gnu1987/gcc/cse.c

/* Common subexpression elimination for GNU compiler.
   Copyright (C) 1987 Free Software Foundation, Inc.

This file is part of GNU CC.

GNU CC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY.  No author or distributor
accepts responsibility to anyone for the consequences of using it
or for whether it serves any particular purpose or works at all,
unless he says so in writing.  Refer to the GNU CC General Public
License for full details.

Everyone is granted permission to copy, modify and redistribute
GNU CC, but only under the conditions described in the
GNU CC General Public License.   A copy of this license is
supposed to have been given to you along with GNU CC so you
can know your rights and responsibilities.  It should be in a
file named COPYING.  Among other things, the copyright notice
and this notice must be preserved on all copies.  */


#include "config.h"
#include "rtl.h"
#include "insn-config.h"
#include "regs.h"

/* The basic idea of common subexpression elimination is to go
   through the code, keeping a record of expressions that would
   have the same value at the current scan point, and replacing
   expressions encountered with the cheapest equivalent expression.

   It is too complicated to keep track of the different possibilities
   when control paths merge; so, at each label, we forget all that is
   known and start fresh.  This can be described as processing each
   basic block separately.  Note, however, that these are not quite
   the same as the basic blocks found by a later pass and used for
   data flow analysis and register packing.  We do not need to start fresh
   after a conditional jump instruction if there is no label there.

   We use two data structures to record the equivalent expressions:
   a hash table for most expressions, and several vectors together
   with "quantity numbers" to record equivalent (pseudo) registers.

   The use of the special data structure for registers is desirable
   because it is faster.  It is possible because registers references
   contain a fairly small number, the register number, taken from
   a contiguously allocated series, and two register references are
   identical if they have the same number.  General expressions
   do not have any such thing, so the only way to retrieve the
   information recorded on an expression other than a register
   is to keep it in a hash table.

Registers and "quantity numbers":
   
   At the start of each basic block, all of the (hardware and pseudo)
   registers used in the function are given distinct quantity
   numbers to indicate their contents.  During scan, when the code
   copies one register into another, we copy the quantity number.
   When a register is loaded in any other way, we allocate a new
   quantity number to describe the value generated by this operation.
   `reg_qty' records what quantity a register is currently thought
   of as containing.

   We also maintain a bidirectional chain of registers for each
   quantity number.  `qty_first_reg', `qty_last_reg',
   `reg_next_eqv' and `reg_prev_eqv' hold these chains.

   The first register in a chain is the one whose lifespan is least local.
   Among equals, it is the one that was seen first.
   We replace any equivalent register with that one.

Constants and quantity numbers

   When a quantity has a known constant value, that value is stored
   in the appropriate element of qty_const.  This is in addition to
   putting the constant in the hash table as is usual for non-regs.

   Regs are preferred to constants as they are to everything else,
   but expressions containing constants can be simplified, by fold_rtx.

Other expressions:

   To record known equivalences among expressions in general
   we use a hash table called `table'.  It has a fixed number of buckets
   that contain chains of `struct table_elt' elements for expressions.
   These chains connect the elements whose expressions have the same
   hash codes.

   Other chains through the same elements connect the elements which
   currently have equivalent values.

   Register references in an expression are canonicalized before hashing
   the expression.  This is done using `reg_qty' and `qty_first_reg'.
   The hash code of a register reference is computed using the quantity
   number, not the register number.

   When the value of an expression changes, it is necessary to remove from the
   hash table not just that expression but all expressions whose values
   could be different as a result.

     1. If the value changing is in memory, except in special cases
     ANYTHING referring to memory could be changed.  That is because
     nobody knows where a pointer does not point.
     The function `invalidate_memory' removes what is necessary.

     The special cases are when the address is constant or is
     a constant plus a fixed register such as the frame pointer
     or a static chain pointer.  When such addresses are stored in,
     we can tell exactly which other such addresses must be invalidated
     due to overlap.  `invalidate' does this.
     All expressions that refer to non-constant
     memory addresses are also invalidated.  `invalidate_memory' does this.

     2. If the value changing is a register, all expressions
     containing references to that register, and only those,
     must be removed.

   Because searching the entire hash table for expressions that contain
   a register is very slow, we try to figure out when it isn't necessary.
   Precisely, this is necessary only when there expressions have been
   entered in the hash table using this register, and then the value has
   changed, and then another expression wants to be added to refer to
   teh register's new value.  This sequence of circumstances is rare
   within any one basic block.

   The vectors `reg_tick' and `reg_in_table' are used to detect this case.
   reg_tick[i] is incremented whenever a value is stored in register i.
   reg_in_table[i] holds -1 if no references to register i have been
   entered in the table; otherwise, it contains the value reg_tick[i] had
   when the references were entered.  If we want to enter a reference
   and reg_in_table[i] != reg_tick[i], we must scan and remove old references.
   Until we want to enter a new entry, the mere fact that the twovectors
   don't match makes the entries be ignored if anyone tries to match them.

   Registers themselves are entered in the hash table as well as in
   the equivalent-register chains.  However, the vectors `reg_tick'
   and `reg_in_table' do not apply to expressions which are simple
   register references.  These expressions are removed from the table
   immediately when they become invalid, and this can be done even if
   we do not immediately search for all the expressions that refer to
   the register.

   A CLOBBER rtx in an instruction invalidates its operand for further
   reuse.  A CLOBBER rtx whose operand is 0 (a null pointer)
   invalidates everything that resides in memory.

Related expressions:

   Constant expressions that differ only by an additive integer
   are called related.  When a constant expression is put in
   the table, the related expression with no constant term
   is also entered.  These are made to point at each other
   so that it is possible to find out if there exists any
   register equivalent to an expression related to a given expression.  */
   
/* One plus largest register number used in this function.  */

static int max_reg;

/* Length of vectors indexed by quantity number.
   We know in advance we will not need a quantity number this big.  */

static int max_qty;

/* Next quantity number to be allocated.
   This is 1 + the largest number needed so far.  */

static int next_qty;

/* Indexed by quantity number, gives the first (or last) (pseudo) register 
   in the chain of registers that currently contain this quantity.  */

static int *qty_first_reg;
static int *qty_last_reg;

/* Indexed by quantity number, gives the rtx of the constant value of the
   quantity, or zero if it does not have a known value.  */

static rtx *qty_const;

/* Value stored in CC0 by previous insn:
   0 if previous insn didn't store in CC0.
   else 100 + (M&7)<<3 + (N&7)
   where M is 1, 0 or -1 if result was pos, zero or neg as signed number
   and N is 1, 0 or -1 if result was pos, zero or neg as unsigned number.  */

static int prev_insn_cc0;

/* Previous actual insn.  0 if at first insn of basic block.  */

static rtx prev_insn;

/* Index by (pseudo) register number, gives the quantity number
   of the register's current contents.  */

static int *reg_qty;

/* Index by (pseudo) register number, gives the number of the next
   (pseudo) register in the chain of registers sharing the same value.
   Or -1 if this register is at the end of the chain.  */

static int *reg_next_eqv;

/* Index by (pseudo) register number, gives the number of the previous
   (pseudo) register in the chain of registers sharing the same value.
   Or -1 if this register is at the beginning of the chain.  */

static int *reg_prev_eqv;

/* Index by (pseudo) register number, gives the latest rtx
   to use to insert a ref to that register.  */

static rtx *reg_rtx;

/* Index by (pseudo) register number, gives the number of times
   that register has been altered in the current basic block.  */

static int *reg_tick;

/* Index by (pseudo) register number, gives the reg_tick value at which
   rtx's containing this register are valid in the hash table.
   If this does not equal the current reg_tick value, such expressions
   existing in the hash table are invalid.
   If this is -1, no expressions containing this register have been
   entered in the table.  */

static int *reg_in_table;

/* Two vectors of max_reg ints:
   one containing all -1's; in the other, element i contains i.
   These are used to initialize various other vectors fast.  */

static int *all_minus_one;
static int *consec_ints;

/* Indexed by hardware register number, contains nonzero
   for registers that may be clobbered by subroutine calls
   (and also for the fixed-use registers,
   which we must check for and refrain from invalidating).  */

static char call_clobbers_reg[] = CALL_USED_REGISTERS;

/* UID of insn that ends the basic block currently being cse-processed.  */

static int cse_basic_block_end;

/* Nonzero if cse has altered conditional jump insns
   in such a way that jump optimization should be redone.  */

static int cse_jumps_altered;

/* canon_hash stores 1 in do_not_record
   if it notices a reference to CC0, CC1 or PC.  */

static int do_not_record;

/* canon_hash stores 1 in hash_arg_in_memory
   if it notices a reference to memory within the expression being hashed.  */

static int hash_arg_in_memory;

/* The hash table contains buckets which are chains of `struct table_elt's,
   each recording one expression's information.
   That expression is in the `exp' field.

   Those elements with the same hash code are chained in both directions
   through the `next_same_hash' and `prev_same_hash' fields.

   Each set of expressions with equivalent values
   are on a two-way chain through the `next_same_value'
   and `prev_same_value' fields, and all point with
   the `first_same_value' field at the first element in
   that chain.  The chain is in order of increasing cost.
   Each element's cost value is in its `cost' field.

   The `in_memory' field is nonzero for elements that
   involve any reference to memory.  These elements are removed
   whenever a write is done to an unidentified location in memory.
   To be safe, we assume that a memory address is unidentified unless
   the address is either a symbol constant or a constant plus
   the frame pointer or argument pointer.

   The `related_value' field is used to connect related expressions
   (that differ by adding an integer).
   The related expressions are chained in a circular fashion.
   `related_value' is zero for expressions for which this
   chain is not useful.  */

struct table_elt
{
  rtx exp;
  struct table_elt *next_same_hash;
  struct table_elt *prev_same_hash;
  struct table_elt *next_same_value;
  struct table_elt *prev_same_value;
  struct table_elt *first_same_value;
  struct table_elt *related_value;
  int cost;
  char in_memory;
};

#define HASH(x) (canon_hash (x) % NBUCKETS)
/* We don't want a lot of buckets, because we rarely have very many
   things stored in the hash table, and a lot of buckets slows
   down a lot of loops that happen frequently.  */
#define NBUCKETS 31

static struct table_elt *table[NBUCKETS];

/* Chain of `struct table_elt's made so far for this function
   but currently removed from the table.  */

static struct table_elt *free_element_chain;

/* Number of `struct table_elt' structures made so far for this function.  */

static int n_elements_made;

/* Maximum value `n_elements_made' has had so far in this compilation
   for functions previously processed.  */

static int max_elements_made;

/* Bits describing what kind of values in memory must be invalidated
   for a particular instruction.  If all three bits are zero,
   no memory refs need to be invalidated.  Each bit is more powerful
   than the preceding ones, and if a bit is set then the preceding
   bits are also set.

   Here is how the bits are set.
   Writing at a fixed address invalidates only variable addresses,
   writing in a structure element at variable address
     invalidates all but scalar variables,
   and writing in anything else at variable address invalidates everything.  */

struct write_data
{
  int var : 1;			/* Invalidate variable addresses.  */
  int nonscalar : 1;		/* Invalidate all but scalar variables.  */
  int all : 1;			/* Invalidate all memory refs.  */
};

static struct table_elt *lookup ();
static void free_element ();

static void remove_invalid_refs ();
static int exp_equiv_p ();
int refers_to_p ();
int refers_to_mem_p ();
static void invalidate_from_clobbers ();
static int safe_hash ();
static int get_integer_term ();
static rtx get_related_value ();
static void note_mem_written ();

/* Return an estimate of the cost of computing rtx X.
   The only use of this is to compare the costs of two expressions
   to decide whether to replace one with the other.  */

static int
rtx_cost (x)
     rtx x;
{
  register int i;
  register RTX_CODE code = GET_CODE (x);
  register char *fmt;
  register int total;

  switch (code)
    {
    case REG:
      return 1;
    case SUBREG:
      return 2;
    CONST_COSTS (x, code);
    }

  total = 2;

  /* Compare the elements.  If any pair of corresponding elements
     fail to match, return 0 for the whole things.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    if (fmt[i] == 'e')
      total += rtx_cost (XEXP (x, i));

  return total;
}

/* Clear the hash table and initialize each register with its own quantity,
   for a new basic block.  */

static void
new_basic_block ()
{
  register int i;
  register int vecsize = max_reg * sizeof (rtx);
  next_qty = max_reg;

  bzero (reg_rtx, vecsize);
  bzero (reg_tick, vecsize);

  bcopy (all_minus_one, reg_in_table, vecsize);
  bcopy (all_minus_one, reg_next_eqv, vecsize);
  bcopy (all_minus_one, reg_prev_eqv, vecsize);
  bcopy (consec_ints, reg_qty, vecsize);

  for (i = 0; i < max_qty; i++)
    {
      qty_first_reg[i] = i;
      qty_last_reg[i] = i;
      qty_const[i] = 0;
    }

  for (i = 0; i < NBUCKETS; i++)
    {
      register struct table_elt *this, *next;
      for (this = table[i]; this; this = next)
	{
	  next = this->next_same_hash;
	  free_element (this);
	}
    }

  bzero (table, sizeof table);

  prev_insn_cc0 = 0;
  prev_insn = 0;
}

/* Say that register REG contains a quantity not in any register before.  */

static void
make_new_qty (reg)
     register int reg;
{
  register int q;

  q = reg_qty[reg] = next_qty++;
  qty_first_reg[q] = reg;
  qty_last_reg[q] = reg;
}

/* Make reg NEW equivalent to reg OLD.
   OLD is not changing; NEW is.  */

static void
make_regs_eqv (new, old)
     register int new, old;
{
  register int lastr, firstr;
  register int q;

  q = reg_qty[new] = reg_qty[old];
  firstr = qty_first_reg[q];

  /* Prefer pseudo regs to hard regs with the same value.
     Among pseudos, if NEW will live longer than any other reg of the same qty,
     and that is beyond the current basic block,
     make it the new canonical replacement for this qty.  */
  if (new >= FIRST_PSEUDO_REGISTER
      && (firstr < FIRST_PSEUDO_REGISTER
	  || ((regno_last_uid[new] > cse_basic_block_end)
	      &&
	      (regno_last_uid[new] > regno_last_uid[firstr]))))
    {
      lastr = qty_first_reg[q];
      reg_prev_eqv[lastr] = new;
      reg_next_eqv[new] = lastr;
      reg_prev_eqv[new] = -1;
      qty_first_reg[q] = new;
    }
  else
    {
      lastr = qty_last_reg[q];
      reg_next_eqv[lastr] = new;
      reg_prev_eqv[new] = lastr;
      reg_next_eqv[new] = -1;
      qty_last_reg[q] = new;
    }
}

/* Discard the records of what is in register REG.  */

static void
reg_invalidate (reg)
     register int reg;
{
  register int n = reg_next_eqv[reg];
  register int p = reg_prev_eqv[reg];
  register int q = reg_qty[reg];

  reg_tick[reg]++;

  if (q == reg)
    return;			/* Save time if already invalid */

  if (n != -1)
    reg_prev_eqv[n] = p;
  else
    qty_last_reg[q] = p;
  if (p != -1)
    reg_next_eqv[p] = n;
  else
    qty_first_reg[q] = n;

  reg_qty[reg] = reg;
  qty_first_reg[reg] = reg;
  qty_last_reg[reg] = reg;
  reg_next_eqv[reg] = -1;
  reg_prev_eqv[reg] = -1;
}

/* Remove any invalid expressions from the hash table
   that refer to any of the registers contained in expression X.

   Make sure that newly inserted references to those registers
   as subexpressions will be considered valid.

   mention_regs is not called when a register itself
   is being stored in the table.  */

static void
mention_regs (x)
     rtx x;
{
  register RTX_CODE code = GET_CODE (x);
  register int i;
  register char *fmt;

  if (code == REG)
    {
      register int regno = REGNO (x);
      reg_rtx[regno] = x;

      if (reg_in_table[regno] >= 0 && reg_in_table[regno] != reg_tick[regno])
	remove_invalid_refs (regno);

      reg_in_table[regno] = reg_tick[regno];

      return;
    }

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    if (fmt[i] == 'e')
      mention_regs (XEXP (x, i));
}

/* Update the register quantities for inserting X into the hash table
   with a value equivalent to CLASSP.
   If MODIFIED is nonzero, X is a destination; it is being modified.
   Note that reg_invalidate should be called on a register
   before insert_regs is done on that register with MODIFIED != 0.  */

static int
insert_regs (x, classp, modified)
     rtx x;
     struct table_elt *classp;
     int modified;
{
  if (GET_CODE (x) == REG)
    {
      register int regno = REGNO (x);
      reg_rtx[regno] = x;
      if (modified || reg_qty[regno] == regno)
	{
	  if (classp && GET_CODE (classp->exp) == REG)
	    {
	      make_regs_eqv (regno, REGNO (classp->exp));
	      /* Make sure reg_rtx is set up even for regs
		 not explicitly set (such as function value).  */
	      reg_rtx[REGNO (classp->exp)] = classp->exp;
	    }
	  else
	    make_new_qty (regno);
	  return 1;
	}
    }
  else
    mention_regs (x);
  return 0;
}

/* Look in or update the hash table.  */

/* Put the element ELT on the list of free elements.  */

static void
free_element (elt)
     struct table_elt *elt;
{
  elt->next_same_hash = free_element_chain;
  free_element_chain = elt;
}

/* Return an element that is free for use.  */

static struct table_elt *
get_element ()
{
  struct table_elt *elt = free_element_chain;
  if (elt)
    {
      free_element_chain = elt->next_same_hash;
      return elt;
    }
  n_elements_made++;
  return (struct table_elt *) oballoc (sizeof (struct table_elt));
}

/* Remove table element ELT from use in the table.
   HASH is its hash code, made using the HASH macro.
   It's an argument because often that is known in advance
   and we save much time not recomputing it.  */

static void
remove (elt, hash)
     register struct table_elt *elt;
     int hash;
{
  if (elt == 0)
    return;

  /* Mark this element as removed.  See cse_insn.  */
  elt->first_same_value = 0;

  /* Remove the table element from its equivalence class.  */
     
  {
    register struct table_elt *prev = elt->prev_same_value;
    register struct table_elt *next = elt->next_same_value;

    if (next) next->prev_same_value = prev;

    if (prev)
      prev->next_same_value = next;
    else
      {
	register struct table_elt *newfirst = next;
	while (next)
	  {
	    next->first_same_value = newfirst;
	    next = next->next_same_value;
	  }
      }
  }

  /* Remove the table element from its hash bucket.  */

  {
    register struct table_elt *prev = elt->prev_same_hash;
    register struct table_elt *next = elt->next_same_hash;

    if (next) next->prev_same_hash = prev;

    if (prev)
      prev->next_same_hash = next;
    else
      table[hash] = next; 
  }

  /* Remove the table element from its related-value circular chain.  */

  if (elt->related_value != 0 && elt->related_value != elt)
    {
      register struct table_elt *p = elt->related_value;
      while (p->related_value != elt)
	p = p->related_value;
      p->related_value = elt->related_value;
      if (p->related_value == p)
	p->related_value = 0;
    }

  free_element (elt);
}

/* Look up X in the hash table and return its table element,
   or 0 if X is not in the table.
   Here we are satisfied to find an expression equivalent to X.  */

static struct table_elt *
lookup (x, hash)
     rtx x;
     int hash;
{
  register struct table_elt *p;

  for (p = table[hash]; p; p = p->next_same_hash)
    if (x == p->exp || exp_equiv_p (x, p->exp, 1))
      return p;

  return 0;
}

/* Look for an expression equivalent to X and of the form (CODE Y).
   If one is found, return Y.  */

static rtx
lookup_as_function (x, code)
     rtx x;
     enum rtx_code code;
{
  register struct table_elt *p = lookup (x, safe_hash (x) % NBUCKETS);
  if (p == 0)
    return 0;

  for (p = p->first_same_value; p; p = p->next_same_value)
    {
      if (GET_CODE (p->exp) == code
	  /* Make sure this is a valid entry in the table.  */
	  && (exp_equiv_p (XEXP (p->exp, 0), XEXP (p->exp, 0))))
	return XEXP (p->exp, 0);
    }
  
  return 0;
}

/* Insert X in the hash table, assuming HASH is its hash code
   and CLASSP is the current first element of the class it should go in
   (or 0 if a new class should be made).
   It is inserted at the proper position to keep the class in
   the order cheapest first.

   For elements of equal cheapness, the most recent one
   goes in front, except that the first element in the list
   remains first unless a cheaper element is added.

   The in_memory field in the hash table element is set to 0.
   The caller must set it nonzero if appropriate.

   You should call insert_regs (X, CLASSP, MODIFY) before calling here,
   and if insert_regs returns a nonzero value
   you must then recompute its hash code before calling here.

   If necessary, update table showing constant values of quantities.  */

#define CHEAPER(X,Y)	\
   (((X)->cost < (Y)->cost) ||						\
    (GET_CODE ((X)->exp) == REG && GET_CODE ((Y)->exp) == REG		\
     && (regno_last_uid[REGNO ((X)->exp)]				\
	 > cse_basic_block_end)						\
     && (regno_last_uid[REGNO ((X)->exp)]				\
	 > regno_last_uid[REGNO ((Y)->exp)])))

static struct table_elt *
insert (x, classp, hash)
     register rtx x;
     register struct table_elt *classp;
     int hash;
{
  register struct table_elt *elt;

  /* Put an element for X into the right hash bucket.  */

  elt = get_element ();
  elt->exp = x;
  elt->cost = rtx_cost (x);
  elt->next_same_value = 0;
  elt->prev_same_value = 0;
  elt->next_same_hash = table[hash];
  elt->prev_same_hash = 0;
  elt->related_value = 0;
  elt->in_memory = 0;
  if (table[hash])
    table[hash]->prev_same_hash = elt;
  table[hash] = elt;

  /* Put it into the proper value-class.  */
  if (classp)
    {
      if (CHEAPER (elt, classp))
	/** Insert at the head of the class */
	{
	  register struct table_elt *p;
	  elt->next_same_value = classp;
	  classp->prev_same_value = elt;
	  elt->first_same_value = elt;

	  for (p = classp; p; p = p->next_same_value)
	    p->first_same_value = elt;
	}
      else
	{
	  /* Insert not at head of the class.  */
	  /* Put it after the last element cheaper than X.  */
	  register struct table_elt *p, *next;
	  for (p = classp; (next = p->next_same_value) && CHEAPER (p, elt);
	       p = next);
	  elt->next_same_value = next;
	  if (next)
	    next->prev_same_value = elt;
	  elt->prev_same_value = p;
	  p->next_same_value = elt;
	  elt->first_same_value = classp;
	}
    }
  else
    elt->first_same_value = elt;

  if (GET_CODE (x) == CONST_INT)
    {
      if (GET_CODE (elt->first_same_value->exp) == REG)
	qty_const[reg_qty[REGNO (elt->first_same_value->exp)]] = x;
    }

  if (GET_CODE (x) == REG)
    {
      if (elt->next_same_value != 0
	  && GET_CODE (elt->next_same_value->exp) == CONST_INT)
	qty_const[reg_qty[REGNO (x)]] = elt->next_same_value->exp;
      if (GET_CODE (elt->first_same_value->exp) == CONST_INT)
	qty_const[reg_qty[REGNO (x)]] = elt->first_same_value->exp;
    }

  /* If this is a constant with symbolic value,
     and it has a term with an explicit integer value,
     link it up with related expressions.  */
  if (GET_CODE (x) == CONST)
    {
      rtx subexp = get_related_value (x);
      int subhash;
      struct table_elt *subelt;

      if (subexp != 0)
	{
	  /* Get the integer-free subexpression in the hash table.  */
	  subhash = safe_hash (subexp) % NBUCKETS;
	  subelt = lookup (subexp, subhash);
	  if (subelt == 0)
	    subelt = insert (subexp, 0, subhash);
	  /* Initialize SUBELT's circular chain if it has none.  */
	  if (subelt->related_value == 0)
	    subelt->related_value = subelt;
	  /* Put new ELT into SUBELT's circular chain.  */
	  elt->related_value = subelt->related_value;
	  subelt->related_value = elt;
	}
    }

  return elt;
}

/* Remove from the hash table all expressions that
   refer to expression X.  X is either a register
   or a memory reference with nonvarying address
   (because, when a memory reference with a varying address is stored in,
   all memory references are removed by invalidate_memory
   so specific invalidation is superfluous).

   A nonvarying address may be just a register or just
   a symbol reference, or it may be either of those plus
   a numeric offset.  */

static void
invalidate (x)
     rtx x;
{
  register int i;
  register struct table_elt *p;
  register rtx base;
  register int start, end;

  /* If X is a register, dependencies on its contents
     are recorded through the qty number mechanism.
     Just change the qty number of the register,
     mark it as invalid for expressions that refer to it,
     and remove it itself.  */

  if (GET_CODE (x) == REG)
    {
      register int hash = HASH (x);
      reg_invalidate (REGNO (x));
      remove (lookup (x, hash), hash);
      return;
    }

  /* X is not a register; it must be a memory reference with
     a nonvarying address.  Remove all hash table elements
     that refer to overlapping pieces of memory.  */

  if (GET_CODE (x) != MEM)
    abort ();
  base = XEXP (x, 0);
  start = 0;

  if (GET_CODE (base) == CONST)
    base = XEXP (base, 0);
  if (GET_CODE (base) == PLUS
      && GET_CODE (XEXP (base, 1)) == CONST_INT)
    {
      start = INTVAL (XEXP (base, 1));
      base = XEXP (base, 0);
    }

  end = start + GET_MODE_SIZE (GET_MODE (x));
  for (i = 0; i < NBUCKETS; i++)
    {
      register struct table_elt *next;
      for (p = table[i]; p; p = next)
	{
	  next = p->next_same_hash;
	  if (refers_to_mem_p (p->exp, base, start, end))
	    remove (p, i);
	}
    }
}

/* Remove all expressions that refer to register REGNO,
   since they are already invalid, and we are about to
   mark that register valid again and don't want the old
   expressions to reappear as valid.  */

static void
remove_invalid_refs (regno)
     int regno;
{
  register int i;
  register struct table_elt *p, *next;
  register rtx x = reg_rtx[regno];

  for (i = 0; i < NBUCKETS; i++)
    for (p = table[i]; p; p = next)
      {
	next = p->next_same_hash;
	if (GET_CODE (p->exp) != REG && refers_to_p (p->exp, x))
	  remove (p, i);
      }
}

/* Remove from the hash table all expressions that reference memory,
   or some of them as specified by *WRITES.  */

static void
invalidate_memory (writes)
     struct write_data *writes;
{
  register int i;
  register struct table_elt *p, *next;
  int all = writes->all;
  int nonscalar = writes->nonscalar;

  for (i = 0; i < NBUCKETS; i++)
    for (p = table[i]; p; p = next)
      {
	next = p->next_same_hash;
	if (p->in_memory
	    && (all
		|| (nonscalar && p->exp->in_struct)
		|| rtx_addr_varies_p (p->exp)))
	  remove (p, i);
      }
}

/* Return the value of the integer term in X, if one is apparent;
   otherwise return 0.
   We do not check extremely carefully for the presence of integer terms
   but rather consider only the cases that `insert' notices
   for the `related_value' field.  */

static int
get_integer_term (x)
     rtx x;
{
  if (GET_CODE (x) == CONST)
    x = XEXP (x, 0);

  if (GET_CODE (x) == MINUS
      && GET_CODE (XEXP (x, 1)) == CONST_INT)
    return - INTVAL (XEXP (x, 1));
  if (GET_CODE (x) != PLUS)
    return 0;
  if (GET_CODE (XEXP (x, 0)) == CONST_INT)
    return INTVAL (XEXP (x, 0));
  if (GET_CODE (XEXP (x, 1)) == CONST_INT)
    return INTVAL (XEXP (x, 1));
  return 0;
}

static rtx
get_related_value (x)
     rtx x;
{
  if (GET_CODE (x) != CONST)
    return 0;
  x = XEXP (x, 0);
  if (GET_CODE (x) == PLUS)
    {
      if (GET_CODE (XEXP (x, 0)) == CONST_INT)
	return XEXP (x, 1);
      if (GET_CODE (XEXP (x, 1)) == CONST_INT)
	return XEXP (x, 0);
    }
  else if (GET_CODE (x) == MINUS
	   && GET_CODE (XEXP (x, 1)) == CONST_INT)
    return XEXP (x, 0);
  return 0;
}

/* Given an expression X of type CONST,
   and ELT which is its table entry (or 0 if it
   is not in the hash table),
   return an alternate expression for X as a register plus integer.
   If none can be found or it would not be a valid address, return 0.  */

static rtx
use_related_value (x, elt)
     rtx x;
     struct table_elt *elt;
{
  register struct table_elt *relt = 0;
  register struct table_elt *p;
  int offset;
  rtx addr;

  /* First, is there anything related known?
     If we have a table element, we can tell from that.
     Otherwise, must look it up.  */

  if (elt != 0 && elt->related_value != 0)
    relt = elt;
  else if (elt == 0 && GET_CODE (x) == CONST)
    {
      rtx subexp = get_related_value (x);
      if (subexp != 0)
	relt = lookup (subexp, safe_hash (subexp) % NBUCKETS);
    }

  if (relt == 0)
    return 0;

  /* Search all related table entries for one that has an
     equivalent register.  */

  p = relt;
  while (1)
    {
      if (p->first_same_value != 0
	  && GET_CODE (p->first_same_value->exp) == REG)
	break;
      p = p->related_value;

      /* We went all the way around, so there is nothing to be found.
	 Return failure.  */
      if (p == relt)
	return 0;
    }

  offset = (get_integer_term (x) - get_integer_term (p->exp));
  if (offset == 0)
    abort ();

  addr = plus_constant (p->first_same_value->exp, offset);
  GO_IF_LEGITIMATE_ADDRESS (QImode, addr, win1);
  return 0;

win1:
  return addr;
}

/* Hash an rtx.  We are careful to make sure the value is never negative.
   Equivalent registers hash identically.

   Store 1 in do_not_record if any subexpression is volatile.

   Store 1 in hash_arg_in_memory if there is a use of MEM
   anywhere within X.

   Note that cse_insn knows that the hash code of a MEM expression
   is just (int) MEM plus the hash code of the address.
   It also knows it can use HASHREG to get the hash code of (REG n).  */

#define HASHBITS 16

#define HASHREG(RTX) \
 ((((int) REG << 7) + reg_qty[REGNO (RTX)]) % NBUCKETS)

static int
canon_hash (x)
     rtx x;
{
  register int i;
  register int hash = 0;
  register RTX_CODE code;
  register char *fmt;

  /* repeat is used to turn tail-recursion into iteration.  */
 repeat:
  code = GET_CODE (x);
  switch (code)
    {
    case REG:
      {
	/* We do not invalidate anything on pushing or popping
	   because they cannot change anything but the stack pointer;
	   but that means we must consider the stack pointer volatile
	   since it can be changed "mysteriously".

	   We consider the function value register volatile
	   because it can be referred to in multi-word modes.
	   If it were live across basic block boundaries, that
	   could cause trouble in register allocation.
	   The only way this can happen is if it were substituted
	   as a cse.  This way we prevent that.  */

	register int regno = REGNO (x);
	if (regno == STACK_POINTER_REGNUM
	    || regno == FUNCTION_VALUE_REGNUM)
	  {
	    do_not_record = 1;
	    return 0;
	  }
	return hash + ((int) REG << 7) + reg_qty[regno];
      }

      /* Assume there is only one rtx object for any given label.  */
    case LABEL_REF:
      return hash + ((int) LABEL_REF << 7) + (int) XEXP (x, 0);

    case SYMBOL_REF:
      return hash + ((int) SYMBOL_REF << 7) + (int) XEXP (x, 0);

    case MEM:
      hash_arg_in_memory = 1;
      /* Now that we have already found this special case,
	 might as well speed it up as much as possible.  */
      hash += (int) MEM;
      x = XEXP (x, 0);
      goto repeat;

    case VOLATILE:
      do_not_record = 1;
      return 0;

    case PRE_DEC:
    case PRE_INC:
    case POST_DEC:
    case POST_INC:
    case PC:
    case CC0:
      do_not_record = 1;
      return 0;
    }

  i = GET_RTX_LENGTH (code) - 1;
  hash += (int) code + (int) GET_MODE (x);
  fmt = GET_RTX_FORMAT (code);
  for (; i >= 0; i--)
    {
      if (fmt[i] == 'e')
	{
	  /* If we are about to do the last recursive call
	     needed at this level, change it into iteration.
	     This function  is called enough to be worth it.  */
	  if (i == 0)
	    {
	      x = XEXP (x, 0);
	      goto repeat;
	    }
	  hash += canon_hash (XEXP (x, i));
	}
      else
	{
	  register int tem = XINT (x, i);
	  hash += ((1 << HASHBITS) - 1) & (tem + tem >> HASHBITS);
	}
    }
  return hash;
}

/* Like canon_hash but with no side effects.  */

static int
safe_hash (x)
     rtx x;
{
  int save_do_not_record = do_not_record;
  int save_hash_arg_in_memory = hash_arg_in_memory;
  int hash = canon_hash (x);
  hash_arg_in_memory = save_hash_arg_in_memory;
  do_not_record = save_do_not_record;
  return hash;
}

/* Return 1 iff X and Y would canonicalize into the same thing,
   without actually constructing the canonicalization of either one.
   If VALIDATE is nonzero,
   we assume X is an expression being processed from the rtl
   and Y was found in the hash table.  We check register refs
   in Y for being marked as valid.  */

static int
exp_equiv_p (x, y, validate)
     rtx x, y;
     int validate;
{
  register int i;
  register int hash = 0;
  register RTX_CODE code = GET_CODE (x);
  register char *fmt;

  /* An expression is usually equivalent to itself,
     but not if it's a register that's invalid.  */
  if (x == y)
    return (code != REG
	    || !validate
	    || reg_in_table[REGNO (y)] == reg_tick[REGNO (y)]);

  if (code != GET_CODE (y))
    return 0;
  if (code == REG)
    return (reg_qty[REGNO (x)] == reg_qty[REGNO (y)]
	    && (!validate
		|| reg_in_table[REGNO (y)] == reg_tick[REGNO (y)]));

  /* Assume there is only one rtx object to refer
     to any given label.
     We already know that X and Y are not the same object
     so they must differ.  */

  if (code == LABEL_REF || code == SYMBOL_REF)
    return XEXP (x, 0) == XEXP (y, 0);

  /* (MULT:SI x y) and (MULT:HI x y) are NOT equivalent.  */

  if (GET_MODE (x) != GET_MODE (y))
    return 0;

  /* Compare the elements.  If any pair of corresponding elements
     fail to match, return 0 for the whole things.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      if (fmt[i] == 'e')
	{
	  if (exp_equiv_p (XEXP (x, i), XEXP (y, i), validate) == 0)
	    return 0;
	}
      else
	{
	  if (XINT (x, i) != XINT (y, i))
	    return 0;
	}
    }
  return 1;
}

/* Return 1 iff any subexpression of X matches Y.
   Here we do not require that X or Y be valid (for registers referred to)
   for being in the hash table.  */

int
refers_to_p (x, y)
     rtx x, y;
{
  register int i;
  register RTX_CODE code;
  register char *fmt;

 repeat:
  if (x == y)
    return 1;

  code = GET_CODE (x);
  /* If X as a whole has the same code as Y, they may match.
     If so, return 1.  */
  if (code == GET_CODE (y))
    {
      if (exp_equiv_p (x, y, 0))
	return 1;
    }

  /* X does not match, so try its subexpressions.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    if (fmt[i] == 'e')
      if (i == 0)
	{
	  x = XEXP (x, 0);
	  goto repeat;
	}
      else
	if (refers_to_p (XEXP (x, i), y))
	  return 1;

  return 0;
}

/* Return 1 iff any subexpression of X refers to memory
   at an address of REG plus some offset
   such that any of the bytes' offsets fall between START (inclusive)
   and END (exclusive).

   The value is undefined if X is a varying address.
   This function is not used in such cases.  */

int
refers_to_mem_p (x, reg, start, end)
     rtx x, reg;
     int start, end;
{
  register int i;
  register RTX_CODE code;
  register char *fmt;

 repeat:
  code = GET_CODE (x);
  if (code == MEM)
    {
      register rtx addr = XEXP (x, 0);	/* Get the address.  */
      int myend;
      if (GET_CODE (addr) == CONST)
	addr = XEXP (addr, 0);
      if (addr == reg)
	i = 0;
      else if (GET_CODE (addr) == PLUS
	       && XEXP (addr, 0) == reg
	       && GET_CODE (XEXP (addr, 1)) == CONST_INT)
	i = INTVAL (XEXP (addr, 1));
      else
	return 0;

      myend = i + GET_MODE_SIZE (GET_MODE (x));
      return myend > start && i < end;
    }

  /* X does not match, so try its subexpressions.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    if (fmt[i] == 'e')
      if (i == 0)
	{
	  x = XEXP (x, 0);
	  goto repeat;
	}
      else
	if (refers_to_mem_p (XEXP (x, i), reg, start, end))
	  return 1;

  return 0;
}

/* Canonicalize an expression:
   replace each register reference inside it
   with the "oldest" equivalent register.  */

static rtx
canon_reg (x)
     rtx x;
{
  register int i;
  register RTX_CODE code = GET_CODE (x);
  register char *fmt;

  if (code == REG)
    {
      register int qty = reg_qty[REGNO (x)];
      register rtx new = reg_rtx[qty_first_reg[qty]];
      return new ? new : x;
    }

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      if (fmt[i] == 'E')
	abort ();
      if (fmt[i] == 'e')
	XEXP (x, i) = canon_reg (XEXP (x, i));
    }

  return x;
}

/* If X is a nontrivial arithmetic operation on an argument
   for which a constant value can be determined, return
   the result of operating on that value, as a constant.
   Otherwise, return X, possibly with one or more operands
   modified by recursive calls to this function.  */

static rtx
fold_rtx (x)
     rtx x;
{
  register RTX_CODE code = GET_CODE (x);
  register char *fmt;
  register int i, val;
  rtx const_arg[2];

  const_arg[0] = 0;
  const_arg[1] = 0;

  /* Try folding our operands.
     Then see which ones have constant values known.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    if (fmt[i] == 'e')
      {
	register rtx tem = fold_rtx (XEXP (x, i));
	XEXP (x, i) = tem;
	if (i < 2)
	  {
	    if (GET_CODE (tem) == CONST_INT)
	      const_arg[i] = tem;
	    else if (GET_CODE (tem) == REG
		     && qty_const[reg_qty[REGNO (tem)]] != 0)
	      const_arg[i] = qty_const[reg_qty[REGNO (tem)]];
	  }
      }

  /* Now decode the kind of rtx X is
     and either return X (if nothing can be done)
     or store a value in VAL and drop through
     (to return a CONST_INT for the integer VAL).  */

  if (GET_RTX_LENGTH (code) == 1)
    {
      register int arg0;

      if (const_arg[0] == 0)
	{
	  return x;
	}

      arg0 = INTVAL (const_arg[0]);

      switch (GET_CODE (x))
	{
	case NOT:
	  val = ~ arg0;
	  break;

	case NEG:
	  val = - arg0;
	  break;

	case SIGN_EXTEND:
	  val = arg0;
	  break;

	default:
	  return x;
	}
    }
  else if (GET_RTX_LENGTH (code) == 2)
    {
      register int arg0, arg1;

      /* If 1st arg is the condition codes, 2nd must be zero
	 and this must be a comparison.
	 Decode the info on how the previous insn set the cc0
	 and use that to deduce result of comparison.  */
      if (XEXP (x, 0) == cc0_rtx)
	{
	  if (prev_insn_cc0 == 0
	      || const_arg[1] != const0_rtx)
	    return x;
	  if (code == LEU || code == LTU || code == GEU || code == GTU)
	    arg0 = prev_insn_cc0 & 7;
	  else
	    arg0 = (prev_insn_cc0 >> 3) & 7;
	  if (arg0 == 7) arg0 = -1;

	  switch (code)
	    {
	    case LE:
	    case LEU:
	      return (arg0 <= 0) ? const1_rtx : const0_rtx;
	    case LT:
	    case LTU:
	      return (arg0 < 0) ? const1_rtx : const0_rtx;
	    case GE:
	    case GEU:
	      return (arg0 >= 0) ? const1_rtx : const0_rtx;
	    case GT:
	    case GTU:
	      return (arg0 > 0) ? const1_rtx : const0_rtx;
	    case NE:
	      return (arg0 != 0) ? const1_rtx : const0_rtx;
	    case EQ:
	      return (arg0 == 0) ? const1_rtx : const0_rtx;
	    default:
	      abort ();
	    }
	}

      if (const_arg[0] == 0 || const_arg[1] == 0)
	{
	  /* Even if we can't compute a constant result,
	     there are some cases worth simplifying.  */
	  if (code == PLUS)
	    {
	      if (const_arg[0] != 0 && INTVAL (const_arg[0]) == 0)
		return XEXP (x, 1);
	      if (const_arg[1] != 0 && INTVAL (const_arg[1]) == 0)
		return XEXP (x, 0);
	      if (const_arg[0] != 0 && GET_CODE (XEXP (x, 1)) == PLUS
		  && (CONSTANT_ADDRESS_P (XEXP (XEXP (x, 1), 0))
		      || CONSTANT_ADDRESS_P (XEXP (XEXP (x, 1), 1))))
		/* constant + (variable + constant)
		   can result if an index register is made constant.
		   We simplify this by adding the constants.
		   If we did not, it would become an invalid address.  */
		return plus_constant (XEXP (x, 1),
				      INTVAL (const_arg[0]));
	      if (const_arg[1] != 0 && GET_CODE (XEXP (x, 0)) == PLUS
		  && (CONSTANT_ADDRESS_P (XEXP (XEXP (x, 0), 0))
		      || CONSTANT_ADDRESS_P (XEXP (XEXP (x, 0), 1))))
		return plus_constant (XEXP (x, 0),
				      INTVAL (const_arg[1]));
	    }
	  if (code == MINUS)
	    {
	      if (const_arg[1] != 0 && INTVAL (const_arg[1]) == 0)
		return XEXP (x, 0);
	    }

	  /* PLUS and MULT can appear inside of a MEM.
	     In such situations, a constant term must come second.  */
	  if (code == MULT || code == PLUS)
	    if (const_arg[0] != 0)
	      {
		XEXP (x, 0) = XEXP (x, 1);
		XEXP (x, 1) = const_arg[0];
	      }

	  return x;
	}

      arg0 = INTVAL (const_arg[0]);
      arg1 = INTVAL (const_arg[1]);

      switch (code)
	{
	case PLUS:
	  val = arg0 + arg1;
	  break;

	case MINUS:
	  if (GET_MODE (x) == VOIDmode)
	    /* Overflowless comparison:
	       cannot represent an exact answer, so don't fold.
	       This is used only to set the CC0,
	       and fold_cc0 will take care of it.  */
	    return x;
	  val = arg0 - arg1;
	  break;

	case MULT:
	  val = arg0 * arg1;
	  break;

	case DIV:
	  val = arg0 / arg1;
	  break;

	case MOD:
	  val = arg0 % arg1;
	  break;

	case UMULT:
	  val = (unsigned) arg0 * arg1;
	  break;

	case UDIV:
	  val = (unsigned) arg0 / arg1;
	  break;

	case UMOD:
	  val = (unsigned) arg0 % arg1;
	  break;

	case AND:
	  val = arg0 & arg1;
	  break;

	case IOR:
	  val = arg0 | arg1;
	  break;

	case XOR:
	  val = arg0 ^ arg1;
	  break;

	case NE:
	  val = arg0 != arg1;
	  break;

	case EQ:
	  val = arg0 == arg1;
	  break;

	case LE:
	  val = arg0 <= arg1;
	  break;

	case LT:
	  val = arg0 < arg1;
	  break;

	case GE:
	  val = arg0 >= arg1;
	  break;

	case GT:
	  val = arg0 > arg1;
	  break;

	case LEU:
	  val = ((unsigned) arg0) <= ((unsigned) arg1);
	  break;

	case LTU:
	  val = ((unsigned) arg0) < ((unsigned) arg1);
	  break;

	case GEU:
	  val = ((unsigned) arg0) >= ((unsigned) arg1);
	  break;

	case GTU:
	  val = ((unsigned) arg0) > ((unsigned) arg1);
	  break;

	case LSHIFT:
	  val = ((unsigned) arg0) << arg1;
	  break;

	case ASHIFT:
	  val = arg0 << arg1;
	  break;

	case ROTATERT:
	  arg1 = - arg1;
	case ROTATE:
	  {
	    int size = GET_MODE_SIZE (GET_MODE (x)) * BITS_PER_UNIT;
	    if (arg1 > 0)
	      {
		arg1 %= size;
		val = ((((unsigned) arg0) << arg1)
		       | (((unsigned) arg0) >> (size - arg1)));
	      }
	    else if (arg1 < 0)
	      {
		arg1 = (- arg1) % size;
		val = ((((unsigned) arg0) >> arg1)
		       | (((unsigned) arg0) << (size - arg1)));
	      }
	    else
	      val = arg0;
	  }
	  break;

	case LSHIFTRT:
	  val = ((unsigned) arg0) >> arg1;
	  break;

	case ASHIFTRT:
	  val = arg0 >> arg1;
	  break;

	default:
	  return x;
	}
    }
  else if (code == IF_THEN_ELSE && const_arg[0] != 0)
    return XEXP (x, ((INTVAL (const_arg[0]) != 0) ? 1 : 2));
  else
    return x;

/* ??? Here must clear or set the bits that don't belong in our mode.  */
  return gen_rtx (CONST_INT, VOIDmode, val);
}

/* Given an expression X which is used to set CC0,
   return an integer recording (in the encoding used for prev_insn_cc0)
   how the condition codes would be set by that expression.
   Return 0 if the value is not constant.  */

static int
fold_cc0 (x)
     rtx x;
{
  if (GET_CODE (x) == MINUS && GET_MODE (x) == VOIDmode)
    {
      rtx y0 = fold_rtx (XEXP (x, 0));
      rtx y1 = fold_rtx (XEXP (x, 1));
      int i0, i1;
      if (GET_CODE (y0) != CONST_INT || GET_CODE (y1) != CONST_INT)
	return 0;
      i0 = INTVAL (y0);
      i1 = INTVAL (y1);
      return 100 + ((i0 < i1 ? 7 : i0 > i1) << 3)
	+ (((unsigned) i0 < (unsigned) i1) ? 7
	   : ((unsigned) i0 > (unsigned) i1));
    }
  x = fold_rtx (x);
  if (GET_CODE (x) != CONST_INT)
    return 0;
  return 100 + ((INTVAL (x) < 0 ? 7 : INTVAL (x) > 0) << 3) + INTVAL (x) != 0;
}

/* Attempt to prove that a loop will be executed >= 1 times,
   or prove it will be executed 0 times.
   If either can be proved, delete some of the code.  */

predecide_loop_entry (insn)
     register rtx insn;
{
  register rtx jump = NEXT_INSN (insn);
  register rtx p = JUMP_LABEL (jump);
  register rtx loop_top_label = NEXT_INSN (NEXT_INSN (jump));
  enum { UNK, DELETE_LOOP, DELETE_JUMP } disposition = UNK;

  /* Trace the flow of control through the end test,
     propagating constants, to see if result is determined.  */
  prev_insn_cc0 = 0;
  while (1)
    {
      /* Arriving at end of loop means endtest will drop out.  */
      if (GET_CODE (p) == NOTE
	  && NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)
	{
	  disposition = DELETE_LOOP;
	  break;
	}
      else if (GET_CODE (p) == CODE_LABEL || GET_CODE (p) == NOTE)
	;
      /* We only know how to handle two kinds of insns:
	 conditional jumps, and those that set the condition codes. */
      else if (GET_CODE (p) == INSN && GET_CODE (PATTERN (p)) == SET
	       && SET_DEST (PATTERN (p)) == cc0_rtx)
	{
	  prev_insn_cc0 = fold_cc0 (copy_rtx (SET_SRC (PATTERN (p))));
	}
      else if (GET_CODE (p) == JUMP_INSN
	       && GET_CODE (PATTERN (p)) == SET
	       && SET_DEST (PATTERN (p)) == pc_rtx)
	{
	  register rtx target
	    = fold_rtx (copy_rtx (SET_SRC (PATTERN (p))));
	  if (GET_CODE (target) == LABEL_REF)
	    p = XEXP (target, 0);
	  else if (target != pc_rtx)
	    /* If destination of jump is not fixed, give up.  */
	    break;
	}
      /* Any other kind of insn means we don't know
	 what result the test will have.  */
      else
	break;

      /* Arriving at top of loop means we can drop straight in.
	 Check here because we can arrive only via a jump insn
	 which would have changed P above.  */
      if (p == loop_top_label)
	{
	  disposition = DELETE_JUMP;
	  break;
	}
      /* We went past one insn; consider the next.  */
      p = NEXT_INSN (p);
    }
  if (disposition == DELETE_JUMP)
    {
      /* We know the loop test will succeed the first time,
	 so delete the jump to the test; drop right into loop.
	 Note that one call to delete_insn gets the BARRIER as well.  */
      delete_insn (jump);
    }
  if (disposition == DELETE_LOOP)
    {
      /* We know the endtest will fail and drop right out of the loop,
	 but it isn't safe to delete the loop here.
	 There could be jumps into it from outside.
	 So make the entry-jump jump around the loop.
	 This will cause find_basic_blocks to delete it if appropriate.  */
      register rtx label = gen_label_rtx ();
      emit_label_after (label, p);
      redirect_jump (jump, label);
    }
}

/* CSE processing for one instruction.
   First simplify sources and addresses of all assignments
   in the instruction, using previously-computed equivalents values.
   Then install the new sources and destinations in the table
   of available values.  */

static rtx set[MAX_SETS_PER_INSN];
static struct table_elt *src_elt[MAX_SETS_PER_INSN];
static int src_hash_code[MAX_SETS_PER_INSN];
static int dest_hash_code[MAX_SETS_PER_INSN];
static char src_in_memory[MAX_SETS_PER_INSN];
static char src_volatile[MAX_SETS_PER_INSN];

static void
cse_insn (insn)
     rtx insn;
{
  register rtx x = PATTERN (insn);
  register int i;
  register int n_sets = 0;

  /* Records what this insn does to set CC0,
     using same encoding used for prev_insn_cc0.  */
  int this_insn_cc0 = 0;
  struct write_data writes_memory;
  static struct write_data init = {0, 0, 0};

  writes_memory = init;

  /* Find all the SETs and CLOBBERs in this instruction.
     Record all the SETs in the array `set' and count them.
     Also determine whether there is a CLOBBER that invalidates
     all memory references, or all references at varying addresses.  */

  if (GET_CODE (x) == SET)
    {
      n_sets = 1;
      set[0] = x;
      /* Return now for unconditional jumps.
	 The never need cse processing, so this does not hurt.
	 The reason is not efficiency but rather
	 so that we can test at the end for instructions
	 that have been simplified to unconditional jumps
	 and not be misled by unchanged instructions
	 that were unconditional jumps to begin with.  */
      if (SET_DEST (x) == pc_rtx
	  && GET_CODE (SET_SRC (x)) == LABEL_REF)
	return;
    }
  else if (GET_CODE (x) == PARALLEL)
    {
      register int lim;
      lim = XVECLEN (x, 0);
      for (i = 0; i < lim; i++)
	{
	  register rtx y = XVECEXP (x, 0, i);
	  if (GET_CODE (y) == SET)
	    set[n_sets++] = y;
	  else if (GET_CODE (y) == CLOBBER)
	    note_mem_written (XEXP (y, 0), &writes_memory);
	  else if (GET_CODE (y) == CALL)
	    canon_reg (y);
	}
    }
  else if (GET_CODE (x) == CLOBBER)
    note_mem_written (XEXP (x, 0), &writes_memory);
  else if (GET_CODE (x) == CALL)
    canon_reg (x);

  if (n_sets == 0)
    {
      invalidate_from_clobbers (&writes_memory, x);
      return;
    }

  /* Canonicalize sources and addresses of destinations.
     set src_elt[i] to the class each source belongs to.
     Detect assignments from or to volatile things
     and set set[i] to zero so they will be ignored
     in the rest of this function.

     Nothing in this loop changes the hash table or the register chains.  */

  for (i = 0; i < n_sets; i++)
    {
      register rtx src, dest, addr;
      register struct table_elt *elt;

      dest = SET_DEST (set[i]);
      src = SET_SRC (set[i]);

      /* Replace each registers in SRC with oldest equivalent register,
	 but if DEST is a register do not replace it if it appears in SRC.  */

      if (GET_CODE (dest) == REG)
	{
	  int tem = reg_qty[REGNO (dest)];
	  reg_qty[REGNO (dest)] = REGNO (dest);
	  src = canon_reg (src);
	  reg_qty[REGNO (dest)] = tem;
	}
      else
	src = canon_reg (src);

      /* Compute SRC's hash code, and also notice if it
	 should not be recorded at all.  In that case,
	 prevent any further processing of this assignment.  */
      do_not_record = 0;
      hash_arg_in_memory = 0;
      src = fold_rtx (src);
      /* If we have (NOT Y), see if Y is known to be (NOT Z).
	 If so, (NOT Y) simplifies to Z.  */
      if (GET_CODE (src) == NOT || GET_CODE (src) == NEG)
	{
	  rtx y = lookup_as_function (XEXP (src, 0), GET_CODE (src));
	  if (y != 0)
	    src = y;
	}

      /* If storing a constant value in a register that
	 previously held the constant value 0,
	 record this fact with a REG_WAS_0 note on this insn.  */
      if (GET_CODE (src) == CONST_INT
	  && GET_CODE (dest) == REG
	  && qty_const[reg_qty[REGNO (dest)]] == const0_rtx)
	REG_NOTES (insn) = gen_rtx (EXPR_LIST, REG_WAS_0,
				    dest, REG_NOTES (insn));

      src_hash_code[i] = HASH (src);
      src_volatile[i] = do_not_record;

      if (GET_CODE (src) == REG)
	{
	  src_in_memory[i] = 0;
	  src_elt[i] = 0;
	}
      else if (!src_volatile[i])
	{
	  /* Replace the source with its cheapest equivalent.  */

	  elt = lookup (src, src_hash_code[i]);
	  if (elt && elt != elt->first_same_value)
	    {
	      elt = elt->first_same_value;
	      /* Find the cheapest one that is still valid.  */
	      while (GET_CODE (elt->exp) != REG
		     && !exp_equiv_p (elt->exp, elt->exp, 1))
		elt = elt->next_same_value;
	      src = copy_rtx (elt->exp);
	      hash_arg_in_memory = 0;
	      src_hash_code[i] = HASH (src);
	    }

	  /* If ELT is a constant, is there a register
	     linearly related to it?  If so, replace it
	     with the sum of that register plus an offset.  */

	  if (GET_CODE (src) == CONST && n_sets == 1)
	    {
	      rtx newsrc = use_related_value (src, elt);
	      if (newsrc)
		{
		  src = newsrc;
		  hash_arg_in_memory = 0;
		  src_hash_code[i] = HASH (src);
		  /* The new expression for the SRC has the same value
		     as the previous one; so if the previous one is in
		     the hash table, put the new one in as equivalent.  */
		  if (elt != 0)
		    elt = insert (src, elt->first_same_value, src_hash_code[i]);
		  /* The reg this insn sets is no longer a "constant" reg
		     because SRC now depends on a register that may not
		     be invariant.  */
		  REG_NOTES (insn) = 0;
		}
	    }

	  src_elt[i] = elt;
	  src_in_memory[i] = hash_arg_in_memory;
	}

      /* Either canon_reg or the copy_rtx may have changed this.  */
      /* Note it is not safe to replace the sources if there
	 is more than one set.  We could get an insn
	 [(set (reg) (reg)) (set (reg) (reg))], which is probably
	 not in the machine description.
	 This case we could handle by breaking into several insns.
	 Cases of partial substituttion cannot win at all.  */
      /* Also, if this insn is setting a "constant" register,
	 we may not replace the value that is given to it.  */
      if (n_sets == 1)
	if (REG_NOTES (insn) == 0
	    || (enum reg_note) GET_MODE (REG_NOTES (insn)) != REG_CONST)
	  SET_SRC (set[i]) = src;

      do_not_record = 0;

      /* If storing into memory, do cse on the memory address.
	 Also compute the hash code of the destination now,
	 before the effects of this instruction are recorded,
	 since the register values used in the address computation
	 are those before this instruction.  */
      /* We can discard a VOLATILE around the destination of the set
	 because we never coalesce, reorder or delete stores into memory.  */
      if (GET_CODE (dest) == VOLATILE)
	SET_SRC (set[i]) = dest = XEXP (dest, 0);

      if (GET_CODE (dest) == MEM)
	{
	  register rtx addr;
	  register int hash;

	  canon_reg (dest);
	  addr = fold_rtx (XEXP (dest, 0));
	  XEXP (dest, 0) = addr;

	  /* Pushing or popping does not invalidate anything.  */
	  if ((GET_CODE (addr) == PRE_DEC || GET_CODE (addr) == PRE_INC
	       || GET_CODE (addr) == POST_DEC || GET_CODE (addr) == POST_INC)
	      && GET_CODE (XEXP (addr, 0)) == REG
	      && REGNO (XEXP (addr, 0)) == STACK_POINTER_REGNUM)
	    ;
	  else
	    /* Otherwise, decide whether we invalidate
	       everything in memory, or just things at non-fixed places.
	       Writing a large aggregate must invalidate everything
	       because we don't know how long it is.  */
	    note_mem_written (dest, &writes_memory);

	  /* Do not consider addresses of local and argument slots.
	     The MEM expressions for args and non-register local variables
	     are made only once and inserted in many instructions,
	     as well as being used to control symbol table output.
	     It is not safe to clobber them.  */
	  if ((GET_CODE (addr) == PLUS
	       && GET_CODE (XEXP (addr, 0)) == REG
	       && GET_CODE (XEXP (addr, 1)) == CONST_INT
	       && (hash = REGNO (XEXP (addr, 0)),
		   hash == FRAME_POINTER_REGNUM || hash == ARG_POINTER_REGNUM))
	      || (GET_CODE (addr) == REG
		  && (hash = REGNO (addr),
		      hash == FRAME_POINTER_REGNUM || hash == ARG_POINTER_REGNUM)))
	    ;
	  else
	    {
	      hash = HASH (addr);
	      if (! do_not_record)
		{
		  elt = lookup (addr, hash);
		  dest_hash_code[i] = ((int) MEM + hash) % NBUCKETS;

		  if (elt && elt != elt->first_same_value)
		    {
		      elt = elt->first_same_value;
		      /* Find the cheapest one that is still valid.  */
		      while (GET_CODE (elt->exp) != REG
			     && !exp_equiv_p (elt->exp, elt->exp, 1))
			elt = elt->next_same_value;

		      XEXP (dest, 0) = addr
			= copy_rtx (elt->exp);
		    }
		}
	    }
	}

      /* For compound destinations that modify only a part of
	  register or memory location, we cannot enter them
	  in the hash table since we know only part of the value.
	  Set src_volatile to prevent this.
	  But we must invalidate the entire value.
	  So prevent set[i] from becoming zero.  */

      if (GET_CODE (dest) == SUBREG
	  || GET_CODE (dest) == STRICT_LOW_PART
	  || GET_CODE (dest) == ZERO_EXTRACT
	  || GET_CODE (dest) == SIGN_EXTRACT)
	src_volatile[i] = 1;

      /* No further processing for this assignment
	 if destination is volatile.  */

      else if (do_not_record
	       || (GET_CODE (dest) == REG 
		   ? REGNO (dest) == STACK_POINTER_REGNUM
		   : GET_CODE (dest) != MEM))
	set[i] = 0;

      if (dest == cc0_rtx && GET_CODE (src) == MINUS)
	this_insn_cc0 = fold_cc0 (src);
    }

  /* Now enter all non-volatile source expressions in the hash table
     if they are not already present.
     Record in src_elt the heads of their equivalence classes.
     This way we can insert the corresponding destinations into
     the same classes even if the actual sources are no longer in them
     (having been invalidated).  */

  for (i = 0; i < n_sets; i++)
    if (set[i] && src_elt[i] == 0 && ! src_volatile[i])
      {
	register rtx src = SET_SRC (set[i]);
	register struct table_elt *elt;

	/* Note that these insert_regs calls cannot remove
	   any of the src_elt's, because they would have failed to match
	   if not still valid.  */
	if (insert_regs (src, 0, 0))
	  src_hash_code[i] = HASH (src);
	elt = insert (src, 0, src_hash_code[i]);
	elt->in_memory = src_in_memory[i];
	src_elt[i] = elt->first_same_value;
      }

  invalidate_from_clobbers (&writes_memory, x);

  /* Now invalidate everything set by this instruction.
     If a SUBREG or other funny destination is being set,
     set[i] is still nonzero, so here we invalidate the reg
     a part of which is being set.  */

  for (i = 0; i < n_sets; i++)
    if (set[i])
      {
	register rtx dest = SET_DEST (set[i]);

	while (GET_CODE (dest) == SUBREG
	       || GET_CODE (dest) == STRICT_LOW_PART
	       || GET_CODE (dest) == ZERO_EXTRACT
	       || GET_CODE (dest) == SIGN_EXTRACT)
	  dest = XEXP (dest, 0);

	/* Needed for registers to remove the register from its
	   previous quantity's chain.
	   Needed for memory if this is a nonvarying address, unless
	   we have just done an invalidate_memory that covers even those.  */
	if (GET_CODE (dest) == REG
	    || (! writes_memory.all && ! rtx_addr_varies_p (dest)))
	  invalidate (dest);
      }

  /* We may have just removed some of the src_elt's.
     So replace each one with the current head of the same class.  */

  for (i = 0; i < n_sets; i++)
    if (set[i] && ! src_volatile[i] && src_elt[i]->first_same_value == 0)
      {
	register struct table_elt *elt = src_elt[i];

	/* If elt was removed, find current head of same class,
	   or 0 if nothing remains of that class.  */

	while (elt && elt->first_same_value == 0)
	  elt = elt->next_same_value;
	src_elt[i] = elt ? elt->first_same_value : 0;
      }

  /* Now insert the destinations into their equivalence classes.
     If the source is volatile, record the destination in
     a class of its own.  */

  for (i = 0; i < n_sets; i++)
    if (set[i])
      {
	register rtx dest = SET_DEST (set[i]);
	register struct table_elt *elt = src_elt[i];

	if (src_volatile[i])
	  elt = 0;
	if (insert_regs (dest, elt, 1))
	  dest_hash_code[i] = HASHREG (dest);
	elt = insert (dest, elt, dest_hash_code[i]);
	elt->in_memory = GET_CODE (dest) == MEM;
      }

  /* Did this insn become an unconditional branch or become a no-op?  */
  if (GET_CODE (insn) == JUMP_INSN
      && GET_CODE (x) == SET
      && SET_DEST (x) == pc_rtx)
    {
      if (SET_SRC (x) == pc_rtx)
	{
	  PUT_CODE (insn, NOTE);
	  NOTE_LINE_NUMBER (insn) = NOTE_INSN_DELETED;
	  NOTE_SOURCE_FILE (insn) = 0;
	  cse_jumps_altered = 1;
	  /* If previous insn just set CC0 for us, delete it too.  */
	  if (prev_insn_cc0 != 0)
	    {
	      PUT_CODE (prev_insn, NOTE);
	      NOTE_LINE_NUMBER (prev_insn) = NOTE_INSN_DELETED;
	      NOTE_SOURCE_FILE (prev_insn) = 0;
	    }
	}
      else if (GET_CODE (SET_SRC (x)) == LABEL_REF)
	{
	  emit_barrier_after (insn);
	  cse_jumps_altered = 1;
	  /* If previous insn just set CC0 for us, delete it too.  */
	  if (prev_insn_cc0 != 0)
	    {
	      PUT_CODE (prev_insn, NOTE);
	      NOTE_LINE_NUMBER (prev_insn) = NOTE_INSN_DELETED;
	      NOTE_SOURCE_FILE (prev_insn) = 0;
	    }
	}
    }

  prev_insn_cc0 = this_insn_cc0;
  prev_insn = insn;
}

/* Store 1 in *WRITES_PTR for those categories of memory ref
   that must be invalidated when the expression WRITTEN is stored in.
   If WRITTEN is null, say everything must be invalidated.  */

static void
note_mem_written (written, writes_ptr)
     rtx written;
     struct write_data *writes_ptr;
{
  static struct write_data everything = {1, 1, 1};

  if (written == 0)
    *writes_ptr = everything;
  else if (GET_CODE (written) == MEM)
    {
      if (rtx_addr_varies_p (written))
	{
	  /* A varying address that is a sum indicates an array element,
	     and that's just as good as a structure element
	     in implying that we need not invalidate scalar variables.  */
	  if (!(written->in_struct
		|| GET_CODE (XEXP (written, 0)) == PLUS))
	    writes_ptr->all = 1;
	  writes_ptr->nonscalar = 1;
	}
      writes_ptr->var = 1;
    }
}

/* Perform invalidation on the basis of everything about an insn
   except for invalidating the actual places that are SET in it.
   This includes the places CLOBBERed, and anything that might
   alias with something that is SET or CLOBBERed.

   W points to the writes_memory for this insn, a struct write_data
   saying which kinds of memory references must be invalidated.
   X is the pattern of the insn.  */

static void
invalidate_from_clobbers (w, x)
     struct write_data *w;
     rtx x;
{
  /* If W->var is not set, W specifies no action.
     If W->all is set, this step gets all memory refs
     so they can be ignored in the rest of this function.  */
  if (w->var)
    invalidate_memory (w);

  if (GET_CODE (x) == CLOBBER)
    {
      rtx ref = XEXP (x, 0);
      if (ref
	  && (GET_CODE (ref) == REG
	      || (GET_CODE (ref) == MEM && ! w->all)))
	invalidate (ref);
    }
  else if (GET_CODE (x) == PARALLEL)
    {
      register int i;
      for (i = XVECLEN (x, 0) - 1; i >= 0; i--)
	{
	  register rtx y = XVECEXP (x, 0, i);
	  if (GET_CODE (y) == CLOBBER)
	    {
	      rtx ref = XEXP (y, 0);
	      if (ref
		  &&(GET_CODE (ref) == REG
		     || (GET_CODE (ref) == MEM && !w->all)))
		invalidate (ref);
	    }
	}
    }
}

static void cse_basic_block ();

/* Perform cse on the instructions of a function.
   F is the first instruction.
   NREGS is one plus the highest pseudo-reg number used in the instruction.

   Returns 1 if jump_optimize should be redone due to simplifications
   in conditional jump instructions.  */

int
cse_main (f, nregs)
     /* f is the first instruction of a chain of insns for one function */
     rtx f;
     /* nregs is the total number of registers used in it */
     int nregs;
{
  register rtx insn = f;
  register int i;

  cse_jumps_altered = 0;

  init_recog ();

  max_reg = nregs;

  all_minus_one = (int *) alloca (nregs * sizeof (int));
  consec_ints = (int *) alloca (nregs * sizeof (int));
  for (i = 0; i < nregs; i++)
    {
      all_minus_one[i] = -1;
      consec_ints[i] = i;
    }

  reg_next_eqv = (int *) alloca (nregs * sizeof (int));
  reg_prev_eqv = (int *) alloca (nregs * sizeof (int));
  reg_qty = (int *) alloca (nregs * sizeof (int));
  reg_rtx = (rtx *) alloca (nregs * sizeof (rtx));
  reg_in_table = (int *) alloca (nregs * sizeof (int));
  reg_tick = (int *) alloca (nregs * sizeof (int));

  /* Discard all the free elements of the previous function
     since they are allocated in the temporarily obstack.  */
  bzero (table, sizeof table);
  free_element_chain = 0;
  n_elements_made = 0;

  /* Loop over basic blocks */
  while (insn)
    {
      register rtx p = insn;
      register int i = 0;
      register int last_uid;

      /* Find end of next basic block */
      while (p && GET_CODE (p) != CODE_LABEL)
	{
	  last_uid = INSN_UID (p);
	  p = NEXT_INSN (p);
	  i++;
	}

      cse_basic_block_end = last_uid;

      max_qty = max_reg + i * MAX_SETS_PER_INSN;

      cse_basic_block (insn, p);

      insn = p ? NEXT_INSN (p) : 0;
    }

  if (max_elements_made < n_elements_made)
    max_elements_made = n_elements_made;

  return cse_jumps_altered;
}

static void
cse_basic_block (from, to)
     register rtx from, to;
{
  register rtx insn;
  int *qv1 = (int *) alloca (max_qty * sizeof (int));
  int *qv2 = (int *) alloca (max_qty * sizeof (int));
  rtx *qv3 = (rtx *) alloca (max_qty * sizeof (rtx));

  qty_first_reg = qv1;
  qty_last_reg = qv2;
  qty_const = qv3;

  new_basic_block ();

  for (insn = from; insn != to; insn = NEXT_INSN (insn))
    {
      register RTX_CODE code = GET_CODE (insn);
      if (code == INSN || code == JUMP_INSN || code == CALL_INSN)
	cse_insn (insn);
      /* Memory, and some registers, are invalidate by subroutine calls.  */
      if (code == CALL_INSN)
	{
	  register int i;
	  static struct write_data everything = {1, 1, 1};
	  invalidate_memory (&everything);
	  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
	    if (call_clobbers_reg[i] && reg_rtx[i]
		&& i != FRAME_POINTER_REGNUM
		&& i != ARG_POINTER_REGNUM)
	      invalidate (reg_rtx[i]);
	}
      /* Loop beginnings are often followed by jumps
	 (that enter the loop above the endtest).
	 See if we can prove the loop will be executed at least once;
	 if so, delete the jump.  Also perhaps we can prove loop
	 will never be executed and delete the entire thing.  */
      if (code == NOTE
	  && NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG
	  && GET_CODE (NEXT_INSN (insn)) == JUMP_INSN)
	{
	  predecide_loop_entry (insn);
	  /* Whether that jump was deleted or not,
	     it certainly is the end of the basic block.
	     Since the jump is unconditional,
	     it requires no further processing here.  */
	  break;
	}
    }
}