define MI_MAX_ALLOC_SIZE as PTRDIFF_MAX (related to #877)

Author: daanx

include/mimalloc/types.h

@@ -195,6 +195,8 @@ typedef int32_t  mi_ssize_t;
 // Alignments over MI_BLOCK_ALIGNMENT_MAX are allocated in dedicated huge page segments
 #define MI_BLOCK_ALIGNMENT_MAX   (MI_SEGMENT_SIZE >> 1)
 
+// we never allocate more than PTRDIFF_MAX (see also <https://sourceware.org/ml/libc-announce/2019/msg00001.html>)
+#define MI_MAX_ALLOC_SIZE   PTRDIFF_MAX
 
 // ------------------------------------------------------
 // Mimalloc pages contain allocated blocks

src/page.c

@@ -857,7 +857,7 @@ static mi_page_t* mi_find_page(mi_heap_t* heap, size_t size, size_t huge_alignme
   // huge allocation?
   const size_t req_size = size - MI_PADDING_SIZE;  // correct for padding_size in case of an overflow on `size`
   if mi_unlikely(req_size > (MI_LARGE_OBJ_SIZE_MAX - MI_PADDING_SIZE) || huge_alignment > 0) {
-    if mi_unlikely(req_size > PTRDIFF_MAX) {  // we don't allocate more than PTRDIFF_MAX (see <https://sourceware.org/ml/libc-announce/2019/msg00001.html>)
+    if mi_unlikely(req_size > MI_MAX_ALLOC_SIZE) {  
       _mi_error_message(EOVERFLOW, "allocation request is too large (%zu bytes)\n", req_size);
       return NULL;
     }