emacs/mps/code/seg.c

/* impl.c.seg: SEGMENTS
 *
 * $Id$
 * $HopeName: MMsrc!seg.c(trunk.30) $
 * Copyright (c) 2001 Ravenbrook Limited.
 *
 * .design: The design for this module is design.mps.seg.
 *
 * PURPOSE
 *
 * .purpose: This is the implementation of the generic segment interface.
 * It defines the interface functions and two useful segment classes:
 * .purpose.class.seg: Class Seg is a class which is as simple
 * as efficiency demands permit.  (It includes fields for storing colour
 * for efficiency).  It may be subclassed by clients of the module.
 * .purpose.class.seg-gc: Class GCSeg is a concrete class support all
 * all current GC features, and providing full backwards compatibility
 * with "old-style" segments.  It may be subclassed by clients of the
 * module. 
 *
 * TRANSGRESSIONS
 *
 * .check.shield: The "pm", "sm", and "depth" fields are not checked by
 * SegCheck, because I haven't spent time working out the invariants.
 * We should certainly work them out, by studying impl.c.shield, and
 * assert things about shielding, protection, shield cache consistency,
 * etc.  richard 1997-04-03
 */

#include "tract.h"
#include "mpm.h"

SRCID(seg, "$Id$");


/* SegGCSeg -- convert generic Seg to GCSeg */

#define SegGCSeg(seg)             ((GCSeg)(seg))

/* SegPoolRing -- Pool ring accessor */

#define SegPoolRing(seg)          (&(seg)->poolRing)


/* forward declarations */

static void SegFinish(Seg seg);

static Res SegInit(Seg seg, Pool pool, Addr base, Size size,
                   Bool withReservoirPermit, va_list args);


/* Generic interface support */


/* SegAlloc -- allocate a segment from the arena */

Res SegAlloc(Seg *segReturn, SegClass class, SegPref pref, 
             Size size, Pool pool, Bool withReservoirPermit, ...)
{
  Res res;
  Arena arena;
  Seg seg;
  Addr base;
  va_list args;

  AVER(segReturn != NULL);
  AVERT(SegClass, class);
  AVERT(SegPref, pref);
  AVER(size > (Size)0);
  AVERT(Pool, pool);
  AVER(BoolCheck(withReservoirPermit));

  arena = PoolArena(pool);
  AVERT(Arena, arena);
  AVER(SizeIsAligned(size, arena->alignment));

  /* allocate the memory from the arena */
  res = ArenaAlloc(&base, pref, size, pool, withReservoirPermit);
  if (res != ResOK)
    goto failArena;

  /* allocate the segment object from the control pool */
  res = ControlAlloc((void **)&seg, arena, class->size, withReservoirPermit);
  if (res != ResOK)
    goto failControl;

  va_start(args, withReservoirPermit);
  seg->class = class;
  res = SegInit(seg, pool, base, size, withReservoirPermit, args);
  va_end(args);
  if (res != ResOK)
    goto failInit;

  EVENT_PPAWP(SegAlloc, arena, seg, SegBase(seg), size, pool);
  *segReturn = seg;
  return ResOK;

failInit:
  ControlFree(arena, seg, class->size);
failControl:
  ArenaFree(base, size, pool);
failArena:
  EVENT_PWP(SegAllocFail, arena, size, pool);
  return res;
}


/* SegFree -- free a segment to the arena */

void SegFree(Seg seg)
{
  Arena arena;
  Pool pool;
  Addr base;
  Size size;
  SegClass class;

  AVERT(Seg, seg);
  pool = SegPool(seg);
  AVERT(Pool, pool);
  arena = PoolArena(pool);
  AVERT(Arena, arena);
  base = SegBase(seg);
  size = SegSize(seg);
  class = seg->class;

  SegFinish(seg);
  ControlFree(arena, seg, class->size);
  ArenaFree(base, size, pool);

  EVENT_PP(SegFree, arena, seg);
  return;
}


/* SegInit -- initialize a segment */

static Res SegInit(Seg seg, Pool pool, Addr base, Size size,
                   Bool withReservoirPermit, va_list args)
{
  Tract tract;
  Addr addr, limit;
  Size align;
  Arena arena;
  SegClass class;
  Res res;

  AVER(seg != NULL);
  AVERT(Pool, pool);
  arena = PoolArena(pool);
  align = ArenaAlign(arena);
  AVER(AddrIsAligned(base, align));
  AVER(SizeIsAligned(size, align));
  class = seg->class;
  AVERT(SegClass, class);
  AVER(BoolCheck(withReservoirPermit));

  limit = AddrAdd(base, size);
  seg->limit = limit;
  seg->rankSet = RankSetEMPTY;
  seg->white = TraceSetEMPTY;
  seg->nailed = TraceSetEMPTY;
  seg->grey = TraceSetEMPTY;
  seg->pm = AccessSetEMPTY;
  seg->sm = AccessSetEMPTY;
  seg->depth = 0;
  seg->firstTract = NULL;

  seg->sig = SegSig;  /* set sig now so tract checks will see it */

  TRACT_FOR(tract, addr, arena, base, limit) {
    AVER(TractCheck(tract));  /* design.mps.check.type.no-sig */
    AVER(TractP(tract) == NULL);
    AVER(!TractHasSeg(tract));
    AVER(TractPool(tract) == pool);
    AVER(TractWhite(tract) == TraceSetEMPTY);
    TRACT_SET_SEG(tract, seg);
    if (addr == base) {
      AVER(seg->firstTract == NULL);
      seg->firstTract = tract;
    }
    AVER(seg->firstTract != NULL);
  }
  AVER(addr == seg->limit);

  RingInit(SegPoolRing(seg));

  /* Class specific initialization comes last */
  res = class->init(seg, pool, base, size, withReservoirPermit, args);
  if (res != ResOK)
    goto failInit;
    
  AVERT(Seg, seg);
  RingAppend(&pool->segRing, SegPoolRing(seg));
  return ResOK;

failInit:
  RingFinish(SegPoolRing(seg));
  TRACT_FOR(tract, addr, arena, base, limit) {
    AVER(TractCheck(tract));  /* design.mps.check.type.no-sig */
    TRACT_UNSET_SEG(tract);
  }
  seg->sig = SigInvalid;
  return res;
}


/* SegFinish -- finish a segment */

static void SegFinish(Seg seg)
{
  Arena arena;
  Addr addr, base, limit;
  Tract tract;
  SegClass class;

  AVERT(Seg, seg);
  class = seg->class;
  AVERT(SegClass, class);

  arena = PoolArena(SegPool(seg));
  if (seg->sm != AccessSetEMPTY) {
    ShieldLower(arena, seg, seg->sm);
  }

  /* Class specific finishing cames first */
  class->finish(seg);

  seg->rankSet = RankSetEMPTY;

  /* See impl.c.shield.shield.flush */
  ShieldFlush(PoolArena(SegPool(seg)));

  base = SegBase(seg);
  limit = SegLimit(seg);
  TRACT_TRACT_FOR(tract, addr, arena, seg->firstTract, limit) {
    AVER(TractCheck(tract));  /* design.mps.check.type.no-sig */
    TractSetWhite(tract, TraceSetEMPTY);
    TRACT_UNSET_SEG(tract);
  }
  AVER(addr == seg->limit);

  RingRemove(SegPoolRing(seg));
  RingFinish(SegPoolRing(seg));

  seg->sig = SigInvalid;

  /* Check that the segment is not exposed, or in the shield */
  /* cache (see impl.c.shield.def.depth). */
  AVER(seg->depth == 0);
  /* Check not shielded or protected (so that pages in hysteresis */
  /* fund are not protected) */
  AVER(seg->sm == AccessSetEMPTY);
  AVER(seg->pm == AccessSetEMPTY);
  
}


/* SegSetGrey -- change the greyness of a segment
 *
 * Sets the segment greyness to the trace set grey.
 */

void SegSetGrey(Seg seg, TraceSet grey)
{
  AVERT(Seg, seg);
  AVER(TraceSetCheck(grey));
  seg->class->setGrey(seg, grey);
}


/* SegSetWhite -- change the whiteness of a segment
 *
 * Sets the segment whiteness to the trace set ts.
 */

void SegSetWhite(Seg seg, TraceSet white)
{
  AVERT(Seg, seg);
  AVER(TraceSetCheck(white));
  seg->class->setWhite(seg, white);
}


/* SegSetRankSet -- set the rank set of a segment
 *
 * The caller must set the summary to empty before setting the rank
 * set to empty.  The caller must set the rank set to non-empty before
 * setting the summary to non-empty.
 */

void SegSetRankSet(Seg seg, RankSet rankSet)
{
  AVERT(Seg, seg);
  AVER(RankSetCheck(rankSet));
  seg->class->setRankSet(seg, rankSet);
}


/* SegSetSummary -- change the summary on a segment */

void SegSetSummary(Seg seg, RefSet summary)
{
  AVERT(Seg, seg);

#ifdef PROTECTION_NONE
  summary = RefSetUNIV;
#endif
  seg->class->setSummary(seg, summary);
}


/* SegSetRankAndSummary -- set both the rank set and the summary */

void SegSetRankAndSummary(Seg seg, RankSet rankSet, RefSet summary)
{
  AVERT(Seg, seg);  
  AVER(RankSetCheck(rankSet));

#ifdef PROTECTION_NONE
  if (rankSet != RankSetEMPTY) {
    summary = RefSetUNIV;
  }
#endif
  seg->class->setRankSummary(seg, rankSet, summary);
}


/* SegBuffer -- return the buffer of a segment */

Buffer SegBuffer(Seg seg)
{
  AVERT_CRITICAL(Seg, seg);  /* .seg.critical */
  return seg->class->buffer(seg);
}


/* SegSetBuffer -- change the buffer on a segment */

void SegSetBuffer(Seg seg, Buffer buffer)
{
  AVERT(Seg, seg);
  if (buffer != NULL)
    AVERT(Buffer, buffer);
  seg->class->setBuffer(seg, buffer);
}


/* SegDescribe -- describe a segment */

Res SegDescribe(Seg seg, mps_lib_FILE *stream)
{
  Res res;
  Pool pool;

  if (!CHECKT(Seg, seg)) return ResFAIL;
  if (stream == NULL) return ResFAIL;

  pool = SegPool(seg);

  res = WriteF(stream,
               "Segment $P [$A,$A) {\n", (WriteFP)seg,
               (WriteFA)SegBase(seg), (WriteFA)SegLimit(seg),
               "  class $P (\"$S\")\n", 
               (WriteFP)seg->class, seg->class->name,
               "  pool $P ($U)\n",
               (WriteFP)pool, (WriteFU)pool->serial,
               NULL);
  if (res != ResOK) return res;

  res = seg->class->describe(seg, stream);
  if (res != ResOK) return res;

  res = WriteF(stream, "\n",
               "} Segment $P\n", (WriteFP)seg, NULL);
  return res;
}


/* .seg.critical: These seg functions are low-level and used 
 * through-out. They are therefore on the critical path and their 
 * AVERs are so-marked.
 */

/* SegBase -- return the base address of a seg */

Addr (SegBase)(Seg seg)
{
  AVERT_CRITICAL(Seg, seg);
  return SegBase(seg);
}


/* SegLimit -- return the limit address of a segment */

Addr (SegLimit)(Seg seg)
{
  AVERT_CRITICAL(Seg, seg);
  return SegLimit(seg);
}


/* SegSize -- return the size of a seg */

Size SegSize(Seg seg)
{
  AVERT_CRITICAL(Seg, seg);
  return AddrOffset(SegBase(seg), SegLimit(seg));
}
 

/* SegOfAddr -- return the seg the given address is in, if any */

Bool SegOfAddr(Seg *segReturn, Arena arena, Addr addr)
{
  Tract tract;
  AVER_CRITICAL(segReturn != NULL);   /* .seg.critical */
  AVERT_CRITICAL(Arena, arena);       /* .seg.critical */
  if (TractOfAddr(&tract, arena, addr)) {
    return TRACT_SEG(segReturn, tract);
  } else {
    return FALSE;
  }
}


/* SegFirst -- return the first seg in the arena
 *
 * This is used to start an iteration over all segs in the arena.
 */

Bool SegFirst(Seg *segReturn, Arena arena)
{
  Tract tract;
  AVER(segReturn != NULL);
  AVERT(Arena, arena);

  if (TractFirst(&tract, arena)) {
    do {
      Seg seg;
      if (TRACT_SEG(&seg, tract)) {
        *segReturn = seg;
        return TRUE;
      } 
    } while (TractNext(&tract, arena, TractBase(tract)));
  } 
  return FALSE;
}


/* SegNext -- return the "next" seg in the arena
 *
 * This is used as the iteration step when iterating over all
 * segs in the arena.
 *
 * SegNext finds the seg with the lowest base address which is
 * greater than a specified address.  The address must be (or once
 * have been) the base address of a seg.
 */

Bool SegNext(Seg *segReturn, Arena arena, Addr addr)
{
  Tract tract;
  Addr base = addr;
  AVER_CRITICAL(segReturn != NULL); /* .seg.critical */
  AVERT_CRITICAL(Arena, arena);

  while (TractNext(&tract, arena, base)) {
    Seg seg;
    if (TRACT_SEG(&seg, tract)) {
      if (tract == seg->firstTract) {
        *segReturn = seg;
        return TRUE;
      } else {
        /* found the next tract in a large segment */
        /* base & addr must be the base of this segment */
        AVER_CRITICAL(TractBase(seg->firstTract) == addr);
        AVER_CRITICAL(addr == base);
        /* set base to the last tract in the segment */
        base = AddrSub(seg->limit, ArenaAlign(arena));
        AVER_CRITICAL(base > addr);
      }
    } else {
      base = TractBase(tract);
    }
  } 
  return FALSE;
}



/* SegMerge -- Merge two adjacent segments
 *
 * See design.mps.seg.merge
 */

Res SegMerge(Seg *mergedSegReturn, Seg segLo, Seg segHi,
             Bool withReservoirPermit, ...)
{
  SegClass class;
  Addr base, mid, limit;
  Arena arena;
  Res res;
  va_list args;

  AVER(NULL != mergedSegReturn);
  AVERT(Seg, segLo);
  AVERT(Seg, segHi);
  class = segLo->class;
  AVER(segHi->class == class);
  AVER(SegPool(segLo) == SegPool(segHi));
  base = SegBase(segLo);
  mid = SegLimit(segLo);
  limit = SegLimit(segHi);
  AVER(SegBase(segHi) == SegLimit(segLo));
  AVER(BoolCheck(withReservoirPermit));
  arena = PoolArena(SegPool(segLo));

  ShieldFlush(arena);  /* see design.mps.seg.split-merge.shield */

  /* Invoke class-specific methods to do the merge */
  va_start(args, withReservoirPermit);
  res = class->merge(segLo, segHi, base, mid, limit, 
                     withReservoirPermit, args);
  va_end(args);
  if (ResOK != res)
    goto failMerge;

  EVENT_PPP(SegMerge, segLo, segLo, segHi);
  /* Deallocate segHi object */
  ControlFree(arena, segHi, class->size);
  AVERT(Seg, segLo);
  *mergedSegReturn = segLo;
  return ResOK;

failMerge:
  AVERT(Seg, segLo); /* check original segs are still valid */
  AVERT(Seg, segHi);
  return res;
}


/* SegSplit -- Split a segment
 *
 * The segment is split at the indicated position.
 * See design.mps.seg.split
 */

Res SegSplit(Seg *segLoReturn, Seg *segHiReturn, Seg seg, Addr at,
             Bool withReservoirPermit, ...)
{
  Addr base, limit;
  SegClass class;
  Seg segNew;
  Arena arena;
  Res res;
  va_list args;

  AVER(NULL != segLoReturn);
  AVER(NULL != segHiReturn);
  AVERT(Seg, seg);
  class = seg->class;
  arena = PoolArena(SegPool(seg));
  base = SegBase(seg);
  limit = SegLimit(seg);
  AVERT(Arena, arena);
  AVER(AddrIsAligned(at, arena->alignment));
  AVER(at > base);
  AVER(at < limit);
  AVER(BoolCheck(withReservoirPermit));

  ShieldFlush(arena);  /* see design.mps.seg.split-merge.shield */

  /* Allocate the new segment object from the control pool */
  res = ControlAlloc((void **)&segNew, arena, class->size,
                     withReservoirPermit);
  if (ResOK != res)
    goto failControl;

  /* Invoke class-specific methods to do the split */
  va_start(args, withReservoirPermit);
  res = class->split(seg, segNew, base, at, limit, 
                     withReservoirPermit, args);
  va_end(args);
  if (ResOK != res)
    goto failSplit;

  EVENT_PPPA(SegSplit, seg, segNew, seg, at);
  AVERT(Seg, seg);
  AVERT(Seg, segNew);
  *segLoReturn = seg;
  *segHiReturn = segNew;
  return ResOK;

failSplit:
  ControlFree(arena, segNew, class->size);
failControl:
  AVERT(Seg, seg); /* check the original seg is still valid */
  return res;
}


/* Class Seg -- The most basic segment class
 *
 * .seg.method.check: Many seg methods are lightweight and used 
 * frequently. Their parameters are checked by the corresponding
 * dispatching function, and so the their parameter AVERs are 
 * marked as critical. 
 */


/* SegCheck -- check the integrity of a segment */

Bool SegCheck(Seg seg)
{
  Tract tract;
  Arena arena;
  Pool pool;
  Addr addr;
  Size align;
  
  CHECKS(Seg, seg);
  CHECKL(TraceSetCheck(seg->white));

  /* can't assume nailed is subset of white - mightn't be during whiten */
  /* CHECKL(TraceSetSub(seg->nailed, seg->white)); */
  CHECKL(TraceSetCheck(seg->grey));
  CHECKL(TractCheck(seg->firstTract));  /* design.mps.check.type.no-sig */
  pool = SegPool(seg);
  CHECKU(Pool, pool);
  arena = PoolArena(pool);
  CHECKU(Arena, arena);
  align = ArenaAlign(arena);
  CHECKL(AddrIsAligned(TractBase(seg->firstTract), align));
  CHECKL(AddrIsAligned(seg->limit, align));
  CHECKL(seg->limit > TractBase(seg->firstTract));

  /* Each tract of the segment must agree about white traces */
  TRACT_TRACT_FOR(tract, addr, arena, seg->firstTract, seg->limit) {
    Seg trseg;

    UNUSED(trseg); /* @@@@ unused in hot varieties */
    CHECKL(TractCheck(tract));  /* design.mps.check.type.no-sig */
    CHECKL(TRACT_SEG(&trseg, tract) && (trseg == seg));
    CHECKL(TractWhite(tract) == seg->white);
    CHECKL(TractPool(tract) == pool);
  }
  CHECKL(addr == seg->limit);

  /* The segment must belong to some pool, so it should be on a */
  /* pool's segment ring.  (Actually, this isn't true just after */
  /* the segment is initialized.) */
  /*  CHECKL(RingNext(&seg->poolRing) != &seg->poolRing); */

  CHECKL(RingCheck(&seg->poolRing));
    
  /* "pm", "sm", and "depth" not checked.  See .check.shield. */
  CHECKL(RankSetCheck(seg->rankSet));
  if (seg->rankSet == RankSetEMPTY) {
    /* design.mps.seg.field.rankSet.empty: If there are no refs */
    /* in the segment then it cannot contain black or grey refs. */
    CHECKL(seg->grey == TraceSetEMPTY);
    CHECKL(seg->sm == AccessSetEMPTY);
    CHECKL(seg->pm == AccessSetEMPTY);
  } else {
    /* design.mps.seg.field.rankSet.single: The Tracer only permits */
    /* one rank per segment [ref?] so this field is either empty or a */
    /* singleton. */
    CHECKL(RankSetIsSingle(seg->rankSet));
    /* Can't check barrier invariants because SegCheck is called */
    /* when raising or lowering the barrier. */
    /* .check.wb: If summary isn't universal then it must be */
    /* write shielded. */
    /* CHECKL(seg->_summary == RefSetUNIV || (seg->_sm & AccessWRITE)); */
    /* @@@@ What can be checked about the read barrier? */
  }
  return TRUE;
}


/* segTrivInit -- method to initialize the base fields of a segment */

static Res segTrivInit(Seg seg, Pool pool, Addr base, Size size, 
                       Bool reservoirPermit, va_list args)
{
  /* all the initialization happens in SegInit so checks are safe */
  Size align;
  Arena arena;

  AVERT(Seg, seg);
  AVERT(Pool, pool);
  arena = PoolArena(pool);
  align = ArenaAlign(arena);
  AVER(AddrIsAligned(base, align));
  AVER(SizeIsAligned(size, align));
  AVER(SegBase(seg) == base);
  AVER(SegSize(seg) == size);
  AVER(SegPool(seg) == pool);
  AVER(BoolCheck(reservoirPermit));
  UNUSED(args);
  return ResOK;
}


/* segTrivFinish -- finish the base fields of a segment */

static void segTrivFinish(Seg seg)
{
  /* all the generic finishing happens in SegFinish  */
  AVERT(Seg, seg);
}


/* segNoSetGrey -- non-method to change the greyness of a segment */

static void segNoSetGrey(Seg seg, TraceSet grey)
{
  AVERT(Seg, seg);
  AVER(TraceSetCheck(grey));
  AVER(seg->rankSet != RankSetEMPTY);
  NOTREACHED;
}


/* segNoSetWhite -- non-method to change the whiteness of a segment */

static void segNoSetWhite(Seg seg, TraceSet white)
{
  AVERT(Seg, seg);
  AVER(TraceSetCheck(white));
  NOTREACHED;
}


/* segNoSetRankSet -- non-method to set the rank set of a segment */

static void segNoSetRankSet(Seg seg, RankSet rankSet)
{
  AVERT(Seg, seg);
  AVER(RankSetCheck(rankSet));
  NOTREACHED;
}


/* segNoSetSummary -- non-method to set the summary of a segment */

static void segNoSetSummary(Seg seg, RefSet summary)
{
  AVERT(Seg, seg);
  UNUSED(summary);
  NOTREACHED;
}


/* segNoSetRankSummary -- non-method to set the rank set & summary */

static void segNoSetRankSummary(Seg seg, RankSet rankSet, RefSet summary)
{
  AVERT(Seg, seg);
  AVER(RankSetCheck(rankSet));
  UNUSED(summary);
  NOTREACHED;
}


/* segNoBuffer -- non-method to return the buffer of a segment */

static Buffer segNoBuffer(Seg seg)
{
  AVERT(Seg, seg);
  NOTREACHED;
  return NULL;
}


/* segNoSetBuffer -- non-method to set the buffer of a segment */

static void segNoSetBuffer(Seg seg, Buffer buffer)
{
  AVERT(Seg, seg);
  if (buffer != NULL)
    AVERT(Buffer, buffer);
  NOTREACHED;
}



/* segNoMerge -- merge method for segs which don't support merge */

static Res segNoMerge(Seg seg, Seg segHi, 
                      Addr base, Addr mid, Addr limit,
                      Bool withReservoirPermit, va_list args)
{
  AVERT(Seg, seg);
  AVERT(Seg, segHi);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == mid);
  AVER(SegBase(segHi) == mid);
  AVER(SegLimit(segHi) == limit);
  AVER(BoolCheck(withReservoirPermit));
  UNUSED(args);
  NOTREACHED;
  return ResFAIL;
}


/* segTrivMerge -- Basic Seg merge method 
 *
 * .similar: Segments must be "sufficiently similar". 
 * See design.mps.seg.merge.inv.similar 
 */

static Res segTrivMerge(Seg seg, Seg segHi, 
                        Addr base, Addr mid, Addr limit,
                        Bool withReservoirPermit, va_list args)
{
  Pool pool;
  Size align;
  Arena arena;
  Tract tract;
  Addr addr;

  AVERT(Seg, seg);
  AVERT(Seg, segHi);
  pool = SegPool(seg);
  arena = PoolArena(pool);
  align = ArenaAlign(arena);
  AVER(AddrIsAligned(base, align));
  AVER(AddrIsAligned(mid, align));
  AVER(AddrIsAligned(limit, align));
  AVER(base < mid);
  AVER(mid < limit);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == mid);
  AVER(SegBase(segHi) == mid);
  AVER(SegLimit(segHi) == limit);
  AVER(BoolCheck(withReservoirPermit));
  UNUSED(args);

  /* .similar.  */
  AVER(seg->rankSet == segHi->rankSet);
  AVER(seg->white == segHi->white);
  AVER(seg->nailed == segHi->nailed);
  AVER(seg->grey == segHi->grey);
  AVER(seg->pm == segHi->pm);
  AVER(seg->sm == segHi->sm);
  AVER(seg->depth == segHi->depth);
  /* Neither segment may be exposed, or in the shield cache */
  /* See design.mps.seg.split-merge.shield & impl.c.shield.def.depth */
  AVER(seg->depth == 0);

  /* no need to update fields which match. See .similar */

  seg->limit = limit;
  TRACT_FOR(tract, addr, arena, mid, limit) {
    AVER(TractCheck(tract));  /* design.mps.check.type.no-sig */
    AVER(TractHasSeg(tract));
    AVER(segHi == TractP(tract));
    AVER(TractPool(tract) == pool);
    TRACT_SET_SEG(tract, seg);
  }
  AVER(addr == seg->limit);

  /* Finish segHi. */
  RingRemove(SegPoolRing(segHi));
  RingFinish(SegPoolRing(segHi));
  segHi->sig = SigInvalid;

  AVERT(Seg, seg);
  return ResOK;
}


/* segNoSplit -- split method for segs which don't support splitting */

static Res segNoSplit(Seg seg, Seg segHi, 
                      Addr base, Addr mid, Addr limit,
                      Bool withReservoirPermit, va_list args)
{
  AVERT(Seg, seg);
  AVER(segHi != NULL);  /* can't check fully, it's not initialized */
  AVER(base < mid);
  AVER(mid < limit);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == limit);
  AVER(BoolCheck(withReservoirPermit));
  UNUSED(args);
  NOTREACHED;
  return ResFAIL;

}


/* segTrivSplit -- Basic Seg split method */

static Res segTrivSplit(Seg seg, Seg segHi, 
                        Addr base, Addr mid, Addr limit,
                        Bool withReservoirPermit, va_list args)
{
  Tract tract;
  Pool pool;
  Addr addr;
  Size align;
  Arena arena;

  AVERT(Seg, seg);
  AVER(segHi != NULL);  /* can't check fully, it's not initialized */
  pool = SegPool(seg);
  arena = PoolArena(pool);
  align = ArenaAlign(arena);
  AVER(AddrIsAligned(base, align));
  AVER(AddrIsAligned(mid, align));
  AVER(AddrIsAligned(limit, align));
  AVER(base < mid);
  AVER(mid < limit);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == limit);
  AVER(BoolCheck(withReservoirPermit));
  UNUSED(args);

  /* Segment may not be exposed, or in the shield cache */
  /* See design.mps.seg.split-merge.shield & impl.c.shield.def.depth */
  AVER(seg->depth == 0);
  
  /* Full initialization for segHi. Just modify seg. */
  seg->limit = mid;
  segHi->limit = limit;
  segHi->rankSet = seg->rankSet;
  segHi->white = seg->white;
  segHi->nailed = seg->nailed;
  segHi->grey = seg->grey;
  segHi->pm = seg->pm;
  segHi->sm = seg->sm;
  segHi->depth = seg->depth;
  segHi->firstTract = NULL;
  segHi->class = seg->class;
  segHi->sig = SegSig;
  RingInit(SegPoolRing(segHi));

  TRACT_FOR(tract, addr, arena, mid, limit) {
    AVER(TractCheck(tract));  /* design.mps.check.type.no-sig */
    AVER(TractHasSeg(tract));
    AVER(seg == TractP(tract));
    AVER(TractPool(tract) == pool);
    TRACT_SET_SEG(tract, segHi);
    if (addr == mid) {
      AVER(segHi->firstTract == NULL);
      segHi->firstTract = tract;
    }
    AVER(segHi->firstTract != NULL);
  }
  AVER(addr == segHi->limit);

  RingAppend(&pool->segRing, SegPoolRing(segHi));
  AVERT(Seg, seg);
  AVERT(Seg, segHi);
  return ResOK;
}


/* segTrivDescribe -- Basic Seg description method */

static Res segTrivDescribe(Seg seg, mps_lib_FILE *stream)
{
  Res res;

  if (!CHECKT(Seg, seg)) return ResFAIL;
  if (stream == NULL) return ResFAIL;

  res = WriteF(stream,
               "  shield depth $U\n", (WriteFU)seg->depth,
               "  protection mode:",
               NULL);
  if (res != ResOK) return res;
  if (SegPM(seg) & AccessREAD) {
     res = WriteF(stream, " read", NULL);
     if (res != ResOK) return res;
  }
  if (SegPM(seg) & AccessWRITE) {
     res = WriteF(stream, " write", NULL);
     if (res != ResOK) return res;
  }
  res = WriteF(stream, "\n  shield mode:", NULL);
  if (res != ResOK) return res;
  if (SegSM(seg) & AccessREAD) {
     res = WriteF(stream, " read", NULL);
     if (res != ResOK) return res;
  }
  if (SegSM(seg) & AccessWRITE) {
     res = WriteF(stream, " write", NULL);
     if (res != ResOK) return res;
  }
  res = WriteF(stream, "\n  ranks:", NULL);
  /* This bit ought to be in a RankSetDescribe in ref.c. */
  if (RankSetIsMember(seg->rankSet, RankAMBIG)) {
     res = WriteF(stream, " ambiguous", NULL);
     if (res != ResOK) return res;
  }
  if (RankSetIsMember(seg->rankSet, RankEXACT)) {
     res = WriteF(stream, " exact", NULL);
     if (res != ResOK) return res;
  }
  if (RankSetIsMember(seg->rankSet, RankFINAL)) {
     res = WriteF(stream, " final", NULL);
     if (res != ResOK) return res;
  }
  if (RankSetIsMember(seg->rankSet, RankWEAK)) {
     res = WriteF(stream, " weak", NULL);
     if (res != ResOK) return res;
  }
  res = WriteF(stream, "\n",
               "  white  $B\n", (WriteFB)seg->white,
               "  grey   $B\n", (WriteFB)seg->grey,
               "  nailed $B\n", (WriteFB)seg->nailed,
               NULL);
  return res;
}


/* Class GCSeg -- Segment class with GC support
 */


/* GCSegCheck -- check the integrity of a GCSeg */

Bool GCSegCheck(GCSeg gcseg)
{
  Seg seg;
  CHECKS(GCSeg, gcseg);
  seg = &gcseg->segStruct;
  CHECKL(SegCheck(seg));

  if (gcseg->buffer != NULL) {
    CHECKU(Buffer, gcseg->buffer);
    /* design.mps.seg.field.buffer.owner */
    CHECKL(BufferPool(gcseg->buffer) == SegPool(seg));
    CHECKL(BufferRankSet(gcseg->buffer) == SegRankSet(seg));
  }

  /* The segment should be on a grey ring if and only if it is grey. */
  CHECKL(RingCheck(&gcseg->greyRing));
  CHECKL((seg->grey == TraceSetEMPTY) ==
         RingIsSingle(&gcseg->greyRing));

  if (seg->rankSet == RankSetEMPTY) {
    /* design.mps.seg.field.rankSet.empty */
    CHECKL(gcseg->summary == RefSetEMPTY);
  } 

  return TRUE;
}


/* gcSegInit -- method to initialize a GC segment */

static Res gcSegInit(Seg seg, Pool pool, Addr base, Size size,
                     Bool withReservoirPermit, va_list args)
{
  SegClass super;
  GCSeg gcseg;
  Arena arena;
  Align align;
  Res res;

  AVERT(Seg, seg);
  AVERT(Pool, pool);
  arena = PoolArena(pool);
  align = ArenaAlign(arena);
  AVER(AddrIsAligned(base, align));
  AVER(SizeIsAligned(size, align));
  gcseg = SegGCSeg(seg);
  AVER(&gcseg->segStruct == seg);
  AVER(BoolCheck(withReservoirPermit));

  /* Initialize the superclass fields first via next-method call */
  super = SEG_SUPERCLASS(GCSegClass);
  res = super->init(seg, pool, base, size, withReservoirPermit, args);
  if (ResOK != res)
    return res;

  gcseg->summary = RefSetEMPTY;
  gcseg->buffer = NULL;
  RingInit(&gcseg->greyRing);
  gcseg->sig = GCSegSig;

  AVERT(GCSeg, gcseg);
  return ResOK;
}


/* gcSegFinish -- finish a GC segment */

static void gcSegFinish(Seg seg)
{
  SegClass super;
  GCSeg gcseg;

  AVERT(Seg, seg);
  gcseg = SegGCSeg(seg);
  AVERT(GCSeg, gcseg);
  AVER(&gcseg->segStruct == seg);

  if (SegGrey(seg) != TraceSetEMPTY) {
    RingRemove(&gcseg->greyRing);
    seg->grey = TraceSetEMPTY;
  }
  gcseg->summary = RefSetEMPTY;

  gcseg->sig = SigInvalid;

  /* Don't leave a dangling buffer allocating into hyperspace. */
  AVER(gcseg->buffer == NULL);

  RingFinish(&gcseg->greyRing);

  /* finish the superclass fields last */
  super = SEG_SUPERCLASS(GCSegClass);
  super->finish(seg);
}


/* gcSegSetGreyInternal -- change the greyness of a segment
 *
 * Internal method for updating the greyness of a GCSeg.
 * Updates the grey ring and the grey seg count.
 * Doesn't affect the shield (so it can be used by split
 * & merge methods).
 */

static void gcSegSetGreyInternal(Seg seg, TraceSet oldGrey, TraceSet grey)
{
  GCSeg gcseg;
  Arena arena;
  Rank rank;
  
  /* Internal method. Parameters are checked by caller */
  gcseg = SegGCSeg(seg);
  arena = PoolArena(SegPool(seg));
  seg->grey = grey;

  /* If the segment is now grey and wasn't before, add it to the */
  /* appropriate grey list so that TraceFindGrey can locate it */
  /* quickly later.  If it is no longer grey and was before, */
  /* remove it from the list. */
  if (oldGrey == TraceSetEMPTY) {
    if (grey != TraceSetEMPTY) {
      AVER(RankSetIsSingle(seg->rankSet));
      for(rank = 0; rank < RankLIMIT; ++rank)
	if (RankSetIsMember(seg->rankSet, rank)) {
	  RingInsert(ArenaGreyRing(arena, rank), &gcseg->greyRing);
	  break;
	}
      AVER(rank != RankLIMIT); /* there should've been a match */
    }
  } else {
    if (grey == TraceSetEMPTY)
      RingRemove(&gcseg->greyRing);
  }

  STATISTIC_STAT
    ({
       TraceId ti; Trace trace;
       TraceSet diff;

       diff = TraceSetDiff(grey, oldGrey);
       TRACE_SET_ITER(ti, trace, diff, arena)
         ++trace->greySegCount;
         if (trace->greySegCount > trace->greySegMax)
           trace->greySegMax = trace->greySegCount;
       TRACE_SET_ITER_END(ti, trace, diff, arena);

       diff = TraceSetDiff(oldGrey, grey);
       TRACE_SET_ITER(ti, trace, diff, arena)
         --trace->greySegCount;
       TRACE_SET_ITER_END(ti, trace, diff, arena);
     });

}


/* gcSegSetGrey -- GCSeg method to change the greyness of a segment
 *
 * Sets the segment greyness to the trace set grey and adjusts
 * the shielding on the segment appropriately.
 */

static void gcSegSetGrey(Seg seg, TraceSet grey)
{
  GCSeg gcseg;
  TraceSet oldGrey, flippedTraces;
  Arena arena;
  
  AVERT_CRITICAL(Seg, seg);            /* .seg.method.check */
  AVER_CRITICAL(TraceSetCheck(grey));  /* .seg.method.check */
  AVER(seg->rankSet != RankSetEMPTY);
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);
  UNUSED(gcseg);

  arena = PoolArena(SegPool(seg));
  oldGrey = seg->grey;
  gcSegSetGreyInternal(seg, oldGrey, grey); /* do the work */

  /* The read barrier is raised when the segment is grey for */
  /* some _flipped_ trace, i.e., is grey for a trace for which */
  /* the mutator is black. */
  flippedTraces = arena->flippedTraces;
  if (TraceSetInter(oldGrey, flippedTraces) == TraceSetEMPTY) {
    if (TraceSetInter(grey, flippedTraces) != TraceSetEMPTY)
      ShieldRaise(arena, seg, AccessREAD);
  } else {
    if (TraceSetInter(grey, flippedTraces) == TraceSetEMPTY)
      ShieldLower(arena, seg, AccessREAD);
  }

  EVENT_PPU(SegSetGrey, arena, seg, grey);
}


/* gcSegSetWhite -- GCSeg method to change whiteness of a segment
 *
 * Sets the segment whiteness to the trace set ts.
 */

static void gcSegSetWhite(Seg seg, TraceSet white)
{
  GCSeg gcseg;
  Tract tract;
  Arena arena;
  Addr addr, limit;

  AVERT_CRITICAL(Seg, seg);            /* .seg.method.check */
  AVER_CRITICAL(TraceSetCheck(white)); /* .seg.method.check */
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);

  arena = PoolArena(SegPool(seg));
  AVERT_CRITICAL(Arena, arena);
  limit = SegLimit(seg);
  /* Each tract of the segment records white traces */
  TRACT_TRACT_FOR(tract, addr, arena, seg->firstTract, limit) {
    Seg trseg;

    UNUSED(trseg); /* @@@@ hack: unused in hot varieties */
    AVER_CRITICAL(TractCheck(tract));  /* design.mps.check.type.no-sig */
    AVER_CRITICAL(TRACT_SEG(&trseg, tract) && (trseg == seg));
    TractSetWhite(tract, white);
  }
  AVER(addr == limit);

  seg->white = white;
}


/* gcSegSetRankSet -- GCSeg method to set the rank set of a segment
 *
 * If the rank set is made non-empty then the segment's summary is
 * now a subset of the mutator's (which is assumed to be RefSetUNIV)
 * so the write barrier must be imposed on the segment.  If the
 * rank set is made empty then there are no longer any references
 * on the segment so the barrier is removed.
 *
 * The caller must set the summary to empty before setting the rank
 * set to empty.  The caller must set the rank set to non-empty before
 * setting the summary to non-empty.
 */

static void gcSegSetRankSet(Seg seg, RankSet rankSet)
{
  GCSeg gcseg;
  RankSet oldRankSet;
  Arena arena;

  AVERT_CRITICAL(Seg, seg);                /* .seg.method.check */
  AVER_CRITICAL(RankSetCheck(rankSet));    /* .seg.method.check */
  AVER_CRITICAL(rankSet == RankSetEMPTY
                || RankSetIsSingle(rankSet)); /* .seg.method.check */
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);

  arena = PoolArena(SegPool(seg));
  oldRankSet = seg->rankSet;
  seg->rankSet = rankSet;

  if (oldRankSet == RankSetEMPTY) {
    if (rankSet != RankSetEMPTY) {
      AVER(gcseg->summary == RefSetEMPTY);
      ShieldRaise(arena, seg, AccessWRITE);
    }
  } else {
    if (rankSet == RankSetEMPTY) {
      AVER(gcseg->summary == RefSetEMPTY);
      ShieldLower(arena, seg, AccessWRITE);
    }
  }
}


/* gcSegSetSummary -- GCSeg method to change the summary on a segment
 *
 * In fact, we only need to raise the write barrier if the
 * segment contains references, and its summary is strictly smaller 
 * than the summary of the unprotectable data (i.e. the mutator).
 * We don't maintain such a summary, assuming that the mutator can 
 * access all references, so its summary is RefSetUNIV.
 */

static void gcSegSetSummary(Seg seg, RefSet summary)
{
  GCSeg gcseg;
  RefSet oldSummary;
  Arena arena;

  AVERT_CRITICAL(Seg, seg);                 /* .seg.method.check */
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);

  arena = PoolArena(SegPool(seg));
  oldSummary = gcseg->summary;
  gcseg->summary = summary;

  AVER(seg->rankSet != RankSetEMPTY);

  /* Note: !RefSetSuper is a test for a strict subset */
  if (!RefSetSuper(summary, RefSetUNIV)) {
    if (RefSetSuper(oldSummary, RefSetUNIV))
      ShieldRaise(arena, seg, AccessWRITE);
  } else {
    if (!RefSetSuper(oldSummary, RefSetUNIV))
      ShieldLower(arena, seg, AccessWRITE);
  }
}


/* gcSegSetRankSummary -- GCSeg method to set both rank set and summary */

static void gcSegSetRankSummary(Seg seg, RankSet rankSet, RefSet summary)
{
  GCSeg gcseg;
  Bool wasShielded, willbeShielded;
  Arena arena;

  AVERT_CRITICAL(Seg, seg);                    /* .seg.method.check */
  AVER_CRITICAL(RankSetCheck(rankSet));        /* .seg.method.check */
  AVER_CRITICAL(rankSet == RankSetEMPTY
                || RankSetIsSingle(rankSet));  /* .seg.method.check */
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);

  /* rankSet == RankSetEMPTY implies summary == RefSetEMPTY */
  AVER(rankSet != RankSetEMPTY || summary == RefSetEMPTY);

  arena = PoolArena(SegPool(seg));

  wasShielded = (seg->rankSet != RankSetEMPTY && gcseg->summary != RefSetUNIV);
  willbeShielded = (rankSet != RankSetEMPTY && summary != RefSetUNIV);

  seg->rankSet = rankSet;
  gcseg->summary = summary;

  if (willbeShielded && !wasShielded) {
    ShieldRaise(arena, seg, AccessWRITE);
  } else if (wasShielded && !willbeShielded) {
    ShieldLower(arena, seg, AccessWRITE);
  }
}


/* gcSegBuffer -- GCSeg method to return the buffer of a segment */

static Buffer gcSegBuffer(Seg seg)
{
  GCSeg gcseg;

  AVERT_CRITICAL(Seg, seg);               /* .seg.method.check */
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);           /* .seg.method.check */
  AVER_CRITICAL(&gcseg->segStruct == seg);

  return gcseg->buffer;
}


/* gcSegSetBuffer -- GCSeg method to change the buffer of a segment */

static void gcSegSetBuffer(Seg seg, Buffer buffer)
{
  GCSeg gcseg;

  AVERT_CRITICAL(Seg, seg);              /* .seg.method.check */
  if (buffer != NULL)
    AVERT_CRITICAL(Buffer, buffer);
  gcseg = SegGCSeg(seg);
  AVERT_CRITICAL(GCSeg, gcseg);
  AVER_CRITICAL(&gcseg->segStruct == seg);

  gcseg->buffer = buffer;
}


/* gcSegMerge -- GCSeg merge method 
 *
 * .buffer: Can't merge two segments both with buffers.
 * See design.mps.seg.merge.inv.buffer.
 */

static Res gcSegMerge(Seg seg, Seg segHi, 
                      Addr base, Addr mid, Addr limit,
                      Bool withReservoirPermit, va_list args)
{
  SegClass super;
  GCSeg gcseg, gcsegHi;
  TraceSet grey;
  RefSet summary;
  Buffer buf;
  Res res;

  AVERT(Seg, seg);
  AVERT(Seg, segHi);
  gcseg = SegGCSeg(seg);
  gcsegHi = SegGCSeg(segHi);
  AVERT(GCSeg, gcseg);
  AVERT(GCSeg, gcsegHi);
  AVER(base < mid);
  AVER(mid < limit);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == mid);
  AVER(SegBase(segHi) == mid);
  AVER(SegLimit(segHi) == limit);
  AVER(BoolCheck(withReservoirPermit));

  buf = gcsegHi->buffer;      /* any buffer on segHi must be reassigned */
  AVER(buf == NULL || gcseg->buffer == NULL); /* See .buffer */
  grey = SegGrey(segHi);      /* check greyness */
  AVER(SegGrey(seg) == grey);

  /* Merge the superclass fields via next-method call */
  super = SEG_SUPERCLASS(GCSegClass);
  res = super->merge(seg, segHi, base, mid, limit, 
                     withReservoirPermit, args);
  if (res != ResOK)
    goto failSuper;

  /* Update fields of gcseg. Finish gcsegHi. */
  summary = RefSetUnion(gcseg->summary, gcsegHi->summary);
  if (summary != gcseg->summary) {
    gcSegSetSummary(seg, summary);
    /* design.mps.seg.split-merge.shield.re-flush */
    ShieldFlush(PoolArena(SegPool(seg)));
  }

  gcSegSetGreyInternal(segHi, grey, TraceSetEMPTY);
  gcsegHi->summary = RefSetEMPTY;
  gcsegHi->sig = SigInvalid;
  RingFinish(&gcsegHi->greyRing);

  /* Reassign any buffer that was connected to segHi  */
  if (NULL != buf) {
    AVER(gcseg->buffer == NULL);
    gcseg->buffer = buf;
    gcsegHi->buffer = NULL;
    BufferReassignSeg(buf, seg);
  }

  AVERT(GCSeg, gcseg);
  return ResOK;

failSuper:
  AVERT(GCSeg, gcseg);
  AVERT(GCSeg, gcsegHi);
  return res;
}


/* gcSegSplit -- GCSeg split method */

static Res gcSegSplit(Seg seg, Seg segHi, 
                      Addr base, Addr mid, Addr limit,
                      Bool withReservoirPermit, va_list args)
{
  SegClass super;
  GCSeg gcseg, gcsegHi;
  Buffer buf;
  TraceSet grey;
  Res res;

  AVERT(Seg, seg);
  AVER(segHi != NULL);  /* can't check fully, it's not initialized */
  gcseg = SegGCSeg(seg);
  gcsegHi = SegGCSeg(segHi);
  AVERT(GCSeg, gcseg);
  AVER(base < mid);
  AVER(mid < limit);
  AVER(SegBase(seg) == base);
  AVER(SegLimit(seg) == limit);
  AVER(BoolCheck(withReservoirPermit));
  
  grey = SegGrey(seg);
  buf = gcseg->buffer; /* Look for buffer to reassign to segHi */
  if (buf != NULL) {
    if (BufferLimit(buf) > mid) {
      /* Existing buffer extends above the split point */
      AVER(BufferBase(buf) > mid); /* check it's all above the split */
    } else {
      buf = NULL; /* buffer lies below split and is unaffected */
    }
  }    

  /* Split the superclass fields via next-method call */
  super = SEG_SUPERCLASS(GCSegClass);
  res = super->split(seg, segHi, base, mid, limit, 
                     withReservoirPermit, args);
  if (res != ResOK)
    goto failSuper;

  /* Full initialization for segHi. */
  gcsegHi->summary = gcseg->summary;
  gcsegHi->buffer = NULL;
  RingInit(&gcsegHi->greyRing);
  gcsegHi->sig = GCSegSig;
  gcSegSetGreyInternal(segHi, TraceSetEMPTY, grey);

  /* Reassign buffer if it's now connected to segHi  */
  if (NULL != buf) {
    gcsegHi->buffer = buf;
    gcseg->buffer = NULL;
    BufferReassignSeg(buf, segHi);
  }

  AVERT(GCSeg, gcseg);
  AVERT(GCSeg, gcsegHi);
  return ResOK;

failSuper:
  AVERT(GCSeg, gcseg);
  return res;
}


/* gcSegDescribe -- GCSeg  description method */

static Res gcSegDescribe(Seg seg, mps_lib_FILE *stream)
{
  Res res;
  SegClass super;
  GCSeg gcseg;

  if (!CHECKT(Seg, seg)) return ResFAIL;
  if (stream == NULL) return ResFAIL;
  gcseg = SegGCSeg(seg);
  if (!CHECKT(GCSeg, gcseg)) return ResFAIL;

  /* Describe the superclass fields first via next-method call */
  super = SEG_SUPERCLASS(GCSegClass);
  res = super->describe(seg, stream);
  if (res != ResOK) return res;

  if (gcseg->buffer != NULL) {
    res = BufferDescribe(gcseg->buffer, stream);
    if (res != ResOK) return res;
  }
  res = WriteF(stream,
               "  summary $W\n", (WriteFW)gcseg->summary,
               NULL);
  return res;
}


/* SegClassCheck -- check a segment class */

Bool SegClassCheck(SegClass class)
{
  CHECKL(ProtocolClassCheck(&class->protocol));
  CHECKL(class->name != NULL); /* Should be <= 6 char C identifier */
  CHECKL(class->size >= sizeof(SegStruct));
  CHECKL(FUNCHECK(class->init));
  CHECKL(FUNCHECK(class->finish));
  CHECKL(FUNCHECK(class->setGrey));
  CHECKL(FUNCHECK(class->setWhite));
  CHECKL(FUNCHECK(class->setRankSet));
  CHECKL(FUNCHECK(class->setRankSummary));
  CHECKL(FUNCHECK(class->merge));
  CHECKL(FUNCHECK(class->split));
  CHECKL(FUNCHECK(class->describe));
  CHECKS(SegClass, class);
  return TRUE;
} 


/* SegClass -- the vanilla segment class definition */

DEFINE_CLASS(SegClass, class)
{
  INHERIT_CLASS(&class->protocol, ProtocolClass);
  class->name = "SEG";
  class->size = sizeof(SegStruct);
  class->init = segTrivInit;
  class->finish = segTrivFinish;
  class->setSummary = segNoSetSummary;  
  class->buffer = segNoBuffer;  
  class->setBuffer = segNoSetBuffer;  
  class->setGrey = segNoSetGrey;
  class->setWhite = segNoSetWhite;
  class->setRankSet = segNoSetRankSet;
  class->setRankSummary = segNoSetRankSummary;
  class->merge = segTrivMerge;
  class->split = segTrivSplit;
  class->describe = segTrivDescribe;
  class->sig = SegClassSig;
}


/* GCSegClass -- GC-supporting segment class definition */
 
typedef SegClassStruct GCSegClassStruct;

DEFINE_CLASS(GCSegClass, class)
{
  INHERIT_CLASS(class, SegClass);
  class->name = "GCSEG";
  class->size = sizeof(GCSegStruct);
  class->init = gcSegInit;
  class->finish = gcSegFinish;
  class->setSummary = gcSegSetSummary;  
  class->buffer = gcSegBuffer;  
  class->setBuffer = gcSegSetBuffer;  
  class->setGrey = gcSegSetGrey;
  class->setWhite = gcSegSetWhite;
  class->setRankSet = gcSegSetRankSet;
  class->setRankSummary = gcSegSetRankSummary;
  class->merge = gcSegMerge;
  class->split = gcSegSplit;
  class->describe = gcSegDescribe;
}


/* SegClassMixInNoSplitMerge -- Mix-in for unsupported merge
 *
 * Classes which don't support segment splitting and merging
 * may mix this in to ensure that erroneous calls are checked.
 */

void SegClassMixInNoSplitMerge(SegClass class)
{
  /* Can't check class because it's not initialized yet */
  class->merge = segNoMerge;
  class->split = segNoSplit;
}