From c90d4af8750bef77ce9abe91df80d5d98ba297d7 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Tue, 29 Oct 2024 14:37:19 -0400
Subject: alloc: Don't require size % align == 0

Allowing unaligned sizes will allow us to allocate aligned slabs with
additional metadata in the tail without ballooning the allocation size
for large alignments.

Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2244.htm#dr_460
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2072.htm
---
 src/alloc.c | 15 +++++----------
 src/alloc.h | 34 ++++++++++++++--------------------
 2 files changed, 19 insertions(+), 30 deletions(-)

(limited to 'src')

diff --git a/src/alloc.c b/src/alloc.c
index 779e1d7..4e68e13 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -20,24 +20,22 @@
 #  define ALLOC_MAX (SIZE_MAX / 2)
 #endif
 
-/** Portable aligned_alloc()/posix_memalign(). */
+/** posix_memalign() wrapper. */
 static void *xmemalign(size_t align, size_t size) {
 	bfs_assert(has_single_bit(align));
 	bfs_assert(align >= sizeof(void *));
-	bfs_assert(is_aligned(align, size));
 
-#if BFS_HAS_ALIGNED_ALLOC
-	return aligned_alloc(align, size);
-#else
+	// Since https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2072.htm,
+	// aligned_alloc() doesn't require the size to be a multiple of align.
+	// But the sanitizers don't know about that yet, so always use
+	// posix_memalign().
 	void *ptr = NULL;
 	errno = posix_memalign(&ptr, align, size);
 	return ptr;
-#endif
 }
 
 void *alloc(size_t align, size_t size) {
 	bfs_assert(has_single_bit(align));
-	bfs_assert(is_aligned(align, size));
 
 	if (size > ALLOC_MAX) {
 		errno = EOVERFLOW;
@@ -53,7 +51,6 @@ void *alloc(size_t align, size_t size) {
 
 void *zalloc(size_t align, size_t size) {
 	bfs_assert(has_single_bit(align));
-	bfs_assert(is_aligned(align, size));
 
 	if (size > ALLOC_MAX) {
 		errno = EOVERFLOW;
@@ -73,8 +70,6 @@ void *zalloc(size_t align, size_t size) {
 
 void *xrealloc(void *ptr, size_t align, size_t old_size, size_t new_size) {
 	bfs_assert(has_single_bit(align));
-	bfs_assert(is_aligned(align, old_size));
-	bfs_assert(is_aligned(align, new_size));
 
 	if (new_size == 0) {
 		free(ptr);
diff --git a/src/alloc.h b/src/alloc.h
index 4f119e0..7865d5d 100644
--- a/src/alloc.h
+++ b/src/alloc.h
@@ -30,25 +30,24 @@ static inline size_t align_ceil(size_t align, size_t size) {
 }
 
 /**
- * Saturating array size.
- *
- * @align
- *         Array element alignment.
- * @size
- *         Array element size.
- * @count
- *         Array element count.
- * @return
- *         size * count, saturating to the maximum aligned value on overflow.
+ * Saturating size addition.
  */
-static inline size_t array_size(size_t align, size_t size, size_t count) {
+static inline size_t size_add(size_t lhs, size_t rhs) {
+	size_t ret = lhs + rhs;
+	return ret >= lhs ? ret : (size_t)-1;
+}
+
+/**
+ * Saturating size multiplication.
+ */
+static inline size_t size_mul(size_t size, size_t count) {
 	size_t ret = size * count;
-	return ret / size == count ? ret : ~(align - 1);
+	return ret / size == count ? ret : (size_t)-1;
 }
 
 /** Saturating array sizeof. */
 #define sizeof_array(type, count) \
-	array_size(alignof(type), sizeof(type), count)
+	size_mul(sizeof(type), count)
 
 /** Size of a struct/union field. */
 #define sizeof_member(type, member) \
@@ -72,13 +71,8 @@ static inline size_t array_size(size_t align, size_t size, size_t count) {
  *         to the maximum aligned value on overflow.
  */
 static inline size_t flex_size(size_t align, size_t min, size_t offset, size_t size, size_t count) {
-	size_t ret = size * count;
-	size_t overflow = ret / size != count;
-
-	size_t extra = offset + align - 1;
-	ret += extra;
-	overflow |= ret < extra;
-	ret |= -overflow;
+	size_t ret = size_mul(size, count);
+	ret = size_add(ret, offset + align - 1);
 	ret = align_floor(align, ret);
 
 	// Make sure flex_sizeof(type, member, 0) >= sizeof(type), even if the
-- 
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