xo-alloc2: refactor - explicit AllocHeader class

xo-alloc2/include/xo/alloc2/alloc/AllocHeader.hpp

@@ -0,0 +1,167 @@
+/** @file AllocHeader.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include <type_traits>
+#include <cstdint>
+#include <cstddef>
+
+namespace xo {
+    namespace mm {
+        struct AllocHeader {
+            using repr_type = std::uintptr_t;
+            using size_type = std::size_t;
+
+            explicit AllocHeader(repr_type x) : repr_{x} {}
+
+#ifdef OBSOLETE
+            std::uint32_t tseq(const AllocHeaderConfig & cfg) const noexcept {
+                return cfg.tseq(repr_);
+            }
+
+            std::uint32_t age(const AllocHeaderConfig & cfg) const noexcept {
+                return cfg.age(repr_);
+            }
+
+            size_type size(const AllocHeaderConfig & cfg) const noexcept {
+                return cfg.size(repr_);
+            }
+#endif
+
+            repr_type repr_;
+        };
+
+        static_assert(sizeof(AllocHeader) == sizeof(AllocHeader::repr_type));
+        static_assert(std::is_standard_layout_v<AllocHeader>);
+
+        /*
+         * Each allocation is preceded by a 64-bit header.
+         * Header is split into 3 configurable-width bit fields,
+         * labelled (from hi to lo bit order) {tseq, age, size}.
+         *
+         * 1. tseq. seq# identifying object types; needed for gc.
+         * 2. gen.  age cohort; increases when alloc survives gc.
+         * 3. size. alloc size.
+         *
+         * Arena allocator only uses size.
+         * X1 collector uses {tseq, gen, size}
+         *
+         * alloc header
+         *
+         *  TTTTTTTTTTTTGGGGGZZZZZZZZZZZZ
+         *  <   tseq   ><gen><   size   >
+         *
+         * masking
+         *
+         *  ..432107654321076543210 bit
+         *
+         *                 >       <  .gen_bits
+         *  0..............01111111   gen_mask_unshifted
+         *  0..011111110..........0   gen_mask_shifted
+         *             >           <  gen_shift
+         */
+        struct AllocHeaderConfig {
+            using repr_type = AllocHeader;
+
+            AllocHeaderConfig() = default;
+            AllocHeaderConfig(std::uint8_t t, std::uint8_t a, std::uint8_t z) noexcept
+                : tseq_bits_{t}, age_bits_{a}, size_bits_{z} {}
+
+            std::uint64_t tseq_mask() const noexcept {
+                // e.g.
+                //   FF FF FF 00 00 00 00 00
+                // with tseq_bits=24, age_bits=8, size_bits=32
+                //
+                return ((1ul << tseq_bits_) - 1) << (age_bits_ + size_bits_);
+            }
+
+            std::uint64_t age_mask() const noexcept {
+                // e.g.
+                //   00 00 00 FF 00 00 00 00
+                // with age_bits=8, size_bits=32
+                //
+                return ((1ul << age_bits_) - 1) << size_bits_;
+            }
+
+            std::uint64_t size_mask() const noexcept {
+                // e.g.
+                //   00 00 00 00 FF FF FF FF
+                // with size_bits=32
+                //
+                return ((1ul << size_bits_) - 1);
+            }
+
+            /** extract type id from alloc header @p hdr **/
+            std::uint32_t tseq(repr_type hdr) const noexcept {
+                // e.g.
+                //   0x302010
+                // for header
+                //   30 20 10 -- -- -- -- --
+                // with tseq_bits_ = 24, age_bits_ + size_bits_ = 40
+                //
+                return (hdr.repr_ & tseq_mask()) >> (age_bits_ + size_bits_);
+            }
+
+            /** extract age from alloc header @p hdr **/
+            std::uint32_t age(repr_type hdr) const noexcept {
+                // e.g.
+                //  0xa0
+                // for header
+                //   -- -- -- a0 -- -- -- --
+                // with age_bits_ = 8, size_bits_ = 32
+                //
+                return (hdr.repr_ & age_mask()) >> size_bits_;
+            }
+
+            /** extract size from alloc header @p hdr **/
+            std::size_t size(repr_type hdr) const noexcept {
+                // e.g.
+                //  0x01020300
+                // for header
+                //   -- -- -- -- 01 02 03 00
+                // with size_bits_ = 32
+                //
+                return (hdr.repr_ & size_mask());
+            }
+
+            /** true iff sentinel tseq, flagging a forwarding pointer **/
+            bool is_forwarding_tseq(repr_type hdr) const noexcept {
+                // e.g.
+                //   0xFFFFFF
+                // i.e. header
+                //   FF FF FF -- -- -- -- --
+                // with tseq_bits_ = 24, age_bits + size_bits_ = 40
+                //
+                return (hdr.repr_ & tseq_mask()) == tseq_mask();
+            }
+
+            bool is_size_enabled() const noexcept { return size_bits_ > 0; }
+
+            /** number of bits for tseq **/
+            std::uint8_t tseq_bits_ = 24;
+            /** number of bits for age **/
+            std::uint8_t age_bits_ = 8;
+            /** number of bits for size **/
+            std::uint8_t size_bits_ = 32;
+        };
+
+        struct AllocInfo {
+            using size_type = AllocHeader::size_type;
+
+            AllocInfo(const AllocHeaderConfig * p_cfg, const AllocHeader * p_hdr)
+                : p_config_{p_cfg}, p_header_{p_hdr} {}
+
+            std::uint32_t tseq() const noexcept { return p_config_->tseq(*p_header_); }
+            std::uint32_t  age() const noexcept { return p_config_->age (*p_header_); }
+            size_type     size() const noexcept { return p_config_->size(*p_header_); }
+
+            const AllocHeaderConfig * p_config_ = nullptr;
+            const AllocHeader *       p_header_ = nullptr;
+        };
+    }
+}
+
+/* end AllocHeader.hpp */