From 57e2d8f1ddf356a0bfabec7630c1e7ff0199f800 Mon Sep 17 00:00:00 2001
From: Feoramund <161657516+Feoramund@users.noreply.github.com>
Date: Thu, 19 Jun 2025 19:36:55 -0400
Subject: [PATCH] mem: Fix `Buddy_Allocator` size calculation to truly include
 alignment

This didn't take into account the size of the header plus the size of
the allocation itself by virtue of `align_forward_uint`; this could
result in no change if `size` was equal to `b.alignment` because the
number is aligned, and if `actual_size` and `size` ended up being equal,
no additional space would be requested.

This meant that a block would end up being allocated on top of its
buddy's head.

Fixes #3435
---
 core/mem/allocators.odin | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/core/mem/allocators.odin b/core/mem/allocators.odin
index a40ab245e..e3b96a8f7 100644
--- a/core/mem/allocators.odin
+++ b/core/mem/allocators.odin
@@ -2247,12 +2247,14 @@ Get required block size to fit in the allocation as well as the alignment paddin
 */
 @(require_results)
 buddy_block_size_required :: proc(b: ^Buddy_Allocator, size: uint) -> uint {
-	size := size
-	actual_size := b.alignment
-	size += size_of(Buddy_Block)
-	size = align_forward_uint(size, b.alignment)
-	for size > actual_size {
-		actual_size <<= 1
+	assert(size > 0)
+	// NOTE: `size_of(Buddy_Block)` will be accounted for in `b.alignment`.
+	// This calculation is also previously guarded against being given a `size`
+	// 0 by `buddy_allocator_alloc_bytes_non_zeroed` checking for that.
+	actual_size := b.alignment + size
+	if intrinsics.count_ones(actual_size) != 1 {
+		// We're not a power of two. Let's fix that.
+		actual_size = 1 << (size_of(uint) * 8 - intrinsics.count_leading_zeros(actual_size))
 	}
 	return actual_size
 }