add guarded objects that are sampled (and fit a size range). guarded sample rate etc can be set per heap as well as defaulted with options

include/mimalloc.h

@@ -309,6 +309,12 @@ mi_decl_nodiscard mi_decl_export mi_heap_t* mi_heap_new_ex(int heap_tag, bool al
 // deprecated
 mi_decl_export int mi_reserve_huge_os_pages(size_t pages, double max_secs, size_t* pages_reserved) mi_attr_noexcept;
 
+// Experimental: objects followed by a guard page.
+// A sample rate of 0 disables guarded objects, while 1 uses a guard page for every object.
+// A seed of 0 uses a random start point. Only objects within the size bound are eligable for guard pages.
+mi_decl_export void mi_heap_guarded_set_sample_rate(mi_heap_t* heap, size_t sample_rate, size_t seed);
+mi_decl_export void mi_heap_guarded_set_size_bound(mi_heap_t* heap, size_t min, size_t max);
+
 
 // ------------------------------------------------------
 // Convenience

include/mimalloc/internal.h

@@ -611,17 +611,23 @@ static inline bool mi_block_ptr_is_guarded(const mi_block_t* block, const void*
 }
 
 static inline bool mi_heap_malloc_use_guarded(mi_heap_t* heap, size_t size) {
-  MI_UNUSED(heap);  
-  if (heap->guarded_sample_rate==0 ||
-      size > heap->guarded_size_max ||
-      size < heap->guarded_size_min) {
+  // this code is written to result in fast assembly as it is on the hot path for allocation
+  const size_t count = heap->guarded_sample_count - 1;  // if the rate was 0, this will underflow and count for a long time..
+  if mi_likely(count != 0) {
+    // no sample
+    heap->guarded_sample_count = count;
     return false;
   }
-  if (++heap->guarded_sample_count < heap->guarded_sample_rate) {
-    return false;
+  else if (size >= heap->guarded_size_min && size <= heap->guarded_size_max) {
+    // use guarded allocation
+    heap->guarded_sample_count = heap->guarded_sample_rate;  // reset
+    return (heap->guarded_sample_rate != 0);
   }
-  heap->guarded_sample_count = 0;  // reset
-  return true;
+  else {
+    // failed size criteria, rewind count (but don't write to an empty heap)
+    if (heap->guarded_sample_rate != 0) { heap->guarded_sample_count = 1; } 
+    return false;
+  }  
 }
 
 mi_decl_restrict void* _mi_heap_malloc_guarded(mi_heap_t* heap, size_t size, bool zero) mi_attr_noexcept;

include/mimalloc/types.h

@@ -507,7 +507,7 @@ struct mi_heap_s {
   size_t                guarded_size_max;                    // maximal size for guarded objects
   size_t                guarded_sample_rate;                 // sample rate (set to 0 to disable guarded pages)
   size_t                guarded_sample_seed;                 // starting sample count
-  size_t                guarded_sample_count;                // current sample count (wraps at `sample_rate`)
+  size_t                guarded_sample_count;                // current sample count (counting down to 0)
   #endif
   mi_page_t*            pages_free_direct[MI_PAGES_DIRECT];  // optimize: array where every entry points a page with possibly free blocks in the corresponding queue for that size.
   mi_page_queue_t       pages[MI_BIN_FULL + 1];              // queue of pages for each size class (or "bin")