File: alloca.c

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/* alloca.c -- allocate automatically reclaimed memory
   (Mostly) portable public-domain implementation -- D A Gwyn

   This implementation of the PWB library alloca function,
   which is used to allocate space off the run-time stack so
   that it is automatically reclaimed upon procedure exit,
   was inspired by discussions with J. Q. Johnson of Cornell.
   J.Otto Tennant <jot@cray.com> contributed the Cray support.

   There are some preprocessor constants that can
   be defined when compiling for your specific system, for
   improved efficiency; however, the defaults should be okay.

   The general concept of this implementation is to keep
   track of all alloca-allocated blocks, and reclaim any
   that are found to be deeper in the stack than the current
   invocation.  This heuristic does not reclaim storage as
   soon as it becomes invalid, but it will do so eventually.

   As a special case, alloca(0) reclaims storage without
   allocating any.  It is a good idea to use alloca(0) in
   your main control loop, etc. to force garbage collection.  */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#ifdef emacs
#include "blockinput.h"
#endif

/* If compiling with GCC 2, this file's not needed.  */
#if !defined (__GNUC__) || __GNUC__ < 2

/* If someone has defined alloca as a macro,
   there must be some other way alloca is supposed to work.  */
#ifndef alloca

#ifdef emacs
#ifdef static
/* actually, only want this if static is defined as ""
   -- this is for usg, in which emacs must undefine static
   in order to make unexec workable
   */
#ifndef STACK_DIRECTION
you
lose
-- must know STACK_DIRECTION at compile-time
#endif /* STACK_DIRECTION undefined */
#endif /* static */
#endif /* emacs */

/* If your stack is a linked list of frames, you have to
   provide an "address metric" ADDRESS_FUNCTION macro.  */

#if defined (CRAY) && defined (CRAY_STACKSEG_END)
intptr_t i00afunc ();
#define ADDRESS_FUNCTION(arg) (char *) i00afunc (&(arg))
#else
#define ADDRESS_FUNCTION(arg) &(arg)
#endif

#if __STDC__
typedef void *pointer;
#else
typedef char *pointer;
#endif

#define	NULL	0

/* Different portions of Emacs need to call different versions of
   malloc.  The Emacs executable needs alloca to call xmalloc, because
   ordinary malloc isn't protected from input signals.  On the other
   hand, the utilities in lib-src need alloca to call malloc; some of
   them are very simple, and don't have an xmalloc routine.

   Non-Emacs programs expect this to call use xmalloc.

   Callers below should use malloc.  */

#ifndef emacs
#define malloc xmalloc
#endif
extern pointer malloc ();

/* Define STACK_DIRECTION if you know the direction of stack
   growth for your system; otherwise it will be automatically
   deduced at run-time.

   STACK_DIRECTION > 0 => grows toward higher addresses
   STACK_DIRECTION < 0 => grows toward lower addresses
   STACK_DIRECTION = 0 => direction of growth unknown  */

#ifndef STACK_DIRECTION
#define	STACK_DIRECTION	0	/* Direction unknown.  */
#endif

#if STACK_DIRECTION != 0

#define	STACK_DIR	STACK_DIRECTION	/* Known at compile-time.  */

#else /* STACK_DIRECTION == 0; need run-time code.  */

static int stack_dir;		/* 1 or -1 once known.  */
#define	STACK_DIR	stack_dir

static void
find_stack_direction ()
{
  static char *addr = NULL;	/* Address of first `dummy', once known.  */
  auto char dummy;		/* To get stack address.  */

  if (addr == NULL)
    {				/* Initial entry.  */
      addr = ADDRESS_FUNCTION (dummy);

      find_stack_direction ();	/* Recurse once.  */
    }
  else
    {
      /* Second entry.  */
      if (ADDRESS_FUNCTION (dummy) > addr)
	stack_dir = 1;		/* Stack grew upward.  */
      else
	stack_dir = -1;		/* Stack grew downward.  */
    }
}

#endif /* STACK_DIRECTION == 0 */

/* An "alloca header" is used to:
   (a) chain together all alloca'ed blocks;
   (b) keep track of stack depth.

   It is very important that sizeof(header) agree with malloc
   alignment chunk size.  The following default should work okay.  */

#ifndef	ALIGN_SIZE
#define	ALIGN_SIZE	sizeof(double)
#endif

typedef union hdr
{
  char align[ALIGN_SIZE];	/* To force sizeof(header).  */
  struct
    {
      union hdr *next;		/* For chaining headers.  */
      char *deep;		/* For stack depth measure.  */
    } h;
} header;

static header *last_alloca_header = NULL;	/* -> last alloca header.  */

/* Return a pointer to at least SIZE bytes of storage,
   which will be automatically reclaimed upon exit from
   the procedure that called alloca.  Originally, this space
   was supposed to be taken from the current stack frame of the
   caller, but that method cannot be made to work for some
   implementations of C, for example under Gould's UTX/32.  */

pointer
alloca (size)
     unsigned size;
{
  auto char probe;		/* Probes stack depth: */
  register char *depth = ADDRESS_FUNCTION (probe);

#if STACK_DIRECTION == 0
  if (STACK_DIR == 0)		/* Unknown growth direction.  */
    find_stack_direction ();
#endif

  /* Reclaim garbage, defined as all alloca'd storage that
     was allocated from deeper in the stack than currently. */

  {
    register header *hp;	/* Traverses linked list.  */

#ifdef emacs
    BLOCK_INPUT;
#endif

    for (hp = last_alloca_header; hp != NULL;)
      if ((STACK_DIR > 0 && hp->h.deep > depth)
	  || (STACK_DIR < 0 && hp->h.deep < depth))
	{
	  register header *np = hp->h.next;

	  free ((pointer) hp);	/* Collect garbage.  */

	  hp = np;		/* -> next header.  */
	}
      else
	break;			/* Rest are not deeper.  */

    last_alloca_header = hp;	/* -> last valid storage.  */

#ifdef emacs
    UNBLOCK_INPUT;
#endif
  }

  if (size == 0)
    return NULL;		/* No allocation required.  */

  /* Allocate combined header + user data storage.  */

  {
    register pointer new = malloc (sizeof (header) + size);
    /* Address of header.  */

    ((header *) new)->h.next = last_alloca_header;
    ((header *) new)->h.deep = depth;

    last_alloca_header = (header *) new;

    /* User storage begins just after header.  */

    return (pointer) ((char *) new + sizeof (header));
  }
}

#if defined (CRAY) && defined (CRAY_STACKSEG_END)

#ifdef DEBUG_I00AFUNC
#include <stdio.h>
#endif

#ifndef CRAY_STACK
#define CRAY_STACK
#ifndef CRAY2
/* Stack structures for CRAY-1, CRAY X-MP, and CRAY Y-MP */
struct stack_control_header
  {
    intptr_t shgrow:32;		/* Number of times stack has grown.  */
    intptr_t shaseg:32;		/* Size of increments to stack.  */
    intptr_t shhwm:32;		/* High water mark of stack.  */
    intptr_t shsize:32;		/* Current size of stack (all segments).  */
  };

/* The stack segment linkage control information occurs at
   the high-address end of a stack segment.  (The stack
   grows from low addresses to high addresses.)  The initial
   part of the stack segment linkage control information is
   0200 (octal) words.  This provides for register storage
   for the routine which overflows the stack.  */

struct stack_segment_linkage
  {
    intptr_t ss[0200];		/* 0200 overflow words.  */
    intptr_t sssize:32;		/* Number of words in this segment.  */
    intptr_t ssbase:32;		/* Offset to stack base.  */
    intptr_t:32;
    intptr_t sspseg:32;		/* Offset to linkage control of previous
				   segment of stack.  */
    intptr_t:32;
    intptr_t sstcpt:32;		/* Pointer to task common address block.  */
    intptr_t sscsnm;		/* Private control structure number for
				   microtasking.  */
    intptr_t ssusr1;		/* Reserved for user.  */
    intptr_t ssusr2;		/* Reserved for user.  */
    intptr_t sstpid;		/* Process ID for pid based multi-tasking.  */
    intptr_t ssgvup;		/* Pointer to multitasking thread giveup.  */
    intptr_t sscray[7];		/* Reserved for Cray Research.  */
    intptr_t ssa0;
    intptr_t ssa1;
    intptr_t ssa2;
    intptr_t ssa3;
    intptr_t ssa4;
    intptr_t ssa5;
    intptr_t ssa6;
    intptr_t ssa7;
    intptr_t sss0;
    intptr_t sss1;
    intptr_t sss2;
    intptr_t sss3;
    intptr_t sss4;
    intptr_t sss5;
    intptr_t sss6;
    intptr_t sss7;
  };

#else /* CRAY2 */
/* The following structure defines the vector of words
   returned by the STKSTAT library routine.  */
struct stk_stat
  {
    intptr_t now;			/* Current total stack size.  */
    intptr_t maxc;			/* Amount of contiguous space which would
				   be required to satisfy the maximum
				   stack demand to date.  */
    intptr_t high_water;		/* Stack high-water mark.  */
    intptr_t overflows;		/* Number of stack overflow ($STKOFEN) calls.  */
    intptr_t hits;			/* Number of internal buffer hits.  */
    intptr_t extends;		/* Number of block extensions.  */
    intptr_t stko_mallocs;		/* Block allocations by $STKOFEN.  */
    intptr_t underflows;		/* Number of stack underflow calls ($STKRETN).  */
    intptr_t stko_free;		/* Number of deallocations by $STKRETN.  */
    intptr_t stkm_free;		/* Number of deallocations by $STKMRET.  */
    intptr_t segments;		/* Current number of stack segments.  */
    intptr_t maxs;			/* Maximum number of stack segments so far.  */
    intptr_t pad_size;		/* Stack pad size.  */
    intptr_t current_address;	/* Current stack segment address.  */
    intptr_t current_size;		/* Current stack segment size.  This
				   number is actually corrupted by STKSTAT to
				   include the fifteen word trailer area.  */
    intptr_t initial_address;	/* Address of initial segment.  */
    intptr_t initial_size;		/* Size of initial segment.  */
  };

/* The following structure describes the data structure which trails
   any stack segment.  I think that the description in 'asdef' is
   out of date.  I only describe the parts that I am sure about.  */

struct stk_trailer
  {
    intptr_t this_address;		/* Address of this block.  */
    intptr_t this_size;		/* Size of this block (does not include
				   this trailer).  */
    intptr_t unknown2;
    intptr_t unknown3;
    intptr_t link;			/* Address of trailer block of previous
				   segment.  */
    intptr_t unknown5;
    intptr_t unknown6;
    intptr_t unknown7;
    intptr_t unknown8;
    intptr_t unknown9;
    intptr_t unknown10;
    intptr_t unknown11;
    intptr_t unknown12;
    intptr_t unknown13;
    intptr_t unknown14;
  };

#endif /* CRAY2 */
#endif /* not CRAY_STACK */

#ifdef CRAY2
/* Determine a "stack measure" for an arbitrary ADDRESS.
   I doubt that "lint" will like this much. */

static intptr_t
i00afunc (intptr_t *address)
{
  struct stk_stat status;
  struct stk_trailer *trailer;
  intptr_t *block, size;
  intptr_t result = 0;

  /* We want to iterate through all of the segments.  The first
     step is to get the stack status structure.  We could do this
     more quickly and more directly, perhaps, by referencing the
     $LM00 common block, but I know that this works.  */

  STKSTAT (&status);

  /* Set up the iteration.  */

  trailer = (struct stk_trailer *) (status.current_address
				    + status.current_size
				    - 15);

  /* There must be at least one stack segment.  Therefore it is
     a fatal error if "trailer" is null.  */

  if (trailer == 0)
    abort ();

  /* Discard segments that do not contain our argument address.  */

  while (trailer != 0)
    {
      block = (intptr_t *) trailer->this_address;
      size = trailer->this_size;
      if (block == 0 || size == 0)
	abort ();
      trailer = (struct stk_trailer *) trailer->link;
      if ((block <= address) && (address < (block + size)))
	break;
    }

  /* Set the result to the offset in this segment and add the sizes
     of all predecessor segments.  */

  result = address - block;

  if (trailer == 0)
    {
      return result;
    }

  do
    {
      if (trailer->this_size <= 0)
	abort ();
      result += trailer->this_size;
      trailer = (struct stk_trailer *) trailer->link;
    }
  while (trailer != 0);

  /* We are done.  Note that if you present a bogus address (one
     not in any segment), you will get a different number back, formed
     from subtracting the address of the first block.  This is probably
     not what you want.  */

  return (result);
}

#else /* not CRAY2 */
/* Stack address function for a CRAY-1, CRAY X-MP, or CRAY Y-MP.
   Determine the number of the cell within the stack,
   given the address of the cell.  The purpose of this
   routine is to linearize, in some sense, stack addresses
   for alloca.  */

static intptr_t
i00afunc (intptr_t address)
{
  intptr_t stkl = 0;

  intptr_t size, pseg, this_segment, stack;
  intptr_t result = 0;

  struct stack_segment_linkage *ssptr;

  /* Register B67 contains the address of the end of the
     current stack segment.  If you (as a subprogram) store
     your registers on the stack and find that you are past
     the contents of B67, you have overflowed the segment.

     B67 also points to the stack segment linkage control
     area, which is what we are really interested in.  */

  stkl = CRAY_STACKSEG_END ();
  ssptr = (struct stack_segment_linkage *) stkl;

  /* If one subtracts 'size' from the end of the segment,
     one has the address of the first word of the segment.

     If this is not the first segment, 'pseg' will be
     nonzero.  */

  pseg = ssptr->sspseg;
  size = ssptr->sssize;

  this_segment = stkl - size;

  /* It is possible that calling this routine itself caused
     a stack overflow.  Discard stack segments which do not
     contain the target address.  */

  while (!(this_segment <= address && address <= stkl))
    {
#ifdef DEBUG_I00AFUNC
      fprintf (stderr, "%011o %011o %011o\n", this_segment, address, stkl);
#endif
      if (pseg == 0)
	break;
      stkl = stkl - pseg;
      ssptr = (struct stack_segment_linkage *) stkl;
      size = ssptr->sssize;
      pseg = ssptr->sspseg;
      this_segment = stkl - size;
    }

  result = address - this_segment;

  /* If you subtract pseg from the current end of the stack,
     you get the address of the previous stack segment's end.
     This seems a little convoluted to me, but I'll bet you save
     a cycle somewhere.  */

  while (pseg != 0)
    {
#ifdef DEBUG_I00AFUNC
      fprintf (stderr, "%011o %011o\n", pseg, size);
#endif
      stkl = stkl - pseg;
      ssptr = (struct stack_segment_linkage *) stkl;
      size = ssptr->sssize;
      pseg = ssptr->sspseg;
      result += size;
    }
  return (result);
}

#endif /* not CRAY2 */
#endif /* CRAY */

#endif /* no alloca */
#endif /* not GCC version 2 */