[project @ 1999-01-27 10:11:27 by simonm]

Add missing file.

ghc/rts/Stable.c

+/* -----------------------------------------------------------------------------
+ * $Id: Stable.c,v 1.1 1999/01/27 10:11:27 simonm Exp $
+ *
+ * (c) The GHC Team 1998-1999
+ *
+ * Stable names and stable pointers.
+ *
+ * ---------------------------------------------------------------------------*/
+
+#include "Rts.h"
+#include "Hash.h"
+#include "StablePriv.h"
+#include "GC.h"
+#include "RtsUtils.h"
+#include "Storage.h"
+#include "RtsAPI.h"
+#include "RtsFlags.h"
+
+/* Comment from ADR's implementation in old RTS:
+
+  This files (together with @ghc/runtime/storage/PerformIO.lhc@ and a
+  small change in @HpOverflow.lc@) consists of the changes in the
+  runtime system required to implement "Stable Pointers". But we're
+  getting a bit ahead of ourselves --- what is a stable pointer and what
+  is it used for?
+
+  When Haskell calls C, it normally just passes over primitive integers,
+  floats, bools, strings, etc.  This doesn't cause any problems at all
+  for garbage collection because the act of passing them makes a copy
+  from the heap, stack or wherever they are onto the C-world stack.
+  However, if we were to pass a heap object such as a (Haskell) @String@
+  and a garbage collection occured before we finished using it, we'd run
+  into problems since the heap object might have been moved or even
+  deleted.
+
+  So, if a C call is able to cause a garbage collection or we want to
+  store a pointer to a heap object between C calls, we must be careful
+  when passing heap objects. Our solution is to keep a table of all
+  objects we've given to the C-world and to make sure that the garbage
+  collector collects these objects --- updating the table as required to
+  make sure we can still find the object.
+
+
+  Of course, all this rather begs the question: why would we want to
+  pass a boxed value?
+
+  One very good reason is to preserve laziness across the language
+  interface. Rather than evaluating an integer or a string because it
+  {\em might\/} be required by the C function, we can wait until the C
+  function actually wants the value and then force an evaluation.
+
+  Another very good reason (the motivating reason!) is that the C code
+  might want to execute an object of sort $IO ()$ for the side-effects
+  it will produce. For example, this is used when interfacing to an X
+  widgets library to allow a direct implementation of callbacks.
+
+
+  The @makeStablePointer :: a -> IO (StablePtr a)@ function
+  converts a value into a stable pointer.  It is part of the @PrimIO@
+  monad, because we want to be sure we don't allocate one twice by
+  accident, and then only free one of the copies.
+
+  \begin{verbatim}
+  makeStablePtr#  :: a -> State# RealWorld -> (# RealWorld, a #)
+  freeStablePtr#  :: StablePtr# a -> State# RealWorld -> State# RealWorld
+  deRefStablePtr# :: StablePtr# a -> State# RealWorld -> 
+        (# State# RealWorld, a #)
+  \end{verbatim}
+  There is also a C procedure @FreeStablePtr@ which frees a stable pointer.
+
+  There may be additional functions on the C side to allow evaluation,
+  application, etc of a stable pointer.
+
+  When Haskell calls C, it normally just passes over primitive integers,
+  floats, bools, strings, etc.  This doesn't cause any problems at all
+  for garbage collection because the act of passing them makes a copy
+  from the heap, stack or wherever they are onto the C-world stack.
+  However, if we were to pass a heap object such as a (Haskell) @String@
+  and a garbage collection occured before we finished using it, we'd run
+  into problems since the heap object might have been moved or even
+  deleted.
+
+  So, if a C call is able to cause a garbage collection or we want to
+  store a pointer to a heap object between C calls, we must be careful
+  when passing heap objects. Our solution is to keep a table of all
+  objects we've given to the C-world and to make sure that the garbage
+  collector collects these objects --- updating the table as required to
+  make sure we can still find the object.
+*/
+
+snEntry *stable_ptr_table;
+snEntry *stable_ptr_free;
+
+unsigned int SPT_size;
+
+/* This hash table maps Haskell objects to stable names, so that every
+ * call to lookupStableName on a given object will return the same
+ * stable name.
+ *
+ * Reference counting
+ * ------------------
+ * A plain stable name entry has a zero reference count, which means
+ * the entry will dissappear when the object it points to is
+ * unreachable.  For stable pointers, we need an entry that sticks
+ * around and keeps the object it points to alive, so each stable name
+ * entry has an associated reference count.
+ *
+ * A stable pointer has a weighted reference count N attached to it
+ * (actually in its upper 5 bits), which represents the weight
+ * 2^N.  The stable name entry keeps a 32-bit reference count, which
+ * represents any weight between 1 and 2^32 (represented as zero).
+ * When the weight is 2^32, the stable name table owns "all" of the
+ * stable pointers to this object, and the entry can be garbage
+ * collected if the object isn't reachable.
+ *
+ * A new stable pointer is given the weight log2(W/2), where W is the
+ * weight stored in the table entry.  The new weight in the table is W
+ * - 2^log2(W/2).
+ *
+ * A stable pointer can be "split" into two stable pointers, by
+ * dividing the weight by 2 and giving each pointer half.
+ * When freeing a stable pointer, the weight of the pointer is added
+ * to the weight stored in the table entry.
+ * */
+
+HashTable *addrToStableHash;
+
+#define INIT_SPT_SIZE 64
+
+static inline void
+initFreeList(snEntry *table, nat n, snEntry *free)
+{
+  snEntry *p;
+
+  for (p = table + n - 1; p >= table; p--) {
+    p->addr = (P_)free;
+    free = p;
+  }
+  stable_ptr_free = table;
+}
+
+void
+initStablePtrTable(void)
+{
+  /* the table will be allocated the first time makeStablePtr is
+   * called */
+  stable_ptr_table = NULL;
+  stable_ptr_free  = NULL;
+  addrToStableHash = NULL;
+  SPT_size = 0;
+}
+
+StgWord
+lookupStableName(StgPtr p)
+{
+  StgWord sn;
+
+  if (stable_ptr_free == NULL) {
+    enlargeStablePtrTable();
+  }
+    
+  (void *)sn = lookupHashTable(addrToStableHash,(W_)p);
+  
+  if (sn != 0) {
+    ASSERT(stable_ptr_table[sn].addr == p);
+    IF_DEBUG(stable,fprintf(stderr,"cached stable name %d at %p\n",sn,p));
+    return sn;
+  } else {
+    sn = stable_ptr_free - stable_ptr_table;
+    (P_)stable_ptr_free  = stable_ptr_free->addr;
+    stable_ptr_table[sn].weight = 0;
+    stable_ptr_table[sn].addr = p;
+    /* IF_DEBUG(stable,fprintf(stderr,"new stable name %d at
+       %p\n",sn,p)); */
+    
+    /* add the new stable name to the hash table */
+    insertHashTable(addrToStableHash, (W_)p, (void *)sn);
+
+    return sn;
+  }
+}
+
+static inline void
+freeStableName(snEntry *sn)
+{
+  sn->addr = (P_)stable_ptr_free;
+  stable_ptr_free = sn;
+}
+
+StgStablePtr
+getStablePtr(StgPtr p)
+{
+  StgWord sn = lookupStableName(p);
+  StgWord weight, weight_2;
+
+  weight = stable_ptr_table[sn].weight;
+  if (weight == 0) {
+    weight = 1 << (BITS_IN(StgWord)-1);
+    stable_ptr_table[sn].weight = weight;
+    return (StgStablePtr)(sn + ((BITS_IN(StgWord)-1) << STABLEPTR_WEIGHT_SHIFT));
+  } 
+  else if (weight == 1) {
+    barf("getStablePtr: too light");
+  } 
+  else {
+    weight /= 2;
+    /* find log2(weight) */
+    for (weight_2 = 1; weight != 1; weight_2++) {
+      weight >>= 1;
+    }
+    stable_ptr_table[sn].weight -= 2^weight_2;
+    return (StgStablePtr)(sn + (weight_2 << STABLEPTR_WEIGHT_SHIFT));
+  }
+}
+
+void
+enlargeStablePtrTable(void)
+{
+  nat old_SPT_size = SPT_size;
+  
+  if (SPT_size == 0) {
+    /* 1st time */
+    SPT_size = INIT_SPT_SIZE;
+    stable_ptr_table = stgMallocWords(SPT_size * sizeof(snEntry), 
+				      "initStablePtrTable");
+    
+    initFreeList(stable_ptr_table+1,INIT_SPT_SIZE-1,NULL);
+    addrToStableHash = allocHashTable();
+  }
+  else {
+    /* 2nd and subsequent times */
+    SPT_size *= 2;
+    stable_ptr_table = 
+      stgReallocWords(stable_ptr_table, SPT_size * sizeof(snEntry),
+		      "enlargeStablePtrTable");
+    
+    initFreeList(stable_ptr_table + old_SPT_size, old_SPT_size, NULL);
+  }
+}
+
+/* -----------------------------------------------------------------------------
+ * Treat stable pointers as roots for the garbage collector.
+ *
+ * A stable pointer is any stable name entry with a weight > 0.  We'll
+ * take the opportunity to zero the "keep" flags at the same time.
+ * -------------------------------------------------------------------------- */
+
+void
+markStablePtrTable(rtsBool full)
+{
+  snEntry *p, *end_stable_ptr_table;
+  StgPtr q;
+  StgClosure *new;
+
+  if (SPT_size == 0)
+    return;
+
+  if (full) {
+    freeHashTable(addrToStableHash,NULL);
+    addrToStableHash = allocHashTable();
+  }
+
+  end_stable_ptr_table = &stable_ptr_table[SPT_size];
+
+  /* Mark all the stable *pointers* (not stable names) 
+   */
+  for (p = stable_ptr_table; p < end_stable_ptr_table; p++) {
+    q = p->addr;
+    /* internal pointers or NULL are free slots */
+    if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
+      if (p->weight != 0) {
+	new = MarkRoot((StgClosure *)q);
+	/* Update the hash table */
+	if (full) {
+	  insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
+	  (StgClosure *)p->addr = new;
+	} else if ((P_)new != q) {
+	  removeHashTable(addrToStableHash, (W_)q, NULL);
+	  insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
+	  (StgClosure *)p->addr = new;
+	}
+	/* IF_DEBUG(stable, fprintf(stderr,"Stable ptr %d still alive
+	   at %p, weight %d\n", p - stable_ptr_table, new,
+	   p->weight)); */
+      }
+      else { 
+	/* reset the keep flag */
+	p->keep = rtsFalse;
+      }
+    }
+  }
+}
+
+/* -----------------------------------------------------------------------------
+ * Garbage collect any dead entries in the stable pointer table.
+ *
+ * A dead entry has:
+ *
+ *          - a weight of zero (i.e. 2^32)
+ *          - a false keep flag
+ *
+ * The keep flag is set by the garbage collector whenever it
+ * encounters a StableName object on the heap.  
+ *
+ * The boolean argument 'full' indicates that a major collection is
+ * being done, so we might as well throw away the hash table and build
+ * a new one.  For a minor collection, we just re-hash the elements
+ * that changed.
+ * -------------------------------------------------------------------------- */
+
+void
+gcStablePtrTable(rtsBool full)
+{
+  snEntry *p, *end_stable_ptr_table;
+  StgPtr q, new;
+
+  if (SPT_size == 0) {
+    return;
+  }
+
+  end_stable_ptr_table = &stable_ptr_table[SPT_size];
+
+  for (p = stable_ptr_table; p < end_stable_ptr_table; p++) {
+    q = p->addr;
+
+    if (q && (q < (P_)stable_ptr_table || q >= (P_)end_stable_ptr_table)) {
+
+      /* We're only interested in Stable Names here. */
+      if (p->weight == 0) {
+	
+	if (((StgClosure *)new = isAlive((StgClosure *)q))) {
+	  IF_DEBUG(stable, fprintf(stderr,"Stable name %d still alive at %p, weight %d\n", p - stable_ptr_table, new, p->weight));
+
+	  p->addr = new;
+	  /* Re-hash this stable name */
+	  if (full) {
+	    insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
+	  } else if (new != q) {
+	    removeHashTable(addrToStableHash, (W_)q, NULL);
+	    insertHashTable(addrToStableHash, (W_)new, (void *)(p - stable_ptr_table));
+	  }
+	}
+
+	else {
+	  /* If there are still StableName objects in the heap
+	   * pointing to this entry (p->keep == rtsTrue), then
+	   * don't free the entry just yet.
+	   */
+	  if (p->keep)
+	    p->addr = NULL;
+	  else
+	    freeStableName(p);
+	}
+      }
+    }
+  }
+}