From 571ef605ddf5d8fbf758aa32accc54d2b2a4065b Mon Sep 17 00:00:00 2001
From: Soutaro Matsumoto <matsumoto@soutaro.com>
Date: Wed, 7 Jan 2026 17:17:16 +0900
Subject: [PATCH] Merge pull request #2788 from ruby/fix-alloc-alignment

Fix allocation alignment
---
 src/util/rbs_allocator.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/util/rbs_allocator.c b/src/util/rbs_allocator.c
index b13240940..f088dea37 100644
--- a/src/util/rbs_allocator.c
+++ b/src/util/rbs_allocator.c
@@ -54,6 +54,12 @@ static size_t get_system_page_size(void) {
 #endif
 }
 
+static inline uintptr_t rbs_align_up_uintptr(uintptr_t value, size_t alignment) {
+    // alignment must be a non-zero power of two
+    RBS_ASSERT(alignment != 0 && (alignment & (alignment - 1)) == 0, "alignment must be a non-zero power of two. alignment: %zu", alignment);
+    return (value + (alignment - 1)) & ~(uintptr_t) (alignment - 1);
+}
+
 static rbs_allocator_page_t *rbs_allocator_page_new(size_t payload_size) {
     const size_t page_header_size = sizeof(rbs_allocator_page_t);
 
@@ -98,10 +104,9 @@ void *rbs_allocator_realloc_impl(rbs_allocator_t *allocator, void *ptr, size_t o
 
 // Allocates `size` bytes from `allocator`, aligned to an `alignment`-byte boundary.
 void *rbs_allocator_malloc_impl(rbs_allocator_t *allocator, size_t size, size_t alignment) {
-    RBS_ASSERT(size % alignment == 0, "size must be a multiple of the alignment. size: %zu, alignment: %zu", size, alignment);
-
     if (allocator->default_page_payload_size < size) { // Big allocation, give it its own page.
-        rbs_allocator_page_t *new_page = rbs_allocator_page_new(size);
+        // Add padding to ensure we can align the start pointer within this page
+        rbs_allocator_page_t *new_page = rbs_allocator_page_new(size + (alignment - 1));
 
         // This simple allocator can only put small allocations into the head page.
         // Naively prepending this large allocation page to the head of the allocator before the previous head page
@@ -120,21 +125,29 @@ void *rbs_allocator_malloc_impl(rbs_allocator_t *allocator, size_t size, size_t
         new_page->next = allocator->page->next;
         allocator->page->next = new_page;
 
-        uintptr_t pointer = (uintptr_t) new_page + sizeof(rbs_allocator_page_t);
-        return (void *) pointer;
+        uintptr_t base = (uintptr_t) new_page + sizeof(rbs_allocator_page_t);
+        uintptr_t aligned_ptr = rbs_align_up_uintptr(base, alignment);
+        return (void *) aligned_ptr;
     }
 
     rbs_allocator_page_t *page = allocator->page;
-    if (page->used + size > page->size) {
+    uintptr_t base = (uintptr_t) page + sizeof(rbs_allocator_page_t);
+
+    // Compute aligned offset within the payload
+    size_t used_aligned = (size_t) (rbs_align_up_uintptr(base + page->used, alignment) - base);
+
+    if (used_aligned + size > page->size) {
         // Not enough space. Allocate a new small page and prepend it to the allocator's linked list.
         rbs_allocator_page_t *new_page = rbs_allocator_page_new(allocator->default_page_payload_size);
         new_page->next = allocator->page;
         allocator->page = new_page;
         page = new_page;
+        base = (uintptr_t) page + sizeof(rbs_allocator_page_t);
+        used_aligned = (size_t) (rbs_align_up_uintptr(base, alignment) - base); // start of fresh page (usually 0 if header is aligned)
     }
 
-    uintptr_t pointer = (uintptr_t) page + sizeof(rbs_allocator_page_t) + page->used;
-    page->used += size;
+    uintptr_t pointer = base + used_aligned;
+    page->used = used_aligned + size;
     return (void *) pointer;
 }