xo-alloc2: work on fomo Arena

include/xo/alloc2/DArena.hpp

@@ -0,0 +1,82 @@
+/** @file DArena.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include <string>
+
+namespace xo {
+    namespace mm {
+
+        /** ArenaConfig
+         **/
+        struct ArenaConfig {
+            /** optional name, for diagnostics **/
+            std::string name_;
+            /** arena size -- hard max = reserved virtual memory **/
+            std::size_t size_;
+            /** hugepage size -- using huge pages relieves some TLB pressure
+             *  (provided you use their full extent :)
+             **/
+            std::size_t hugepage_z_ = 2 * 1024 * 1024;
+            /** true to enable debug logging **/
+            bool debug_flag_ = false;
+        };
+
+        /** Arena allocator state
+         *
+         *    <----------------------------size-------------------------->
+         *    <------------committed-----------><-------uncommitted------>
+         *    <--allocated-->
+         *
+         *    XXXXXXXXXXXXXXX___________________..........................
+         *
+         *    allocated:   in use
+         *    committed:   physical memory obtained
+         *    uncommitted: mapped in virtual memory, not backed by memory
+         **/
+        struct DArena {
+            /** [lo, hi) already-mapped address range **/
+            DArena(const ArenaConfig & cfg,
+                   std::byte * lo,
+                   std::byte * hi);
+
+            ~DArena();
+
+            ArenaConfig config_;
+
+            /** size of a VM page (via getpagesize()). Likely 4k **/
+            std::size_t page_z_ = 0;
+
+            /** arena owns memory in range [@ref lo_, @ref hi_)
+             **/
+            std::byte * lo_ = nullptr;
+
+            /** prefix of this size is committed.
+             *  Remainder mapped but uncommitted.
+             **/
+            std::size_t committed_z_ = 0;
+
+            /** free pointer.
+             *  Memory in range [@ref lo_, @ref free_) current in use
+             **/
+            std::byte * free_ = nullptr;
+
+            /** soft limit; end of committed virtual memory
+             *  Memory in range [@ref lo_, @ref limit_) is committed
+             *  (backed by physical memory)
+             **/
+            std::byte * limit_ = nullptr;
+
+            /** hard limit; end of reserved virtual memory
+             *  Memory in range [@ref limit_, @ref hi_) is uncommitted
+             **/
+            std::byte * hi_ = nullptr;
+        };
+
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end DArena.hpp */