text: /* -----------------------------------------------------------------------------
   *
   * (c) The GHC Team 1998-2008
   *
   * Generational garbage collector
   *
   * Documentation on the architecture of the Garbage Collector can be
   * found in the online commentary:
   *
   *   https://gitlab.haskell.org/ghc/ghc/wikis/commentary/rts/storage/gc
   *
   * ---------------------------------------------------------------------------*/
  
  #pragma once
  
  #include "WSDeque.h"
  #include "GetTime.h" // for Ticks
  
  #include "BeginPrivate.h"
  
  /* -----------------------------------------------------------------------------
     General scheme
  
     ToDo: move this to the wiki when the implementation is done.
  
     We're only going to try to parallelise the copying GC for now.  The
     Plan is as follows.
  
     Each thread has a gc_thread structure (see below) which holds its
     thread-local data.  We'll keep a pointer to this in a thread-local
     variable, or possibly in a register.
  
     In the gc_thread structure is a gen_workspace for each generation.  The
     primary purpose of the gen_workspace is to hold evacuated objects;
     when an object is evacuated, it is copied to the "todo" block in
     the thread's workspace for the appropriate generation.  When the todo
     block is full, it is pushed to the global gen->todos list, which
     is protected by a lock.  (in fact we intervene a one-place buffer
     here to reduce contention).
  
     A thread repeatedly grabs a block of work from one of the
     gen->todos lists, scavenges it, and keeps the scavenged block on
     its own ws->scavd_list (this is to avoid unnecessary contention
     returning the completed buffers back to the generation: we can just
     collect them all later).
  
     When there is no global work to do, we start scavenging the todo
     blocks in the workspaces.  This is where the scan_bd field comes
     in: we can scan the contents of the todo block, when we have
     scavenged the contents of the todo block (up to todo_bd->free), we
     don't want to move this block immediately to the scavd_list,
     because it is probably only partially full.  So we remember that we
     have scanned up to this point by saving the block in ws->scan_bd,
     with the current scan pointer in ws->scan.  Later, when more
     objects have been copied to this block, we can come back and scan
     the rest.  When we visit this workspace again in the future,
     scan_bd may still be the same as todo_bd, or it might be different:
     if enough objects were copied into this block that it filled up,
     then we will have allocated a new todo block, but *not* pushed the
     old one to the generation, because it is partially scanned.
  
     The reason to leave scanning the todo blocks until last is that we
     want to deal with full blocks as far as possible.
     ------------------------------------------------------------------------- */
  
  
  /* -----------------------------------------------------------------------------
     Generation Workspace
  
     A generation workspace exists for each generation for each GC
     thread. The GC thread takes a block from the todos list of the
     generation into the scanbd and then scans it.  Objects referred to
     by those in the scan block are copied into the todo or scavd blocks
     of the relevant generation.
  
     ------------------------------------------------------------------------- */
  
  typedef struct gen_workspace_ {
      generation * gen;           // the gen for this workspace
      struct gc_thread_ * my_gct; // the gc_thread that contains this workspace
  
      // where objects to be scavenged go
      bdescr *     todo_bd;
      StgPtr       todo_free;            // free ptr for todo_bd
      StgPtr       todo_lim;             // lim for todo_bd
  
      WSDeque *    todo_q;
      bdescr *     todo_overflow;
      uint32_t     n_todo_overflow;
  
      // where large objects to be scavenged go
      bdescr *     todo_large_objects;
  
      // Objects that have already been scavenged.
      bdescr *     scavd_list;
      StgWord      n_scavd_blocks;     // count of blocks in this list
      StgWord      n_scavd_words;
  
      // Partially-full, scavenged, blocks
      bdescr *     part_list;
      StgWord      n_part_blocks;      // count of above
      StgWord      n_part_words;
  
      StgWord pad[1];
  
  } gen_workspace ATTRIBUTE_ALIGNED(64);
  // align so that computing gct->gens[n] is a shift, not a multiply
  // fails if the size is <64, which is why we need the pad above
  
  /* ----------------------------------------------------------------------------
     GC thread object
  
     Every GC thread has one of these. It contains all the generation
     specific workspaces and other GC thread local information. At some
     later point it maybe useful to move this other into the TLS store
     of the GC threads
     ------------------------------------------------------------------------- */
  
  /* values for the wakeup field */
  #define GC_THREAD_INACTIVE             0
  #define GC_THREAD_STANDING_BY          1
  #define GC_THREAD_RUNNING              2
  #define GC_THREAD_WAITING_TO_CONTINUE  3
  
  typedef struct gc_thread_ {
      Capability *cap;
  
  #if defined(THREADED_RTS)
      OSThreadId id;                 // The OS thread that this struct belongs to
      SpinLock   gc_spin;
      SpinLock   mut_spin;
      volatile StgWord wakeup;       // NB not StgWord8; only StgWord is guaranteed atomic
  #endif
      uint32_t thread_index;         // a zero based index identifying the thread
  
      bdescr * free_blocks;          // a buffer of free blocks for this thread
                                     //  during GC without accessing the block
                                     //   allocators spin lock.
  
      // These two lists are chained through the STATIC_LINK() fields of static
      // objects.  Pointers are tagged with the current static_flag, so before
      // following a pointer, untag it with UNTAG_STATIC_LIST_PTR().
      StgClosure* static_objects;            // live static objects
      StgClosure* scavenged_static_objects;  // static objects scavenged so far
  
      W_ gc_count;                   // number of GCs this thread has done
  
      // block that is currently being scanned
      bdescr *     scan_bd;
  
      // Remembered sets on this CPU.  Each GC thread has its own
      // private per-generation remembered sets, so it can add an item
      // to the remembered set without taking a lock.  The mut_lists
      // array on a gc_thread is the same as the one on the
      // corresponding Capability; we stash it here too for easy access
      // during GC; see recordMutableGen_GC().
      bdescr **    mut_lists;
  
      // --------------------
      // evacuate flags
  
      uint32_t evac_gen_no;          // Youngest generation that objects
                                     // should be evacuated to in
                                     // evacuate().  (Logically an
                                     // argument to evacuate, but it's
                                     // static a lot of the time so we
                                     // optimise it into a per-thread
                                     // variable).
  
      bool failed_to_evac;           // failure to evacuate an object typically
                                     // Causes it to be recorded in the mutable
                                     // object list
  
      bool eager_promotion;          // forces promotion to the evac gen
                                     // instead of the to-space
                                     // corresponding to the object
  
      W_ thunk_selector_depth;       // used to avoid unbounded recursion in
                                     // evacuate() for THUNK_SELECTOR
  
      // -------------------
      // stats
  
      W_ copied;
      W_ scanned;
      W_ any_work;
      W_ no_work;
      W_ scav_find_work;
  
      Time gc_start_cpu;   // process CPU time
      Time gc_sync_start_elapsed;  // start of GC sync
      Time gc_start_elapsed;  // process elapsed time
      W_ gc_start_faults;
  
      // -------------------
      // workspaces
  
      // array of workspaces, indexed by gen->abs_no.  This is placed
      // directly at the end of the gc_thread structure so that we can get from
      // the gc_thread pointer to a workspace using only pointer
      // arithmetic, no memory access.  This happens in the inner loop
      // of the GC, see Evac.c:alloc_for_copy().
      gen_workspace gens[];
  } gc_thread;
  
  
  extern uint32_t n_gc_threads;
  
  extern gc_thread **gc_threads;
  
  #if defined(THREADED_RTS) && defined(llvm_CC_FLAVOR)
  extern ThreadLocalKey gctKey;
  #endif
  
  #include "EndPrivate.h"
  