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/* Part of SWI-Prolog
Author: Jan Wielemaker
E-mail: J.Wielemaker@vu.nl
WWW: http://www.swi-prolog.org
Copyright (C): 1985-2013, University of Amsterdam
VU University Amsterdam
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
/*#define O_DEBUG 1*/
#include "pl-incl.h"
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Segmented stack handling. A segmented stack is a stack that is allocated
in segments, This means we cannot compare addresses otherwise then by
identity. We use a segmented stack for cycle detection.
Measurements with the chunksize on SuSE Linux 10.2 indicate there is no
measurable performance change above approximately 256 bytes. We'll keep
the figure on the safe side for systems with less efficient malloc
implementations.
Note that atom-gc requires completely asynchronous calling of
scanSegStack() and therefore pushSegStack()/popSegStack() must push the
data before updating the pointers.
TBD: Avoid instruction/cache write reordering in push/pop.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
int
pushSegStack_(segstack *stack, void *data)
{ if ( stack->top + stack->unit_size <= stack->max )
{ memcpy(stack->top, data, stack->unit_size);
stack->top += stack->unit_size;
return TRUE;
} else
{ segchunk *chunk = PL_malloc(SEGSTACK_CHUNKSIZE);
if ( !chunk )
return FALSE; /* out of memory */
chunk->allocated = TRUE;
chunk->size = SEGSTACK_CHUNKSIZE;
chunk->next = NULL;
chunk->previous = stack->last;
chunk->top = CHUNK_DATA(chunk); /* async scanning */
if ( stack->last )
{ stack->last->next = chunk;
stack->last->top = stack->top;
stack->top = chunk->top; /* async scanning */
stack->last = chunk;
} else
{ stack->top = chunk->top; /* async scanning */
stack->last = stack->first = chunk;
}
stack->base = CHUNK_DATA(chunk);
stack->max = addPointer(chunk, chunk->size);
memcpy(CHUNK_DATA(chunk), data, stack->unit_size);
stack->top = CHUNK_DATA(chunk) + stack->unit_size;
return TRUE;
}
}
int
pushRecordSegStack(segstack *stack, Record r)
{ if ( stack->top + sizeof(r) <= stack->max )
{ Record *rp = (Record*)stack->top;
*rp++ = r;
stack->top = (char*)rp;
return TRUE;
} else
{ return pushSegStack_(stack, &r);
}
}
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Pop data. Note that we leave the first chunk associated with the stack
to speedup frequent small usage.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
int
popSegStack_(segstack *stack, void *data)
{ again:
if ( stack->top >= stack->base + stack->unit_size )
{ stack->top -= stack->unit_size;
memcpy(data, stack->top, stack->unit_size);
return TRUE;
} else
{ segchunk *chunk = stack->last;
if ( chunk )
{ if ( chunk->previous )
{ stack->last = chunk->previous;
stack->last->next = NULL;
if ( chunk->allocated )
PL_free(chunk);
chunk = stack->last;
stack->base = CHUNK_DATA(chunk);
stack->max = addPointer(chunk, chunk->size);
stack->top = chunk->top;
goto again;
}
}
return FALSE;
}
}
void *
topOfSegStack(segstack *stack)
{ segchunk *chunk;
if ( stack->top >= stack->base + stack->unit_size )
{ return stack->top - stack->unit_size;
} else if ( stack->last && (chunk=stack->last->previous) )
{ assert(chunk->top - stack->unit_size >= CHUNK_DATA(chunk));
return chunk->top - stack->unit_size;
}
return NULL;
}
void
popTopOfSegStack_(segstack *stack)
{ again:
if ( stack->top >= stack->base + stack->unit_size )
{ stack->top -= stack->unit_size;
} else
{ segchunk *chunk = stack->last;
if ( chunk )
{ if ( chunk->previous )
{ segchunk *del = chunk;
stack->last = chunk->previous;
stack->last->next = NULL;
chunk = stack->last;
stack->top = chunk->top;
MemoryBarrier(); /* Sync with scanSegStack() */
stack->base = CHUNK_DATA(chunk);
stack->max = addPointer(chunk, chunk->size);
if ( del->allocated )
PL_free(del);
goto again;
}
}
assert(0);
}
}
/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
scanSegStack(segstack *stack, void (*func)(void *cell))
Walk along all living cells on the stack and call func on them. The stack
is traversed last-to-first.
This is used by markAtomsFindall(), which runs concurrent with findall/3
in the thread being scanned. New records pushed while AGC is in progress
are marked by findall/3 itself. Findall/3 can concurrently pop this
stack. It may do this quicker than the marking, but this is no problem
because the pointers remain valid as all records are reclaimed at the
end of findall/3 by calling clear_mem_pool(). It is only a problem if
popTopOfSegStack_() pops a chunk because the popped chunk is freed.
Therefore $collect_findall_bag/2 locks if it needs to call the expensive
chunk-popping popTopOfSegStack_().
With thanks to Eugeniy Meshcheryakov for finding this and running many
tests. The issue is triggered by Tests/thread_agc_findall.pl, notably on
slow single core hardware.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
static inline void
scan_chunk(segstack *stack, char *top, char *base, void (*func)(void *cell))
{ while(top >= base+stack->unit_size)
{ top -= stack->unit_size;
(*func)((void*)top);
}
}
void
scanSegStack(segstack *stack, void (*func)(void *cell))
{ segchunk *chunk;
if ( (chunk=stack->last) ) /* something there */
{ if ( stack->base == CHUNK_DATA(chunk) )
chunk->top = stack->top; /* close last chunk */
for(; chunk; chunk=chunk->previous)
scan_chunk(stack, chunk->top, CHUNK_DATA(chunk), func);
}
}
void
clearSegStack_(segstack *s)
{ segchunk *c = s->first;
segchunk *n;
if ( !c->allocated ) /* statically allocated first chunk */
{ n = c->next;
c->next = NULL;
s->last = c;
s->base = s->top = c->top;
s->max = addPointer(c, c->size);
for(c=n; c; c = n)
{ n = c->next;
PL_free(c);
}
} else /* all dynamic chunks */
{ for(; c; c = n)
{ n = c->next;
PL_free(c);
}
memset(s, 0, sizeof(*s));
}
}
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