1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 1998-2006
5 * Generational garbage collector: evacuation functions
7 * Documentation on the architecture of the Garbage Collector can be
8 * found in the online commentary:
10 * http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
12 * ---------------------------------------------------------------------------*/
22 #include "LdvProfile.h"
24 /* Used to avoid long recursion due to selector thunks
26 #define MAX_THUNK_SELECTOR_DEPTH 16
28 static void eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool);
29 STATIC_INLINE void evacuate_large(StgPtr p);
31 /* -----------------------------------------------------------------------------
32 Allocate some space in which to copy an object.
33 -------------------------------------------------------------------------- */
36 alloc_for_copy (nat size, step *stp)
41 /* Find out where we're going, using the handy "to" pointer in
42 * the step of the source object. If it turns out we need to
43 * evacuate to an older generation, adjust it here (see comment
46 if (stp < gct->evac_step) {
47 if (gct->eager_promotion) {
50 gct->failed_to_evac = rtsTrue;
54 ws = &gct->steps[stp->gen_no][stp->no];
56 /* chain a new block onto the to-space for the destination step if
60 ASSERT(ws->todo_free >= ws->todo_bd->free && ws->todo_free <= ws->todo_lim);
62 if (to + size >= ws->todo_lim) {
63 to = gc_alloc_todo_block(ws);
65 ws->todo_free = to + size;
66 ASSERT(ws->todo_free >= ws->todo_bd->free && ws->todo_free <= ws->todo_lim);
71 /* -----------------------------------------------------------------------------
73 -------------------------------------------------------------------------- */
81 /* -----------------------------------------------------------------------------
82 Evacuate a large object
84 This just consists of removing the object from the (doubly-linked)
85 step->large_objects list, and linking it on to the (singly-linked)
86 step->new_large_objects list, from where it will be scavenged later.
88 Convention: bd->flags has BF_EVACUATED set for a large object
89 that has been evacuated, or unset otherwise.
90 -------------------------------------------------------------------------- */
93 evacuate_large(StgPtr p)
95 bdescr *bd = Bdescr(p);
99 // object must be at the beginning of the block (or be a ByteArray)
100 ASSERT(get_itbl((StgClosure *)p)->type == ARR_WORDS ||
101 (((W_)p & BLOCK_MASK) == 0));
103 // already evacuated?
104 if (bd->flags & BF_EVACUATED) {
105 /* Don't forget to set the gct->failed_to_evac flag if we didn't get
106 * the desired destination (see comments in evacuate()).
108 if (bd->step < gct->evac_step) {
109 gct->failed_to_evac = rtsTrue;
110 TICK_GC_FAILED_PROMOTION();
117 ACQUIRE_SPIN_LOCK(&stp->sync_large_objects);
118 // remove from large_object list
120 bd->u.back->link = bd->link;
121 } else { // first object in the list
122 stp->large_objects = bd->link;
125 bd->link->u.back = bd->u.back;
127 RELEASE_SPIN_LOCK(&stp->sync_large_objects);
129 /* link it on to the evacuated large object list of the destination step
132 if (stp < gct->evac_step) {
133 if (gct->eager_promotion) {
134 stp = gct->evac_step;
136 gct->failed_to_evac = rtsTrue;
140 ws = &gct->steps[stp->gen_no][stp->no];
142 bd->gen_no = stp->gen_no;
143 bd->link = ws->todo_large_objects;
144 ws->todo_large_objects = bd;
145 bd->flags |= BF_EVACUATED;
148 /* -----------------------------------------------------------------------------
149 Evaluate a THUNK_SELECTOR if possible.
151 p points to a THUNK_SELECTOR that we want to evaluate. The
152 result of "evaluating" it will be evacuated and a pointer to the
153 to-space closure will be returned.
155 If the THUNK_SELECTOR could not be evaluated (its selectee is still
156 a THUNK, for example), then the THUNK_SELECTOR itself will be
158 -------------------------------------------------------------------------- */
160 unchain_thunk_selectors(StgSelector *p, StgClosure *val)
168 ASSERT(p->header.info == &stg_WHITEHOLE_info);
170 ASSERT(p->header.info == &stg_BLACKHOLE_info);
172 prev = (StgSelector*)((StgClosure *)p)->payload[0];
174 // Update the THUNK_SELECTOR with an indirection to the
175 // EVACUATED closure now at p. Why do this rather than
176 // upd_evacuee(q,p)? Because we have an invariant that an
177 // EVACUATED closure always points to an object in the
178 // same or an older generation (required by the short-cut
179 // test in the EVACUATED case, below).
180 SET_INFO(p, &stg_IND_info);
181 ((StgInd *)p)->indirectee = val;
183 // For the purposes of LDV profiling, we have created an
185 LDV_RECORD_CREATE(p);
192 eval_thunk_selector (StgClosure **q, StgSelector * p, rtsBool evac)
193 // NB. for legacy reasons, p & q are swapped around :(
198 StgClosure *selectee;
199 StgSelector *prev_thunk_selector;
203 prev_thunk_selector = NULL;
204 // this is a chain of THUNK_SELECTORs that we are going to update
205 // to point to the value of the current THUNK_SELECTOR. Each
206 // closure on the chain is a BLACKHOLE, and points to the next in the
207 // chain with payload[0].
211 bd = Bdescr((StgPtr)p);
212 if (HEAP_ALLOCED(p)) {
213 // If the THUNK_SELECTOR is in to-space or in a generation that we
214 // are not collecting, then bale out early. We won't be able to
215 // save any space in any case, and updating with an indirection is
216 // trickier in a non-collected gen: we would have to update the
218 if ((bd->gen_no > N) || (bd->flags & BF_EVACUATED)) {
219 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
220 *q = (StgClosure *)p;
223 // we don't update THUNK_SELECTORS in the compacted
224 // generation, because compaction does not remove the INDs
225 // that result, this causes confusion later
226 // (scavenge_mark_stack doesn't deal with IND). BEWARE! This
227 // bit is very tricky to get right. If you make changes
228 // around here, test by compiling stage 3 with +RTS -c -RTS.
229 if (bd->flags & BF_COMPACTED) {
230 // must call evacuate() to mark this closure if evac==rtsTrue
231 *q = (StgClosure *)p;
232 if (evac) evacuate(q);
233 unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
239 // BLACKHOLE the selector thunk, since it is now under evaluation.
240 // This is important to stop us going into an infinite loop if
241 // this selector thunk eventually refers to itself.
242 #if defined(THREADED_RTS)
243 // In threaded mode, we'll use WHITEHOLE to lock the selector
244 // thunk while we evaluate it.
246 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
247 if (info_ptr == (W_)&stg_WHITEHOLE_info) {
249 info_ptr = xchg((StgPtr)&p->header.info, (W_)&stg_WHITEHOLE_info);
250 } while (info_ptr == (W_)&stg_WHITEHOLE_info);
253 // make sure someone else didn't get here first
254 if (INFO_PTR_TO_STRUCT(info_ptr)->type != THUNK_SELECTOR) {
259 // Save the real info pointer (NOTE: not the same as get_itbl()).
260 info_ptr = (StgWord)p->header.info;
261 SET_INFO(p,&stg_BLACKHOLE_info);
264 field = INFO_PTR_TO_STRUCT(info_ptr)->layout.selector_offset;
266 // The selectee might be a constructor closure,
267 // so we untag the pointer.
268 selectee = UNTAG_CLOSURE(p->selectee);
271 // selectee now points to the closure that we're trying to select
272 // a field from. It may or may not be in to-space: we try not to
273 // end up in to-space, but it's impractical to avoid it in
274 // general. The compacting GC scatters to-space pointers in
275 // from-space during marking, for example. We rely on the property
276 // that evacuate() doesn't mind if it gets passed a to-space pointer.
278 info = get_itbl(selectee);
279 switch (info->type) {
281 goto bale_out; // about to be evacuated by another thread (or a loop).
290 case CONSTR_NOCAF_STATIC:
292 // check that the size is in range
293 ASSERT(field < (StgWord32)(info->layout.payload.ptrs +
294 info->layout.payload.nptrs));
296 // Select the right field from the constructor
297 val = selectee->payload[field];
300 // For the purposes of LDV profiling, we have destroyed
301 // the original selector thunk, p.
302 SET_INFO(p, (StgInfoTable *)info_ptr);
303 LDV_RECORD_DEAD_FILL_SLOP_DYNAMIC((StgClosure *)p);
304 SET_INFO(p, &stg_BLACKHOLE_info);
307 // the closure in val is now the "value" of the
308 // THUNK_SELECTOR in p. However, val may itself be a
309 // THUNK_SELECTOR, in which case we want to continue
310 // evaluating until we find the real value, and then
311 // update the whole chain to point to the value.
313 info = get_itbl(UNTAG_CLOSURE(val));
314 switch (info->type) {
318 case IND_OLDGEN_PERM:
320 val = ((StgInd *)val)->indirectee;
323 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
324 prev_thunk_selector = p;
325 p = (StgSelector*)val;
328 ((StgClosure*)p)->payload[0] = (StgClosure *)prev_thunk_selector;
329 prev_thunk_selector = p;
332 if (evac) evacuate(q);
334 // evacuate() cannot recurse through
335 // eval_thunk_selector(), because we know val is not
337 unchain_thunk_selectors(prev_thunk_selector, val);
345 case IND_OLDGEN_PERM:
347 // Again, we might need to untag a constructor.
348 selectee = UNTAG_CLOSURE( ((StgInd *)selectee)->indirectee );
352 // We don't follow pointers into to-space; the constructor
353 // has already been evacuated, so we won't save any space
354 // leaks by evaluating this selector thunk anyhow.
361 // recursively evaluate this selector. We don't want to
362 // recurse indefinitely, so we impose a depth bound.
363 if (gct->thunk_selector_depth >= MAX_THUNK_SELECTOR_DEPTH) {
367 gct->thunk_selector_depth++;
368 // rtsFalse says "don't evacuate the result". It will,
369 // however, update any THUNK_SELECTORs that are evaluated
371 eval_thunk_selector(&val, (StgSelector*)selectee, rtsFalse);
372 gct->thunk_selector_depth--;
374 // did we actually manage to evaluate it?
375 if (val == selectee) goto bale_out;
377 // Of course this pointer might be tagged...
378 selectee = UNTAG_CLOSURE(val);
392 case SE_CAF_BLACKHOLE:
399 barf("eval_thunk_selector: strange selectee %d",
404 // We didn't manage to evaluate this thunk; restore the old info
405 // pointer. But don't forget: we still need to evacuate the thunk itself.
406 SET_INFO(p, (const StgInfoTable *)info_ptr);
408 copy(&val,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->step->to);
410 val = (StgClosure *)p;
413 unchain_thunk_selectors(prev_thunk_selector, val);
417 /* -----------------------------------------------------------------------------
418 move_TSO is called to update the TSO structure after it has been
419 moved from one place to another.
420 -------------------------------------------------------------------------- */
423 move_TSO (StgTSO *src, StgTSO *dest)
427 // relocate the stack pointer...
428 diff = (StgPtr)dest - (StgPtr)src; // In *words*
429 dest->sp = (StgPtr)dest->sp + diff;