c++: Implement for namespace statics CWG 2867 - Order of initialization for structure...

[official-gcc.git] / gcc / ext-dce.cc

/* RTL dead zero/sign extension (code) elimination.
   Copyright (C) 2000-2025 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "backend.h"
#include "rtl.h"
#include "tree.h"
#include "memmodel.h"
#include "insn-config.h"
#include "emit-rtl.h"
#include "recog.h"
#include "cfganal.h"
#include "tree-pass.h"
#include "cfgrtl.h"
#include "rtl-iter.h"
#include "df.h"
#include "print-rtl.h"
#include "dbgcnt.h"
#include "diagnostic-core.h"

/* These should probably move into a C++ class.  */
static vec<bitmap_head> livein;
static bitmap all_blocks;
static bitmap livenow;
static bitmap changed_pseudos;
static bool modify;

/* We consider four bit groups for liveness:
   bit 0..7   (least significant byte)
   bit 8..15  (second least significant byte)
   bit 16..31
   bit 32..BITS_PER_WORD-1  */

/* For the given REG, return the number of bit groups implied by the
   size of the REG's mode, up to a maximum of 4 (number of bit groups
   tracked by this pass).

   For partial integer and variable sized modes also return 4.  This
   could possibly be refined for something like PSI mode, but it
   does not seem worth the effort.  */

static int
group_limit (const_rtx reg)
{
  machine_mode mode = GET_MODE (reg);

  if (!GET_MODE_BITSIZE (mode).is_constant ())
    return 4;

  int size = GET_MODE_SIZE (mode).to_constant ();

  size = exact_log2 (size);

  if (size < 0)
    return 4;

  size++;
  return (size > 4 ? 4 : size);
}

/* Make all bit groups live for REGNO in bitmap BMAP.  For hard regs,
   we assume all groups are live.  For a pseudo we consider the size
   of the pseudo to avoid creating unnecessarily live chunks of data.  */

static void
make_reg_live (bitmap bmap, int regno)
{
  int limit;

  /* For pseudos we can use the mode to limit how many bit groups
     are marked as live since a pseudo only has one mode.  Hard
     registers have to be handled more conservatively.  */
  if (regno > FIRST_PSEUDO_REGISTER)
    {
      rtx reg = regno_reg_rtx[regno];
      limit = group_limit (reg);
    }
  else
    limit = 4;

  for (int i = 0; i < limit; i++)
    bitmap_set_bit (bmap, regno * 4 + i);
}

/* Note this pass could be used to narrow memory loads too.  It's
   not clear if that's profitable or not in general.  */

#define UNSPEC_P(X) (GET_CODE (X) == UNSPEC || GET_CODE (X) == UNSPEC_VOLATILE)

/* If we know the destination of CODE only uses some low bits
   (say just the QI bits of an SI operation), then return true
   if we can propagate the need for just the subset of bits
   from the destination to the sources.

   FIXME: This is safe for operands 1 and 2 of an IF_THEN_ELSE, but not
   operand 0.  Thus is likely would need some special casing to handle.  */

static bool
safe_for_live_propagation (rtx_code code)
{
  /* First handle rtx classes which as a whole are known to
     be either safe or unsafe.  */
  switch (GET_RTX_CLASS (code))
    {
      case RTX_OBJ:
      case RTX_CONST_OBJ:
        return true;

      case RTX_COMPARE:
      case RTX_COMM_COMPARE:
      case RTX_TERNARY:
        return false;

      default:
        break;
    }

  /* What's left are specific codes.  We only need to identify those
     which are safe.   */
  switch (code)
    {
    /* These are trivially safe.  */
    case SUBREG:
    case NOT:
    case ZERO_EXTEND:
    case SIGN_EXTEND:
    case TRUNCATE:
    case PLUS:
    case MINUS:
    case MULT:
    case SMUL_HIGHPART:
    case UMUL_HIGHPART:
    case AND:
    case IOR:
    case XOR:
      return true;

    /* We can propagate for the shifted operand, but not the shift
       count.  The count is handled specially.  */
    case ASHIFT:
    case LSHIFTRT:
    case ASHIFTRT:
    case SS_ASHIFT:
    case US_ASHIFT:
      return true;

    /* There may be other safe codes.  If so they can be added
       individually when discovered.  */
    default:
      return false;
    }
}

/* Clear bits in LIVENOW and set bits in LIVE_TMP for objects
   set/clobbered by OBJ contained in INSN.

   Conceptually it is always safe to ignore a particular destination
   here as that will result in more chunks of data being considered
   live.  That's what happens when we "continue" the main loop when
   we see something we don't know how to handle such as a vector
   mode destination.

   The more accurate we are in identifying what objects (and chunks
   within an object) are set by INSN, the more aggressive the
   optimization phase during use handling will be.  */

static bool
ext_dce_process_sets (rtx_insn *insn, rtx obj, bitmap live_tmp)
{
  bool skipped_dest = false;

  subrtx_iterator::array_type array;
  FOR_EACH_SUBRTX (iter, array, obj, NONCONST)
    {
      const_rtx x = *iter;

      /* An EXPR_LIST (from call fusage) ends in NULL_RTX.  */
      if (x == NULL_RTX)
        continue;

      if (UNSPEC_P (x))
        continue;

      if (GET_CODE (x) == SET || GET_CODE (x) == CLOBBER)
        {
          unsigned bit = 0;
          x = SET_DEST (x);

          /* We don't support vector destinations or destinations
             wider than DImode.  */
          scalar_int_mode outer_mode;
          if (!is_a <scalar_int_mode> (GET_MODE (x), &outer_mode)
              || GET_MODE_BITSIZE (outer_mode) > HOST_BITS_PER_WIDE_INT)
            {
              /* Skip the subrtxs of this destination.  There is
                 little value in iterating into the subobjects, so
                 just skip them for a bit of efficiency.  */
              skipped_dest = true;
              iter.skip_subrtxes ();
              continue;
            }

          /* We could have (strict_low_part (subreg ...)).  We can not just
             strip the STRICT_LOW_PART as that would result in clearing
             some bits in LIVENOW that are still live.  So process the
             STRICT_LOW_PART specially.  */
          if (GET_CODE (x) == STRICT_LOW_PART)
            {
              x = XEXP (x, 0);

              /* The only valid operand of a STRICT_LOW_PART is a non
                 paradoxical SUBREG.  */
              gcc_assert (SUBREG_P (x)
                          && !paradoxical_subreg_p (x)
                          && SUBREG_BYTE (x).is_constant ());

              /* I think we should always see a REG here.  But let's
                 be sure.  */
              gcc_assert (REG_P (SUBREG_REG (x)));

              /* The inner mode might be larger, just punt for
                 that case.  Remember, we can not just continue to process
                 the inner RTXs due to the STRICT_LOW_PART.  */
              if (!is_a <scalar_int_mode> (GET_MODE (SUBREG_REG (x)), &outer_mode)
                  || GET_MODE_BITSIZE (outer_mode) > HOST_BITS_PER_WIDE_INT)
                {
                  /* Skip the subrtxs of the STRICT_LOW_PART.  We can't
                     process them because it'll set objects as no longer
                     live when they are in fact still live.  */
                  skipped_dest = true;
                  iter.skip_subrtxes ();
                  continue;
                }

              /* LIVE_TMP contains the set groups that are live-out and set in
                 this insn.  It is used to narrow the groups live-in for the
                 inputs of this insn.

                 The simple thing to do is mark all the groups as live, but
                 that will significantly inhibit optimization.

                 We also need to be careful in the case where we have an in-out
                 operand.  If we're not careful we'd clear LIVE_TMP
                 incorrectly.  */
              HOST_WIDE_INT rn = REGNO (SUBREG_REG (x));
              int limit = group_limit (SUBREG_REG (x));
              for (HOST_WIDE_INT i = 4 * rn; i < 4 * rn + limit; i++)
                if (bitmap_bit_p (livenow, i))
                  bitmap_set_bit (live_tmp, i);

              if (bitmap_empty_p (live_tmp))
                make_reg_live (live_tmp, rn);

              /* The mode of the SUBREG tells us how many bits we can
                 clear.  */
              machine_mode mode = GET_MODE (x);
              HOST_WIDE_INT size
                = exact_log2 (GET_MODE_SIZE (mode).to_constant ()) + 1;
              bitmap_clear_range (livenow, 4 * rn, size);

              /* We have fully processed this destination.  */
              iter.skip_subrtxes ();
              continue;
            }

          /* Phase one of destination handling.  First remove any wrapper
             such as SUBREG or ZERO_EXTRACT.  */
          unsigned HOST_WIDE_INT mask
            = GET_MODE_MASK (GET_MODE_INNER (GET_MODE (x)));
          if (SUBREG_P (x))
            {
              /* If we have a SUBREG destination that is too wide, just
                 skip the destination rather than continuing this iterator.
                 While continuing would be better, we'd need to strip the
                 subreg and restart within the SET processing rather than
                 the top of the loop which just complicates the flow even
                 more.  */
              if (!is_a <scalar_int_mode> (GET_MODE (SUBREG_REG (x)), &outer_mode)
                  || GET_MODE_BITSIZE (outer_mode) > HOST_BITS_PER_WIDE_INT)
                {
                  skipped_dest = true;
                  iter.skip_subrtxes ();
                  continue;
                }

              /* We can safely strip a paradoxical subreg.  The inner mode will
                 be narrower than the outer mode.  We'll clear fewer bits in
                 LIVENOW than we'd like, but that's always safe.  */
              if (paradoxical_subreg_p (x))
                x = XEXP (x, 0);
              else if (SUBREG_BYTE (x).is_constant ())
                {
                  bit = subreg_lsb (x).to_constant ();
                  mask = GET_MODE_MASK (GET_MODE (SUBREG_REG (x))) << bit;
                  gcc_assert (mask);
                  x = SUBREG_REG (x);
                }
              else
                gcc_unreachable ();
            }

          if (GET_CODE (x) == ZERO_EXTRACT)
            {
              /* Unlike a SUBREG destination, a set of a ZERO_EXTRACT only
                 modifies the bits referenced in the ZERO_EXTRACT, the rest
                 remain the same.  Thus we can not continue here, we must
                 either figure out what part of the destination is modified
                 or skip the sub-rtxs.  */
              skipped_dest = true;
              iter.skip_subrtxes ();
              continue;
            }

          /* BIT >= 64 indicates something went horribly wrong.  */
          gcc_assert (bit <= HOST_BITS_PER_WIDE_INT - 1);

          /* Now handle the actual object that was changed.  */
          if (REG_P (x))
            {
              /* LIVE_TMP contains the set groups that are live-out and set in
                 this insn.  It is used to narrow the groups live-in for the
                 inputs of this insn.

                 The simple thing to do is mark all the groups as live, but
                 that will significantly inhibit optimization.

                 We also need to be careful in the case where we have an in-out
                 operand.  If we're not careful we'd clear LIVE_TMP
                 incorrectly.  */
              HOST_WIDE_INT rn = REGNO (x);
              int limit = group_limit (x);
              for (HOST_WIDE_INT i = 4 * rn; i < 4 * rn + limit; i++)
                if (bitmap_bit_p (livenow, i))
                  bitmap_set_bit (live_tmp, i);

              if (bitmap_empty_p (live_tmp))
                make_reg_live (live_tmp, rn);

              /* Now clear the bits known written by this instruction.
                 Note that BIT need not be a power of two, consider a
                 ZERO_EXTRACT destination.  */
              int start = (bit < 8 ? 0 : bit < 16 ? 1 : bit < 32 ? 2 : 3);
              int end = ((mask & ~HOST_WIDE_INT_UC (0xffffffff)) ? 4
                         : (mask & HOST_WIDE_INT_UC (0xffff0000)) ? 3
                         : (mask & 0xff00) ? 2 : 1);
              bitmap_clear_range (livenow, 4 * rn + start, end - start);
            }
          /* Some ports generate (clobber (const_int)).  */
          else if (CONST_INT_P (x))
            continue;
          else
            gcc_assert (CALL_P (insn)
                        || MEM_P (x)
                        || x == pc_rtx
                        || GET_CODE (x) == SCRATCH);

          iter.skip_subrtxes ();
        }
      else if (GET_CODE (x) == COND_EXEC)
        {
          /* This isn't ideal, but may not be so bad in practice.  */
          skipped_dest = true;
          iter.skip_subrtxes ();
        }
    }
  return skipped_dest;
}

/* INSN has a sign/zero extended source inside SET that we will
   try to turn into a SUBREG.  */
static void
ext_dce_try_optimize_insn (rtx_insn *insn, rtx set)
{
  rtx src = SET_SRC (set);
  rtx inner = XEXP (src, 0);

  /* Avoid (subreg (mem)) and other constructs which may be valid RTL, but
     not useful for this optimization.  */
  if (!(REG_P (inner) || (SUBREG_P (inner) && REG_P (SUBREG_REG (inner)))))
    return;

  rtx new_pattern;
  if (dump_file)
    {
      fprintf (dump_file, "Processing insn:\n");
      dump_insn_slim (dump_file, insn);
      fprintf (dump_file, "Trying to simplify pattern:\n");
      print_rtl_single (dump_file, SET_SRC (set));
    }

  /* We decided to turn do the optimization but allow it to be rejected for
     bisection purposes.  */
  if (!dbg_cnt (::ext_dce))
    {
      if (dump_file)
        fprintf (dump_file, "Rejected due to debug counter.\n");
      return;
    }

  new_pattern = simplify_gen_subreg (GET_MODE (src), inner,
                                     GET_MODE (inner), 0);
  /* simplify_gen_subreg may fail in which case NEW_PATTERN will be NULL.
     We must not pass that as a replacement pattern to validate_change.  */
  if (new_pattern)
    {
      int ok = validate_change (insn, &SET_SRC (set), new_pattern, false);

      rtx x = SET_DEST (set);
      while (SUBREG_P (x) || GET_CODE (x) == ZERO_EXTRACT)
        x = XEXP (x, 0);

      gcc_assert (REG_P (x));
      if (ok)
        bitmap_set_bit (changed_pseudos, REGNO (x));

      if (dump_file)
        {
          if (ok)
            fprintf (dump_file, "Successfully transformed to:\n");
          else
            fprintf (dump_file, "Failed transformation to:\n");

          print_rtl_single (dump_file, new_pattern);
          fprintf (dump_file, "\n");
        }
    }
  else
    {
      if (dump_file)
        fprintf (dump_file, "Unable to generate valid SUBREG expression.\n");
    }
}

/* Some operators imply that their second operand is fully live,
   regardless of how many bits in the output are live.  An example
   would be the shift count on a target without SHIFT_COUNT_TRUNCATED
   defined.

   Return TRUE if CODE is such an operator.  FALSE otherwise.  */

static bool
binop_implies_op2_fully_live (rtx_code code)
{
  switch (code)
    {
    case ASHIFT:
    case LSHIFTRT:
    case ASHIFTRT:
    case ROTATE:
    case ROTATERT:
    case SS_ASHIFT:
    case US_ASHIFT:
      return !SHIFT_COUNT_TRUNCATED;

    default:
      return false;
    }
}

/* X, with code CODE, is an operation for which safe_for_live_propagation
   holds true, and bits set in MASK are live in the result.  Compute a
   mask of (potentially) live bits in the non-constant inputs.  In case of
   binop_implies_op2_fully_live (e.g. shifts), the computed mask may
   exclusively pertain to the first operand.

   This looks wrong as we may have some important operations embedded as
   operands of another operation.  For example, we might have an extension
   wrapping a shift.  It really feels like this needs to be recursing down
   into operands much more often.  */

unsigned HOST_WIDE_INT
carry_backpropagate (unsigned HOST_WIDE_INT mask, enum rtx_code code, rtx x)
{
  if (mask == 0)
    return 0;

  enum machine_mode mode = GET_MODE_INNER (GET_MODE (x));
  unsigned HOST_WIDE_INT mmask = GET_MODE_MASK (mode);

  /* While we don't try to optimize operations on types larger
     than 64 bits, we do want to make sure not to invoke undefined
     behavior when presented with such operations during use
     processing.  The safe thing to do is to just return mmask
     for that scenario indicating every possible chunk is life.  */
  scalar_int_mode smode;
  if (!is_a <scalar_int_mode> (mode, &smode)
      || GET_MODE_BITSIZE (smode) > HOST_BITS_PER_WIDE_INT)
    return mmask;

  switch (code)
    {
    case PLUS:
    case MINUS:
    case MULT:
      return (HOST_WIDE_INT_UC (2) << floor_log2 (mask)) - 1;

    /* We propagate for the shifted operand, but not the shift
       count.  The count is handled specially.  */
    case ASHIFT:
      if (CONST_INT_P (XEXP (x, 1))
          && UINTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (smode))
        return (HOST_WIDE_INT) mask >> INTVAL (XEXP (x, 1));
      return (HOST_WIDE_INT_UC (2) << floor_log2 (mask)) - 1;

    /* We propagate for the shifted operand, but not the shift
       count.  The count is handled specially.  */
    case LSHIFTRT:
      if (CONST_INT_P (XEXP (x, 1))
          && UINTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (smode))
        return mmask & (mask << INTVAL (XEXP (x, 1)));
      return mmask;

    /* We propagate for the shifted operand, but not the shift
       count.  The count is handled specially.  */
    case ASHIFTRT:
      if (CONST_INT_P (XEXP (x, 1))
          && UINTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (smode))
        {
          HOST_WIDE_INT sign = 0;
          if (HOST_BITS_PER_WIDE_INT - clz_hwi (mask) + INTVAL (XEXP (x, 1))
              > GET_MODE_BITSIZE (smode))
            sign = HOST_WIDE_INT_1U << (GET_MODE_BITSIZE (smode) - 1);
          return sign | (mmask & (mask << INTVAL (XEXP (x, 1))));
        }
      return mmask;

    case SMUL_HIGHPART:
    case UMUL_HIGHPART:
      if (XEXP (x, 1) == const0_rtx)
        return 0;
      if (XEXP (x, 1) == const1_rtx)
        return mmask;
      if (CONST_INT_P (XEXP (x, 1)))
        {
          if (pow2p_hwi (INTVAL (XEXP (x, 1))))
            return mmask & (mask << (GET_MODE_BITSIZE (smode)
                                     - exact_log2 (INTVAL (XEXP (x, 1)))));

          int bits = (HOST_BITS_PER_WIDE_INT + GET_MODE_BITSIZE (smode)
                      - clz_hwi (mask) - ctz_hwi (INTVAL (XEXP (x, 1))));
          if (bits < GET_MODE_BITSIZE (smode))
            return (HOST_WIDE_INT_1U << bits) - 1;
        }
      return mmask;

    case SIGN_EXTEND:
      if (!GET_MODE_BITSIZE (GET_MODE (x)).is_constant ()
          || !GET_MODE_BITSIZE (GET_MODE (XEXP (x, 0))).is_constant ())
        return -1;

      /* We want the mode of the inner object.  We need to ensure its
         sign bit is on in MASK.  */
      mode = GET_MODE_INNER (GET_MODE (XEXP (x, 0)));
      if (mask & ~GET_MODE_MASK (mode))
        mask |= HOST_WIDE_INT_1U << (GET_MODE_BITSIZE (mode).to_constant ()
                                     - 1);

      /* Recurse into the operand.  */
      return carry_backpropagate (mask, GET_CODE (XEXP (x, 0)), XEXP (x, 0));

    case ZERO_EXTEND:
      if (!GET_MODE_BITSIZE (GET_MODE (x)).is_constant ()
          || !GET_MODE_BITSIZE (GET_MODE (XEXP (x, 0))).is_constant ())
        return -1;

      /* Recurse into the operand.  */
      return carry_backpropagate (mask, GET_CODE (XEXP (x, 0)), XEXP (x, 0));

    /* We propagate for the shifted operand, but not the shift
       count.  The count is handled specially.  */
    case SS_ASHIFT:
    case US_ASHIFT:
      if (CONST_INT_P (XEXP (x, 1))
          && UINTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (smode))
        {
          return ((mmask & ~((unsigned HOST_WIDE_INT) mmask
                             >> (INTVAL (XEXP (x, 1))
                                 + (XEXP (x, 1) != const0_rtx
                                    && code == SS_ASHIFT))))
                  | ((HOST_WIDE_INT) mask >> INTVAL (XEXP (x, 1))));
        }
      return mmask;

    default:
      return mask;
    }
}

/* Process uses in INSN contained in OBJ.  Set appropriate bits in LIVENOW
   for any chunks of pseudos that become live, potentially filtering using
   bits from LIVE_TMP.

   If MODIFY is true, then optimize sign/zero extensions to SUBREGs when
   the extended bits are never read and mark pseudos which had extensions
   eliminated in CHANGED_PSEUDOS.  */

static void
ext_dce_process_uses (rtx_insn *insn, rtx obj,
                      bitmap live_tmp, bool skipped_dest)
{
  subrtx_var_iterator::array_type array_var;
  FOR_EACH_SUBRTX_VAR (iter, array_var, obj, NONCONST)
    {
      /* An EXPR_LIST (from call fusage) ends in NULL_RTX.  */
      rtx x = *iter;
      if (x == NULL_RTX)
        continue;

      /* So the basic idea in this FOR_EACH_SUBRTX_VAR loop is to
         handle SETs explicitly, possibly propagating live information
         into the uses.

         We may continue the loop at various points which will cause
         iteration into the next level of RTL.  Breaking from the loop
         is never safe as it can lead us to fail to process some of the
         RTL and thus not make objects live when necessary.  */
      enum rtx_code xcode = GET_CODE (x);
      if (xcode == SET)
        {
          const_rtx dst = SET_DEST (x);
          rtx src = SET_SRC (x);
          const_rtx y;
          unsigned HOST_WIDE_INT bit = 0;

          /* The code of the RHS of a SET.  */
          enum rtx_code code = GET_CODE (src);

          /* ?!? How much of this should mirror SET handling, potentially
             being shared?   */
          if (SUBREG_P (dst) && SUBREG_BYTE (dst).is_constant ())
            {
              bit = subreg_lsb (dst).to_constant ();
              if (bit >= HOST_BITS_PER_WIDE_INT)
                bit = HOST_BITS_PER_WIDE_INT - 1;
              dst = SUBREG_REG (dst);
            }
          else if (GET_CODE (dst) == STRICT_LOW_PART)
            dst = XEXP (dst, 0);

          /* Main processing of the uses.  Two major goals here.

             First, we want to try and propagate liveness (or the lack
             thereof) from the destination register to the source
             register(s).

             Second, if the source is an extension, try to optimize
             it into a SUBREG.  The SUBREG form indicates we don't
             care about the upper bits and will usually be copy
             propagated away.

             If we fail to handle something in here, the expectation
             is the iterator will dive into the sub-components and
             mark all the chunks in any found REGs as live.  */
          if (REG_P (dst) && safe_for_live_propagation (code))
            {
              /* Create a mask representing the bits of this output
                 operand that are live after this insn.  We can use
                 this information to refine the live in state of
                 inputs to this insn in many cases.

                 We have to do this on a per SET basis, we might have
                 an INSN with multiple SETS, some of which can narrow
                 the source operand liveness, some of which may not.  */
              unsigned HOST_WIDE_INT dst_mask = 0;
              HOST_WIDE_INT rn = REGNO (dst);
              unsigned HOST_WIDE_INT mask_array[]
                = { 0xff, 0xff00, HOST_WIDE_INT_UC (0xffff0000),
                    -HOST_WIDE_INT_UC (0x100000000) };
              for (int i = 0; i < 4; i++)
                if (bitmap_bit_p (live_tmp, 4 * rn + i))
                  dst_mask |= mask_array[i];
              dst_mask >>= bit;

              /* If we ignored a destination during set processing, then
                 consider all the bits live.  */
              if (skipped_dest)
                dst_mask = -1;

              dst_mask = carry_backpropagate (dst_mask, code, src);

              /* ??? Could also handle ZERO_EXTRACT / SIGN_EXTRACT
                 of the source specially to improve optimization.  */
              if (code == SIGN_EXTEND || code == ZERO_EXTEND)
                {
                  rtx inner = XEXP (src, 0);
                  unsigned HOST_WIDE_INT src_mask
                    = GET_MODE_MASK (GET_MODE_INNER (GET_MODE (inner)));

                  /* DST_MASK could be zero if we had something in the SET
                     that we couldn't handle.  */
                  if (modify && !skipped_dest && (dst_mask & ~src_mask) == 0)
                    ext_dce_try_optimize_insn (insn, x);

                  /* Stripping the extension here just seems wrong on multiple
                     levels.  It's source side handling, so it seems like it
                     belongs in the loop below.  Stripping here also makes it
                     harder than necessary to properly handle live bit groups
                     for (ANY_EXTEND (SUBREG)) where the SUBREG has
                     SUBREG_PROMOTED state.  */
                  dst_mask &= src_mask;
                  src = XEXP (src, 0);
                  code = GET_CODE (src);
                }

              /* Optimization is done at this point.  We just want to make
                 sure everything that should get marked as live is marked
                 from here onward.  */

              /* We will handle the other operand of a binary operator
                 at the bottom of the loop by resetting Y.  */
              if (BINARY_P (src))
                y = XEXP (src, 0);
              else
                y = src;

              /* We're inside a SET and want to process the source operands
                 making things live.  Breaking from this loop will cause
                 the iterator to work on sub-rtxs, so it is safe to break
                 if we see something we don't know how to handle.

                 This code is just hokey as it really just handles trivial
                 unary and binary cases.  Otherwise the loop exits and we
                 continue iterating on sub-rtxs, but outside the set context.  */
              unsigned HOST_WIDE_INT save_mask = dst_mask;
              for (;;)
                {
                  /* In general we want to restore DST_MASK before each loop
                     iteration.  The exception is when the opcode implies that
                     the other operand is fully live.  That's handled by
                     changing SAVE_MASK below.  */
                  dst_mask = save_mask;
                  /* Strip an outer paradoxical subreg.  The bits outside
                     the inner mode are don't cares.  So we can just strip
                     and process the inner object.  */
                  if (paradoxical_subreg_p (y))
                    y = XEXP (y, 0);
                  else if (SUBREG_P (y) && SUBREG_BYTE (y).is_constant ())
                    {
                      /* We really want to know the outer code here, ie do we
                         have (ANY_EXTEND (SUBREG ...)) as we need to know if
                         the extension matches the SUBREG_PROMOTED state.  In
                         that case optimizers can turn the extension into a
                         simple copy.  Which means that bits outside the
                         SUBREG's mode are actually live.

                         We don't want to mark those bits live unnecessarily
                         as that inhibits extension elimination in important
                         cases such as those in Coremark.  So we need that
                         outer code.  */
                      if (!REG_P (SUBREG_REG (y))
                          || (SUBREG_PROMOTED_VAR_P (y)
                              && ((GET_CODE (SET_SRC (x)) == SIGN_EXTEND
                                   && SUBREG_PROMOTED_SIGNED_P (y))
                                  || (GET_CODE (SET_SRC (x)) == ZERO_EXTEND
                                      && SUBREG_PROMOTED_UNSIGNED_P (y)))))
                        break;

                      bit = subreg_lsb (y).to_constant ();

                      /* If this is a wide object (more bits than we can fit
                         in a HOST_WIDE_INT), then just break from the SET
                         context.   That will cause the iterator to walk down
                         into the subrtx and if we land on a REG we'll mark
                         the whole think live.  */
                      if (bit >= HOST_BITS_PER_WIDE_INT)
                        break;

                      /* The SUBREG's mode determines the live width.  */
                      if (dst_mask)
                        {
                          dst_mask <<= bit;
                          if (!dst_mask)
                            dst_mask = -HOST_WIDE_INT_UC (0x100000000);
                        }
                      y = SUBREG_REG (y);
                    }

                  if (REG_P (y))
                    {
                      /* We have found the use of a register.  We need to mark
                         the appropriate chunks of the register live.  The mode
                         of the REG is a starting point.  We may refine that
                         based on what chunks in the output were live.  */
                      rn = 4 * REGNO (y);
                      unsigned HOST_WIDE_INT tmp_mask = dst_mask;

                      /* If the RTX code for the SET_SRC is not one we can
                         propagate destination liveness through, then just
                         set the mask to the mode's mask.  */
                      if (!safe_for_live_propagation (code))
                        tmp_mask
                          = GET_MODE_MASK (GET_MODE_INNER (GET_MODE (y)));

                      if (tmp_mask & 0xff)
                        bitmap_set_bit (livenow, rn);
                      if (tmp_mask & 0xff00)
                        bitmap_set_bit (livenow, rn + 1);
                      if (tmp_mask & HOST_WIDE_INT_UC (0xffff0000))
                        bitmap_set_bit (livenow, rn + 2);
                      if (tmp_mask & -HOST_WIDE_INT_UC (0x100000000))
                        bitmap_set_bit (livenow, rn + 3);
                    }
                  else if (!CONSTANT_P (y))
                    break;

                  /* We might have (ashift (const_int 1) (reg...))
                     By setting dst_mask we can continue iterating on the
                     the next operand and it will be considered fully live.

                     Note that since we restore DST_MASK from SAVE_MASK at the
                     top of the loop, we have to change SAVE_MASK to get the
                     semantics we want.  */
                  if (binop_implies_op2_fully_live (GET_CODE (src)))
                    save_mask = -1;

                  /* If this was anything but a binary operand, break the inner
                     loop.  This is conservatively correct as it will cause the
                     iterator to look at the sub-rtxs outside the SET context.  */
                  if (!BINARY_P (src))
                    break;

                  /* We processed the first operand of a binary operator.  Now
                     handle the second.  */
                  y = XEXP (src, 1), src = pc_rtx;
                }

              /* These are leaf nodes, no need to iterate down into them.  */
              if (REG_P (y) || CONSTANT_P (y))
                iter.skip_subrtxes ();
            }
        }
      /* If we are reading the low part of a SUBREG, then we can
         refine liveness of the input register, otherwise let the
         iterator continue into SUBREG_REG.  */
      else if (SUBREG_P (x)
               && REG_P (SUBREG_REG (x))
               && !paradoxical_subreg_p (x)
               && subreg_lowpart_p (x)
               && GET_MODE_BITSIZE (GET_MODE (x)).is_constant ()
               && GET_MODE_BITSIZE (GET_MODE (x)).to_constant () <= 32)
        {
          HOST_WIDE_INT size = GET_MODE_BITSIZE (GET_MODE (x)).to_constant ();
          HOST_WIDE_INT rn = 4 * REGNO (SUBREG_REG (x));

          /* If this is a promoted subreg, then more of it may be live than
             is otherwise obvious.  */
          if (SUBREG_PROMOTED_VAR_P (x))
            size = GET_MODE_BITSIZE (GET_MODE (SUBREG_REG (x))).to_constant ();

          bitmap_set_bit (livenow, rn);
          if (size > 8)
            bitmap_set_bit (livenow, rn + 1);
          if (size > 16)
            bitmap_set_bit (livenow, rn + 2);
          if (size >= 32)
            bitmap_set_bit (livenow, rn + 3);
          iter.skip_subrtxes ();
        }
      /* If we have a register reference that is not otherwise handled,
         just assume all the chunks are live.  */
      else if (REG_P (x))
        bitmap_set_range (livenow, REGNO (x) * 4, group_limit (x));
    }
}

/* Process a single basic block BB with current liveness information
   in LIVENOW, returning updated liveness information.

   If MODIFY is true, then this is the last pass and unnecessary
   extensions should be eliminated when possible.  If an extension
   is removed, the source pseudo is marked in CHANGED_PSEUDOS.  */

static void
ext_dce_process_bb (basic_block bb)
{
  rtx_insn *insn;

  FOR_BB_INSNS_REVERSE (bb, insn)
    {
      if (!NONDEBUG_INSN_P (insn))
        continue;

      /* Live-out state of the destination of this insn.  We can
         use this to refine the live-in state of the sources of
         this insn in many cases.  */
      bitmap live_tmp = BITMAP_ALLOC (NULL);

      /* First process any sets/clobbers in INSN.  */
      bool skipped_dest = ext_dce_process_sets (insn, PATTERN (insn), live_tmp);

      /* CALL_INSNs need processing their fusage data.  */
      if (CALL_P (insn))
        skipped_dest |= ext_dce_process_sets (insn,
                                              CALL_INSN_FUNCTION_USAGE (insn),
                                              live_tmp);

      /* And now uses, optimizing away SIGN/ZERO extensions as we go.  */
      ext_dce_process_uses (insn, PATTERN (insn), live_tmp, skipped_dest);

      /* A nonlocal goto implicitly uses the frame pointer.  */
      if (JUMP_P (insn) && find_reg_note (insn, REG_NON_LOCAL_GOTO, NULL_RTX))
        {
          bitmap_set_range (livenow, FRAME_POINTER_REGNUM * 4, 4);
          if (!HARD_FRAME_POINTER_IS_FRAME_POINTER)
            bitmap_set_range (livenow, HARD_FRAME_POINTER_REGNUM * 4, 4);
        }

      /* And process fusage data for the use as well.  */
      if (CALL_P (insn))
        {
          if (!FAKE_CALL_P (insn))
            bitmap_set_range (livenow, STACK_POINTER_REGNUM * 4, 4);

          /* If this is not a call to a const fucntion, then assume it
             can read any global register.  */
          if (!RTL_CONST_CALL_P (insn))
            for (unsigned i = 0; i < FIRST_PSEUDO_REGISTER; i++)
              if (global_regs[i])
                bitmap_set_range (livenow, i * 4, 4);

          ext_dce_process_uses (insn, CALL_INSN_FUNCTION_USAGE (insn), live_tmp, false);
        }

      BITMAP_FREE (live_tmp);
    }
}

/* SUBREG_PROMOTED_VAR_P is set by the gimple->rtl optimizers and
   is usually helpful.  However, in some cases setting the value when
   it not strictly needed can cause this pass to miss optimizations.

   Specifically consider (set (mem) (subreg (reg))).  If set in that
   case it will cause more bit groups to be live for REG than would
   be strictly necessary which in turn can inhibit extension removal.

   So do a pass over the IL wiping the SUBREG_PROMOTED_VAR_P when it
   is obviously not needed.  */

static void
maybe_clear_subreg_promoted_p (void)
{
  for (rtx_insn *insn = get_insns(); insn; insn = NEXT_INSN (insn))
    {
      if (!NONDEBUG_INSN_P (insn))
        continue;

      rtx set = single_set (insn);
      if (!set)
        continue;

      /* There may be other cases where we should clear, but for
         now, this is the only known case where it causes problems.  */
      if (MEM_P (SET_DEST (set)) && SUBREG_P (SET_SRC (set))
        && GET_MODE (SET_DEST (set)) <= GET_MODE (SUBREG_REG (SET_SRC (set))))
        SUBREG_PROMOTED_VAR_P (SET_SRC (set)) = 0;
    }
}


/* We optimize away sign/zero extensions in this pass and replace
   them with SUBREGs indicating certain bits are don't cares.

   This changes the SUBREG_PROMOTED_VAR_P state of the object.
   It is fairly painful to fix this on the fly, so we have
   recorded which pseudos are affected and we look for SUBREGs
   of those pseudos and fix them up.  */

static void
reset_subreg_promoted_p (void)
{
  /* If we removed an extension, that changed the promoted state
     of the destination of that extension.  Thus we need to go
     find any SUBREGs that reference that pseudo and adjust their
     SUBREG_PROMOTED_P state.  */
  for (rtx_insn *insn = get_insns(); insn; insn = NEXT_INSN (insn))
    {
      if (!NONDEBUG_INSN_P (insn))
        continue;

      rtx pat = PATTERN (insn);
      subrtx_var_iterator::array_type array;
      FOR_EACH_SUBRTX_VAR (iter, array, pat, NONCONST)
        {
          rtx sub = *iter;

          /* We only care about SUBREGs.  */
          if (GET_CODE (sub) != SUBREG)
            continue;

          const_rtx x = SUBREG_REG (sub);

          /* We only care if the inner object is a REG.  */
          if (!REG_P (x))
            continue;

          /* And only if the SUBREG is a promoted var.  */
          if (!SUBREG_PROMOTED_VAR_P (sub))
            continue;

          if (bitmap_bit_p (changed_pseudos, REGNO (x)))
            SUBREG_PROMOTED_VAR_P (sub) = 0;
        }
    }
}

/* Initialization of the ext-dce pass.  Primarily this means
   setting up the various bitmaps we utilize.  */

static void
ext_dce_init (void)
{
  livein.create (last_basic_block_for_fn (cfun));
  livein.quick_grow_cleared (last_basic_block_for_fn (cfun));
  for (int i = 0; i < last_basic_block_for_fn (cfun); i++)
    bitmap_initialize (&livein[i], &bitmap_default_obstack);

  auto_bitmap refs (&bitmap_default_obstack);
  df_get_exit_block_use_set (refs);

  unsigned i;
  bitmap_iterator bi;
  EXECUTE_IF_SET_IN_BITMAP (refs, 0, i, bi)
    make_reg_live (&livein[EXIT_BLOCK], i);

  livenow = BITMAP_ALLOC (NULL);
  all_blocks = BITMAP_ALLOC (NULL);
  changed_pseudos = BITMAP_ALLOC (NULL);

  for (int i = 0; i < last_basic_block_for_fn (cfun); i++)
    if (i != ENTRY_BLOCK && i != EXIT_BLOCK)
      bitmap_set_bit (all_blocks, i);

  modify = false;
}

/* Finalization of the ext-dce pass.  Primarily this means
   releasing up the various bitmaps we utilize.  */

static void
ext_dce_finish (void)
{
  for (unsigned i = 0; i < livein.length (); i++)
    bitmap_clear (&livein[i]);
  livein.release ();

  BITMAP_FREE (livenow);
  BITMAP_FREE (changed_pseudos);
  BITMAP_FREE (all_blocks);
}

/* Process block number BB_INDEX as part of the backward
   simple dataflow analysis.  Return TRUE if something in
   this block changed or FALSE otherwise.  */

static bool
ext_dce_rd_transfer_n (int bb_index)
{
  /* The ENTRY/EXIT blocks never change.  */
  if (bb_index == ENTRY_BLOCK || bb_index == EXIT_BLOCK)
    return false;

  basic_block bb = BASIC_BLOCK_FOR_FN (cfun, bb_index);

  /* Make everything live that's live in the successors.  */
  bitmap_clear (livenow);
  edge_iterator ei;
  edge e;

  FOR_EACH_EDGE (e, ei, bb->succs)
    bitmap_ior_into (livenow, &livein[e->dest->index]);

  ext_dce_process_bb (bb);

  /* We may have narrowed the set of live objects at the start
     of this block.  If so, update the bitmaps and indicate to
     the generic dataflow code that something changed.  */
  if (!bitmap_equal_p (&livein[bb_index], livenow))
    {
      bitmap_copy (&livein[bb_index], livenow);
      return true;
    }

  return false;
}

/* Dummy function for the df_simple_dataflow API.  */
static bool ext_dce_rd_confluence_n (edge) { return true; }

/* Use lifetime analyis to identify extensions that set bits that
   are never read.  Turn such extensions into SUBREGs instead which
   can often be propagated away.  */

void
ext_dce_execute (void)
{
  /* Limit the amount of memory we use for livein, with 4 bits per
     reg per basic-block including overhead that maps to one byte
     per reg per basic-block.  */
  uint64_t memory_request
    = (uint64_t)n_basic_blocks_for_fn (cfun) * max_reg_num ();
  if (memory_request / 1024 > (uint64_t)param_max_gcse_memory)
    {
      warning (OPT_Wdisabled_optimization,
               "ext-dce disabled: %d basic blocks and %d registers; "
               "increase %<--param max-gcse-memory%> above %wu",
               n_basic_blocks_for_fn (cfun), max_reg_num (),
               memory_request / 1024);
      return;
    }

  /* Some settings of SUBREG_PROMOTED_VAR_P are actively harmful
     to this pass.  Clear it for those cases.  */
  maybe_clear_subreg_promoted_p ();
  df_analyze ();
  ext_dce_init ();

  do
    {
      df_simple_dataflow (DF_BACKWARD, NULL, NULL,
                          ext_dce_rd_confluence_n, ext_dce_rd_transfer_n,
                          all_blocks, df_get_postorder (DF_BACKWARD),
                          df_get_n_blocks (DF_BACKWARD));
      modify = !modify;
    }
  while (modify);

  reset_subreg_promoted_p ();

  ext_dce_finish ();
}


namespace {

const pass_data pass_data_ext_dce =
{
  RTL_PASS, /* type */
  "ext_dce", /* name */
  OPTGROUP_NONE, /* optinfo_flags */
  TV_EXT_DCE, /* tv_id */
  PROP_cfglayout, /* properties_required */
  0, /* properties_provided */
  0, /* properties_destroyed */
  0, /* todo_flags_start */
  TODO_df_finish, /* todo_flags_finish */
};

class pass_ext_dce : public rtl_opt_pass
{
public:
  pass_ext_dce (gcc::context *ctxt)
    : rtl_opt_pass (pass_data_ext_dce, ctxt)
  {}

  /* opt_pass methods: */
  virtual bool gate (function *) { return flag_ext_dce && optimize > 0; }
  virtual unsigned int execute (function *)
    {
      ext_dce_execute ();
      return 0;
    }

}; // class pass_combine

} // anon namespace

rtl_opt_pass *
make_pass_ext_dce (gcc::context *ctxt)
{
  return new pass_ext_dce (ctxt);
}