl.fsc[tl.fsc_count] = idx;
+                tl.fsc_count += 1;
             }
-            tl.fsc[tl.fsc_count] = tl.wm_lo;
-            tl.fsc_count += 1;
         }
-        if (tl.fsc_count != 0) {
-            flush_fsc(info, tl, slab, tl.fsc_count - 1);
+
+        // Now flush the rest, in two iterations if necessary.
+        while (tl.fsc_count != 0) {
+            tl.fsc_count -= 1;
+            const idx = tl.fsc[tl.fsc_count];
+            const ptr = slab[idx..].ptr;
+            free_into_fl(info, tl, ptr, idx, @min(flush_max, tl.fsc_count));
         }
     }
 }
-
-fn flush_fsc(
-    info: *SlabInfo,
-    tl: *TlSlabInfo,
-    slab: *Slab,
-    n_more: UnitIdx,
-) void {
-    tl.fsc_count -= 1;
-    const idx = tl.fsc[tl.fsc_count];
-    const ptr: SlotPtr = slab[idx..].ptr;
-    free_into_fl(info, tl, ptr, idx, n_more);
-}
-- 
cgit v1.2.3