From 4a75eddd8f50fd351d3ddc0a17d675562302a0a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nils=20O=2E=20Sel=C3=A5sdal?= <nos@utel.tech>
Date: Tue, 16 Jun 2026 23:46:52 +0200
Subject: [PATCH] Implement memory decommit

---
 src/arena.c | 13 ++++++++++++-
 src/arena.h |  3 +++
 src/main.c  | 36 +++++++++++++++++++++++++++++++++---
 3 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/src/arena.c b/src/arena.c
index fc0390f..3ca6661 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -115,7 +115,7 @@ void arena_reset_to(Arena *arena, U64 total_offset)
         MemoryBlock *prev = block->prev;
 
         if (block->arena_offset >= total_offset) {
-            ASAN_UNPOISON_MEMORY_REGION(block->base, block->size);
+            ASAN_UNPOISON_MEMORY_REGION(block->base, block->committed_end);
             memory_free(block->base, block->size);
             arena->current = prev;
         } else {
@@ -123,8 +123,19 @@ void arena_reset_to(Arena *arena, U64 total_offset)
             new_pos = Max(MemoryBlock_Header_Size, new_pos);
             // Disallow arbitrary pops to positions that previously were not commited.
             Assert(new_pos <= block->committed_end);
+            Assert(new_pos <= block->size);
             block->first_free = new_pos;
             ASAN_POISON_MEMORY_REGION(&block->base[block->first_free], memoryblock_available(block));
+
+            if ((arena->flags & ArenaFlag_NoDecommit) == 0) {
+                U32 aligned_pos = RoundUpToMultiple(block->first_free, arena->commit_size);
+                if (aligned_pos < block->committed_end) {
+                    ASAN_UNPOISON_MEMORY_REGION(&block->base[aligned_pos], block->committed_end);
+                    memory_decommit(&block->base[aligned_pos], block->committed_end - aligned_pos);
+                    block->committed_end = aligned_pos;
+                }
+            }
+
             break;
         }
 
diff --git a/src/arena.h b/src/arena.h
index 0a666ed..f2f64db 100644
--- a/src/arena.h
+++ b/src/arena.h
@@ -4,6 +4,9 @@
 
 typedef U32 ArenaFlags;
 typedef struct ArenaOptions ArenaOptions;
+enum {
+    ArenaFlag_NoDecommit, // arena_ temp_end/reset/reset_to doesn't decommit memory
+};
 struct ArenaOptions {
     U32 size;
     U32 commit_size;
diff --git a/src/main.c b/src/main.c
index 1284859..8210d79 100644
--- a/src/main.c
+++ b/src/main.c
@@ -162,6 +162,34 @@ static void arena_test_4(void)
 
     arena_free(&arena);
 }
+
+static void arena_test_decommit(void)
+{
+    TRACEF("\n");
+
+    U32 page_size = platform_page_size();
+    ArenaOptions arena_params = {};
+    U32 arena_size  = MB(1) + MemoryBlock_Header_Size;
+    U32 commit_size = KB(64);
+    arena_params.size = arena_size;
+    arena_params.commit_size = commit_size;
+    Arena arena;
+    arena_init(&arena, &arena_params);
+    void *ptr = arena_push(&arena, commit_size * 10, 8);
+    MemoryZero(ptr, commit_size * 10);
+    arena_reset(&arena);
+
+    void *pre_ptr = arena_push(&arena, 5, 4);
+
+    ATemp tmp = arena_temp_begin(&arena);
+    ptr = arena_push(&arena, commit_size * 2 + 13, 8);
+    MemoryZero(ptr, commit_size * 2 + 13);
+    arena_temp_end(&tmp);
+    MemoryZero(pre_ptr, 5);
+
+    arena_free(&arena);
+}
+
 void miniarena_test1(void)
 {
     #define  arena_size  4096
@@ -290,8 +318,10 @@ int main(int argc, char *argv[])
     // arena_test_2();
     // arena_test_3();
     // arena_test_4();
-//    miniarena_test1();
-    apool_test();
-    apool_test_autogrow();
+    arena_test_decommit();
+
+    // miniarena_test1();
+    // apool_test();
+    // apool_test_autogrow();
 }