remove MI_ALIGNMENT_MAX and add comments

include/mimalloc-types.h

@@ -139,7 +139,8 @@ typedef int32_t  mi_ssize_t;
 
 // Derived constants
 #define MI_SEGMENT_SIZE                   (MI_ZU(1)<<MI_SEGMENT_SHIFT)
-#define MI_SEGMENT_MASK                   (MI_SEGMENT_SIZE - 1)
+#define MI_SEGMENT_ALIGN                  (MI_SEGMENT_SIZE)
+#define MI_SEGMENT_MASK                   (MI_SEGMENT_ALIGN - 1)
 
 #define MI_SMALL_PAGE_SIZE                (MI_ZU(1)<<MI_SMALL_PAGE_SHIFT)
 #define MI_MEDIUM_PAGE_SIZE               (MI_ZU(1)<<MI_MEDIUM_PAGE_SHIFT)
@@ -163,13 +164,13 @@ typedef int32_t  mi_ssize_t;
 #if (MI_LARGE_OBJ_WSIZE_MAX >= 655360)
 #error "mimalloc internal: define more bins"
 #endif
-#if (MI_ALIGNMENT_MAX > MI_SEGMENT_SIZE/2)
-#error "mimalloc internal: the max aligned boundary is too large for the segment size"
-#endif
 
 // Used as a special value to encode block sizes in 32 bits.
 #define MI_HUGE_BLOCK_SIZE   ((uint32_t)MI_HUGE_OBJ_SIZE_MAX)
 
+// Alignments over MI_ALIGNMENT_MAX are allocated in dedicated huge page segments 
+#define MI_ALIGNMENT_MAX   (MI_SEGMENT_SIZE >> 1)  
+
 
 // ------------------------------------------------------
 // Mimalloc pages contain allocated blocks

include/mimalloc.h

@@ -166,7 +166,6 @@ mi_decl_export void mi_process_info(size_t* elapsed_msecs, size_t* user_msecs, s
 // Note that `alignment` always follows `size` for consistency with unaligned
 // allocation, but unfortunately this differs from `posix_memalign` and `aligned_alloc`.
 // -------------------------------------------------------------------------------------
-#define MI_ALIGNMENT_MAX   (2*1024*1024UL)    // maximum supported alignment is 1MiB
 
 mi_decl_nodiscard mi_decl_export mi_decl_restrict void* mi_malloc_aligned(size_t size, size_t alignment) mi_attr_noexcept mi_attr_malloc mi_attr_alloc_size(1) mi_attr_alloc_align(2);
 mi_decl_nodiscard mi_decl_export mi_decl_restrict void* mi_malloc_aligned_at(size_t size, size_t alignment, size_t offset) mi_attr_noexcept mi_attr_malloc mi_attr_alloc_size(1);

src/alloc-aligned.c

@@ -36,9 +36,9 @@ static mi_decl_noinline void* mi_heap_malloc_zero_aligned_at_fallback(mi_heap_t*
     // use OS allocation for very large alignment and allocate inside a huge page (dedicated segment with 1 page)
     if mi_unlikely(offset != 0) {
       // todo: cannot support offset alignment for very large alignments yet
-#if MI_DEBUG > 0
+      #if MI_DEBUG > 0
       _mi_error_message(EOVERFLOW, "aligned allocation with a very large alignment cannot be used with an alignment offset (size %zu, alignment %zu, offset %zu)\n", size, alignment, offset);
-#endif
+      #endif
       return NULL;
     }
     oversize = (size <= MI_SMALL_SIZE_MAX ? MI_SMALL_SIZE_MAX + 1 /* ensure we use generic malloc path */ : size);

src/os.c

@@ -842,29 +842,30 @@ void* _mi_os_alloc_aligned(size_t size, size_t alignment, bool commit, bool* lar
 
 /* -----------------------------------------------------------
   OS aligned allocation with an offset. This is used
-  for large alignments > MI_SEGMENT_SIZE so we can align
+  for large alignments > MI_ALIGNMENT_MAX. We use a large mimalloc
-  the first page at an offset from the start of the segment.
+  page where the object can be aligned at an offset from the start of the segment.
-  As we may need to overallocate, we need to free such pointers
+  As we may need to overallocate, we need to free such pointers using `mi_free_aligned` 
-  using `mi_free_aligned` to use the actual start of the 
+  to use the actual start of the memory region.
-  memory region.
 ----------------------------------------------------------- */
 
-
 void* _mi_os_alloc_aligned_offset(size_t size, size_t alignment, size_t offset, bool commit, bool* large, mi_stats_t* tld_stats) {
   mi_assert(offset <= MI_SEGMENT_SIZE);
   mi_assert(offset <= size);
   mi_assert((alignment % _mi_os_page_size()) == 0);
   if (offset > MI_SEGMENT_SIZE) return NULL;
   if (offset == 0) {
+    // regular aligned allocation
     return _mi_os_alloc_aligned(size, alignment, commit, large, tld_stats);
   }
   else {
+    // overallocate to align at an offset
     const size_t extra = _mi_align_up(offset, alignment) - offset;
     const size_t oversize = size + extra;
     void* start = _mi_os_alloc_aligned(oversize, alignment, commit, large, tld_stats);
     if (start == NULL) return NULL;
     void* p = (uint8_t*)start + extra;
     mi_assert(_mi_is_aligned((uint8_t*)p + offset, alignment));
+    // decommit the overallocation at the start
     if (commit && extra > _mi_os_page_size()) {
       _mi_os_decommit(start, extra, tld_stats);
     }

src/segment.c

@@ -1317,9 +1317,7 @@ mi_page_t* _mi_segment_page_alloc(mi_heap_t* heap, size_t block_size, size_t pag
     mi_assert_internal(_mi_is_power_of_two(page_alignment));
     mi_assert_internal(page_alignment >= MI_SEGMENT_SIZE);
     //mi_assert_internal((MI_SEGMENT_SIZE % page_alignment) == 0);
-    if (page_alignment < MI_SEGMENT_SIZE) {
+    if (page_alignment < MI_SEGMENT_SIZE) { page_alignment = MI_SEGMENT_SIZE; }
-      page_alignment = MI_SEGMENT_SIZE;      
-    }
     page = mi_segment_huge_page_alloc(block_size, page_alignment, tld, os_tld);
   }
   else if (block_size <= MI_SMALL_OBJ_SIZE_MAX) {

test/test-api.c

@@ -34,7 +34,7 @@ we therefore test the API over various inputs. Please add more tests :-)
 
 #include "mimalloc.h"
 // #include "mimalloc-internal.h"
-#include "mimalloc-types.h" // for MI_DEBUG
+#include "mimalloc-types.h" // for MI_DEBUG and MI_ALIGNMENT_MAX
 
 #include "testhelper.h"