wip: purgeable arenas; update option names and improve arena reservation

src/os.c

@@ -440,6 +440,25 @@ bool _mi_os_unreset(void* addr, size_t size, bool* is_zero, mi_stats_t* tld_stat
 }
 
 
+// either resets or decommits memory, returns true if the memory was decommitted.
+bool _mi_os_purge(void* p, size_t size, mi_stats_t* stats)
+{
+  if (!mi_option_is_enabled(mi_option_allow_purge)) return false;
+
+  if (mi_option_is_enabled(mi_option_purge_decommits) &&   // should decommit?
+    !_mi_preloading())                                   // don't decommit during preloading (unsafe)
+  {
+    _mi_os_decommit(p, size, stats);
+    return true;  // decommitted
+  }
+  else {
+    _mi_os_reset(p, size, stats);
+    return false;  // not decommitted
+  }
+}
+
+
+
 // Protect a region in memory to be not accessible.
 static  bool mi_os_protectx(void* addr, size_t size, bool protect) {
   // page align conservatively within the range