303 lines
13 KiB
C++
303 lines
13 KiB
C++
/* GC.hpp
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*
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* author: Roland Conybeare, jul 2025
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*/
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#pragma once
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#include "ListAlloc.hpp"
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#include "GcStatistics.hpp"
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#include "xo/indentlog/print/array.hpp"
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#include <vector>
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#include <array>
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namespace xo {
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/** types that can participate in GC inherit from this base class. See Object.hpp in this directory **/
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class Object;
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namespace gc {
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enum class role {
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/** nursery: generation for new objects **/
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from_space,
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/** tenured: generation for objects that have survived two collections **/
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to_space,
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N,
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};
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constexpr std::size_t role2int(role x) { return static_cast<std::size_t>(x); }
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/** @class Config
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* @brief garbage collector configuration
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**/
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struct Config {
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/** initial size in bytes for youngest (Nursery) generation.
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* GC allocates two nursery spaces of this size.
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* Will allocate more space as needed
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**/
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std::size_t initial_nursery_z_ = 0;
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/** initial size in bytes for oldest (Tenured) generation.
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* GC allocates two tenured spaces of this size
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* Will allocate more space as needed
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**/
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std::size_t initial_tenured_z_ = 0;
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/** true to permit incremental garbage collection **/
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bool allow_incremental_gc_ = true;
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/** true to report statistics **/
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bool stats_flag_ = false;
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/** true to enable debug logging **/
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bool debug_flag_ = false;
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};
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/** @class GCRunstate
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* @brief encapsulate state needed while GC is running
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*
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* state pertaining to a single GC invocation.
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* We stash an instance of this in @ref GC as context,
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* so that per-Object-derived-type auxiliary functions can be slightly streamlined
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**/
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class GCRunstate {
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public:
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GCRunstate() = default;
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explicit GCRunstate(bool in_progress, bool full_move)
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: in_progress_{in_progress}, full_move_{full_move} {}
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bool in_progress() const { return in_progress_; }
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bool full_move() const { return full_move_; }
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private:
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/** true when GC begins; remains true until GC cycle complete **/
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bool in_progress_ = false;
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/** true for full GC; false for incremental GC **/
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bool full_move_ = false;
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};
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class MutationLogEntry {
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public:
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MutationLogEntry(Object * parent, Object ** lhs) : parent_{parent}, lhs_{lhs} {}
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Object * parent() const { return parent_; }
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Object ** lhs() const { return lhs_; }
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Object * child() const { return *lhs_; }
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bool is_child_forwarded() const;
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bool is_parent_forwarded() const;
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Object * parent_destination() const;
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/** Flag obsolete mutation.
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* Future proofing, never happens for regular objects
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**/
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bool is_dead() const { return false; }
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MutationLogEntry update_parent_moved(Object * parent_to) const;
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void fixup_parent_child_moved(Object * child_to) { *lhs_ = child_to; }
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private:
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Object * parent_;
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Object ** lhs_;
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};
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using MutationLog = std::vector<MutationLogEntry>;
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/** @class GC
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* @brief generational garbage collector
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*
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* Works with objects of type @ref xo::Object
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**/
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class GC : public IAlloc {
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public:
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/** create new GC instance with configuration @p config **/
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explicit GC(const Config & config);
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virtual ~GC();
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/** create GC allocator.
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*
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* Initial memory consumption:
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* approximately 2x @ref Config::nursery_size_ + 2x @ref Config::tenured_size_
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**/
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static up<GC> make(const Config & config);
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const GCRunstate & runstate() const { return runstate_; }
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const GcStatistics & native_gc_statistics() const { return gc_statistics_; }
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GcStatisticsExt get_gc_statistics() const;
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/** true iff GC permitted in current state **/
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bool is_gc_enabled() const { return gc_enabled_ == 0; }
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/** true during (and only during) a GC cycle **/
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bool gc_in_progress() const { return runstate_.in_progress(); }
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/** @return generation to which object at @p x belongs **/
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generation_result tospace_generation_of(const void * x) const;
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/** @return generation that contains @p x, given it's in from-space **/
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generation_result fromspace_generation_of(const void * x) const;
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/** true iff from-space contains @p x **/
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bool fromspace_contains(const void * x) const;
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/** @return free pointer for generation @p gen, i.e. nursery or tenured space **/
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std::byte * free_ptr(generation gen);
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/** @return current size of (number of entries in) mutation log **/
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std::size_t mlog_size() const;
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/** add gc root at address @p addr . Gc will keep alive anything reachable
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* from @c *addr
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**/
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void add_gc_root(Object ** addr);
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/** request garbage collection. **/
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void request_gc(generation g);
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/** disable garbage collection until matching call to @ref enable_gc.
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*
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* GC is disabled when number of calls to @ref disable_gc exceeds number of
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* calls to @ref enable_gc.
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**/
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void disable_gc();
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/** enable garbage collection
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*
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* GC is enabled when number of calls to @ref enable_gc is at least as large
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* as number of calls to @ref disable_gc.
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**/
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void enable_gc();
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// inherited from IAlloc..
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virtual const std::string & name() const final override;
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/** capacity in bytes (counting both free+allocated) for object storage.
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* only counts one of {to-space, from-space},
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* since one role is always held empty between collections.
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**/
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virtual std::size_t size() const final override;
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virtual std::size_t allocated() const final override;
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virtual std::size_t available() const final override;
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/** only tests to-space **/
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virtual bool contains(const void * x) const final override;
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virtual bool is_before_checkpoint(const void * x) const final override;
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virtual std::size_t before_checkpoint() const final override;
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virtual std::size_t after_checkpoint() const final override;
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virtual bool debug_flag() const final override;
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virtual void clear() final override;
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virtual void checkpoint() final override;
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/** GC bookkeeping for an assignment that modifes an Object reference.
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* Whenever an @ref Object instance P contains a member variable that can refer
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* to another @ref Object, then we need to involve GC to perform the assignment.
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* In particular a side-effect that changes the target of such reference to Q after P
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* has been promoted, may lead to a tenured->nursery cross-generational pointer.
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* GC needs to know about such pointers to it can update them as part of subsequent
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* incremental collections.
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*
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* @param parent. object with member variable being modified
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* @param lhs. address of a member variable within the allocation of @p parent.
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* @param rhs. new target for @p *lhs
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**/
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virtual void assign_member(Object * parent, Object ** lhs, Object* rhs) final override;
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virtual std::byte * alloc(std::size_t z) final override;
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virtual std::byte * alloc_gc_copy(std::size_t z, const void * src) final override;
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virtual void release_redline_memory() final override;
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private:
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/** begin GC now **/
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void execute_gc(generation g);
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/** cleanup phase. aux function for @ref execute_gc **/
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void cleanup_phase(generation g);
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/** swap roles of From/To spaces for nursery generation **/
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void swap_nursery();
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/** swap roles of From/To spaces for tenured generation **/
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void swap_tenured();
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/** swap roles of From/To spaces for mutation log **/
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void swap_mutation_log();
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/** swap roles of FromSpace/ToSpace **/
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void swap_spaces(generation g);
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/** scan to-space for object statistics before GC */
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void capture_object_statistics(generation upto, capture_phase phase);
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/** copy object **/
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void copy_object(Object ** addr, generation upto, ObjectStatistics * object_stats);
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/** copy everything reachable from global gc roots **/
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void copy_globals(generation g);
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/** review mutation log; may discover+rescue reachable objects.
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**/
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void forward_mutation_log(generation upto);
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/** Aux function for @ref execute_gc. Updates bookkeeping for cross-generational
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* (T->N, aka xgen) and (N1->N0, aka xckp) pointers
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**/
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void incremental_gc_forward_mlog(ObjectStatistics * per_type_stats);
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/**
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* Aux function for @ref incremental_gc_forward_mlog. Calls this function until
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* fixpoint.
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*
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* @param from_mlog incoming mutation log. Contains {xgen,xckp} pointers before GC.
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* Contents of this log is consumed (+discarded) before method returns.
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* @param to_mlog outgoing mutation log. Will contain {xgen,xckp} pointers after GC.
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* @param defer_mlog contains log entries associated with possible garbage.
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**/
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void incremental_gc_forward_mlog_phase(MutationLog * from_mlog,
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MutationLog * to_mlog,
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MutationLog * defer_mlog,
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ObjectStatistics * per_type_stats);
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private:
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/** garbage collector configuration **/
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Config config_;
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/** contains allocated objects, along with unreachable garbage to be collected.
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* roles reverse after each incremental, or full, collection.
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**/
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std::array<up<ListAlloc>, role2int(role::N)> nursery_;
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/** empty space, destination for objects that survive collection.
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* roles reverse after each full collection.
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**/
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std::array<up<ListAlloc>, role2int(role::N)> tenured_;
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/** current state of GC activity.
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* @text
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* in_progress full_move descr
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* -----------------------------------------
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* false * gc not running
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* true false incremental gc
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* true true full gc
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* -----------------------------------------
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* @endtext
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**/
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GCRunstate runstate_;
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/** root object handles: targets of handles in this vector are always preserved by GC.
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* Application can introduce new root object pointers at any time provided GC not running,
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* but cannot withdraw them.
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**/
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std::vector<Object**> gc_root_v_;
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/** log cross-generational and cross-checkpoint mutations.
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* These need to be adjusted on next incremental collection
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**/
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std::array<up<MutationLog>, role2int(role::N)> mutation_log_;
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/** temporary mutation log (for deferred entries) **/
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up<MutationLog> defer_mutation_log_;
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/** allocation/collection counters **/
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GcStatistics gc_statistics_;
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/** optional per-object-type counters. snapshot at beginning of collection cycle **/
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std::array<ObjectStatistics, gen2int(generation::N)> object_statistics_sab_;
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/** optional per-object-type counters. snapshot at end of collection cycle **/
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std::array<ObjectStatistics, gen2int(generation::N)> object_statistics_sae_;
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/** trigger full GC whenever this much data arrives in tenured generation **/
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std::size_t full_gc_threshold_ = 0;
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/** trigger incr GC whenever this much data arrives in nuresery generation **/
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std::size_t incr_gc_threshold_ = 0;
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/** true when GC requested,
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* remains true until GC.. completes? begins?
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**/
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bool incr_gc_pending_ = false;
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bool full_gc_pending_ = false;
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/** enabled when 0. disabled when <0 **/
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int gc_enabled_ = 0;
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};
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} /*namespace gc*/
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} /*namespace xo*/
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/* end GC.hpp */
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