From 13c5783e5e1cc0119c424bedd0bddf97fe14f815 Mon Sep 17 00:00:00 2001
From: overlookmotel <557937+overlookmotel@users.noreply.github.com>
Date: Tue, 15 Jul 2025 15:09:52 +0000
Subject: [PATCH] fix(allocator): fix `FixedSizeAllocator` pointer maths
 (#12299)

Fix a bug with calculating pointer to start of `Allocator` which crept in when buffer size changed in #12277.
---
 crates/oxc_allocator/src/fixed_size.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/crates/oxc_allocator/src/fixed_size.rs b/crates/oxc_allocator/src/fixed_size.rs
index a52e2032b2e17..dd04b6eff103d 100644
--- a/crates/oxc_allocator/src/fixed_size.rs
+++ b/crates/oxc_allocator/src/fixed_size.rs
@@ -11,6 +11,7 @@ use crate::{
 };
 
 const TWO_GIB: usize = 1 << 31;
+const FOUR_GIB: usize = 1 << 32;
 
 // What we ideally want is an allocation 2 GiB in size, aligned on 4 GiB.
 // But system allocator on Mac OS refuses allocations with 4 GiB alignment.
@@ -68,7 +69,7 @@ impl FixedSizeAllocator {
         // SAFETY: `offset` is either 0 or `TWO_GIB`.
         // We allocated 4 GiB of memory, so adding `offset` to `alloc_ptr` is in bounds.
         let chunk_ptr = unsafe {
-            let offset = alloc_ptr.as_ptr() as usize % ALLOC_SIZE;
+            let offset = alloc_ptr.as_ptr() as usize % FOUR_GIB;
             alloc_ptr.add(offset)
         };