Commit b15b4a17 authored by Simon Marlow's avatar Simon Marlow

Small refactoring, and add comments

I discovered a new invariant while experimenting (blackholing is not
optional when using parallel GC), so documented it.
parent db12b8ea
......@@ -195,6 +195,10 @@ threadPaused(Capability *cap, StgTSO *tso)
maybePerformBlockedException (cap, tso);
if (tso->what_next == ThreadKilled) { return; }
// NB. Blackholing is *not* optional, we must either do lazy
// blackholing, or eager blackholing consistently. See Note
// [upd-black-hole] in sm/Scav.c.
stack_end = &tso->stack[tso->stack_size];
frame = (StgClosure *)tso->sp;
......@@ -1680,24 +1680,34 @@ scavenge_stack(StgPtr p, StgPtr stack_end)
// the indirection into an IND_PERM, so that evacuate will
// copy the indirection into the old generation instead of
// discarding it.
// Note [upd-black-hole]
// One slight hiccup is that the THUNK_SELECTOR machinery can
// overwrite the updatee with an IND. In parallel GC, this
// could even be happening concurrently, so we can't check for
// the IND. Fortunately if we assume that blackholing is
// happening (either lazy or eager), then we can be sure that
// the updatee is never a THUNK_SELECTOR and we're ok.
// NB. this is a new invariant: blackholing is not optional.
nat type;
const StgInfoTable *i;
StgClosure *updatee;
i = ((StgUpdateFrame *)p)->updatee->;
updatee = ((StgUpdateFrame *)p)->updatee;
i = updatee->;
type = get_itbl(((StgUpdateFrame *)p)->updatee)->type;
type = get_itbl(updatee)->type;
if (type == IND) {
((StgUpdateFrame *)p)->updatee-> =
(StgInfoTable *)&stg_IND_PERM_info;
updatee-> = &stg_IND_PERM_info;
} else if (type == IND_OLDGEN) {
((StgUpdateFrame *)p)->updatee-> =
(StgInfoTable *)&stg_IND_OLDGEN_PERM_info;
updatee-> = &stg_IND_OLDGEN_PERM_info;
evacuate(&((StgUpdateFrame *)p)->updatee);
p += sizeofW(StgUpdateFrame);
evacuate(&((StgUpdateFrame *)p)->updatee);
ASSERT(GET_CLOSURE_TAG(((StgUpdateFrame *)p)->updatee) == 0);
p += sizeofW(StgUpdateFrame);
// small bitmap (< 32 entries, or 64 on a 64-bit machine)
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