#include "expr.h"
#include "recog.h"
#include "dse.h"
+#include "optabs.h"
#include "dbgcnt.h"
/* This file contains three techniques for performing Dead Store
}
+/* If the modes are different and the value's source and target do not
+ line up, we need to extract the value from lower part of the rhs of
+ the store, shift it, and then put it into a form that can be shoved
+ into the read_insn. This function generates a right SHIFT of a
+ value that is at least ACCESS_SIZE bytes wide of READ_MODE. The
+ shift sequence is returned or NULL if we failed to find a
+ shift. */
+
+static rtx
+find_shift_sequence (rtx read_reg,
+ int access_size,
+ store_info_t store_info,
+ read_info_t read_info,
+ int shift)
+{
+ enum machine_mode store_mode = GET_MODE (store_info->mem);
+ enum machine_mode read_mode = GET_MODE (read_info->mem);
+
+ /* Some machines like the x86 have shift insns for each size of
+ operand. Other machines like the ppc or the ia-64 may only have
+ shift insns that shift values within 32 or 64 bit registers.
+ This loop tries to find the smallest shift insn that will right
+ justify the value we want to read but is available in one insn on
+ the machine. */
+
+ while (access_size < UNITS_PER_WORD)
+ {
+ rtx target;
+ enum machine_mode new_mode
+ = smallest_mode_for_size (access_size * BITS_PER_UNIT,
+ GET_MODE_CLASS (read_mode));
+ rtx new_reg = gen_reg_rtx (new_mode);
+
+ start_sequence ();
+
+ /* In theory we could also check for an ashr. Ian Taylor knows
+ of one dsp where the cost of these two was not the same. But
+ this really is a rare case anyway. */
+ target = expand_binop (new_mode, lshr_optab, new_reg,
+ GEN_INT (shift), new_reg, 1, OPTAB_DIRECT);
+
+ if (target == new_reg)
+ {
+ rtx shift_seq = get_insns ();
+ end_sequence ();
+
+ /* If cost is too great, set target to NULL and
+ let the iteration happen. */
+ if (shift_seq != NULL)
+ {
+ int cost = 0;
+ rtx insn;
+
+ for (insn = shift_seq; insn != NULL_RTX; insn = NEXT_INSN (insn))
+ cost += insn_rtx_cost (insn);
+
+ /* The computation up to here is essentially independent
+ of the arguments and could be precomputed. It may
+ not be worth doing so. We could precompute if
+ worthwhile or at least cache the results. The result
+ technically depends on SHIFT, ACCESS_SIZE, and
+ GET_MODE_CLASS (READ_MODE). But in practice the
+ answer will depend only on ACCESS_SIZE. */
+
+ if (cost <= COSTS_N_INSNS (1))
+ {
+ /* We found an acceptable shift. Generate a move to
+ take the value from the store and put it into the
+ shift pseudo, then shift it, then generate another
+ move to put in into the target of the read. */
+ start_sequence ();
+ emit_move_insn (new_reg, gen_lowpart (new_mode, store_info->rhs));
+ emit_insn (shift_seq);
+ emit_move_insn (read_reg, gen_lowpart (read_mode, new_reg));
+
+ if (dump_file)
+ {
+ fprintf (dump_file, " -- adding extract insn r%d:%s = r%d:%s\n",
+ REGNO (new_reg), GET_MODE_NAME (new_mode),
+ REGNO (store_info->rhs), GET_MODE_NAME (store_mode));
+
+ fprintf (dump_file, " -- with shift of r%d by %d\n",
+ REGNO(new_reg), shift);
+ fprintf (dump_file, " -- and second extract insn r%d:%s = r%d:%s\n",
+ REGNO (read_reg), GET_MODE_NAME (read_mode),
+ REGNO (new_reg), GET_MODE_NAME (new_mode));
+ }
+
+ /* Get the three insn sequence and return it. */
+ shift_seq = get_insns ();
+ end_sequence ();
+ return shift_seq;
+ }
+ }
+ }
+ else
+ /* End the sequence. */
+ end_sequence ();
+
+ access_size = access_size * 2;
+ }
+
+ return NULL;
+}
+
+
/* Take a sequence of:
A <- r1
...
...
... <- r2
- The STORE_INFO and STORE_INFO are for the store and the READ_INFO
+ or
+
+ r3 <- extract (r1)
+ r3 <- r3 >> shift
+ r2 <- extract (r3)
+ ... <- r2
+
+ or
+
+ r2 <- extract (r1)
+ ... <- r2
+
+ Depending on the alignment and the mode of the store and
+ subsequent load.
+
+
+ The STORE_INFO and STORE_INSN are for the store and READ_INFO
and READ_INSN are for the read. Return true if the replacement
went ok. */
replace_read (store_info_t store_info, insn_info_t store_insn,
read_info_t read_info, insn_info_t read_insn, rtx *loc)
{
+ enum machine_mode store_mode = GET_MODE (store_info->mem);
+ enum machine_mode read_mode = GET_MODE (read_info->mem);
+ int shift;
+ int access_size; /* In bytes. */
+ rtx read_reg = gen_reg_rtx (read_mode);
+ rtx shift_seq = NULL;
+
if (!dbg_cnt (dse))
return false;
+ if (GET_MODE_CLASS (read_mode) != GET_MODE_CLASS (store_mode))
+ return false;
+
+ /* To get here the read is within the boundaries of the write so
+ shift will never be negative. Start out with the shift being in
+ bytes. */
+ if (BYTES_BIG_ENDIAN)
+ shift = store_info->end - read_info->end;
+ else
+ shift = read_info->begin - store_info->begin;
+
+ access_size = shift + GET_MODE_SIZE (read_mode);
+
+ /* From now on it is bits. */
+ shift *= BITS_PER_UNIT;
+
+ /* We need to keep this in perspective. We are replacing a read
+ with a sequence of insns, but the read will almost certainly be
+ in cache, so it is not going to be an expensive one. Thus, we
+ are not willing to do a multi insn shift or worse a subroutine
+ call to get rid of the read. */
+ if (shift)
+ {
+ if (access_size > UNITS_PER_WORD || FLOAT_MODE_P (store_mode))
+ return false;
+
+ shift_seq = find_shift_sequence (read_reg, access_size, store_info,
+ read_info, shift);
+ if (!shift_seq)
+ return false;
+ }
+
if (dump_file)
- fprintf (dump_file, "generating move to replace load at %d from store at %d\n",
+ fprintf (dump_file, "replacing load at %d from store at %d\n",
INSN_UID (read_insn->insn), INSN_UID (store_insn->insn));
- if (GET_MODE (store_info->mem) == GET_MODE (read_info->mem))
+
+ if (validate_change (read_insn->insn, loc, read_reg, 0))
{
- rtx new_reg = gen_reg_rtx (GET_MODE (store_info->mem));
- if (validate_change (read_insn->insn, loc, new_reg, 0))
+ rtx insns;
+ deferred_change_t deferred_change = pool_alloc (deferred_change_pool);
+
+ if (read_mode == store_mode)
{
- rtx insns;
- deferred_change_t deferred_change = pool_alloc (deferred_change_pool);
-
start_sequence ();
- emit_move_insn (new_reg, store_info->rhs);
+
+ /* The modes are the same and everything lines up. Just
+ generate a simple move. */
+ emit_move_insn (read_reg, store_info->rhs);
+ if (dump_file)
+ fprintf (dump_file, " -- adding move insn r%d = r%d\n",
+ REGNO (read_reg), REGNO (store_info->rhs));
insns = get_insns ();
end_sequence ();
- emit_insn_before (insns, store_insn->insn);
-
- if (dump_file)
- fprintf (dump_file, " -- adding move insn %d: r%d = r%d\n",
- INSN_UID (insns), REGNO (new_reg), REGNO (store_info->rhs));
-
- /* And now for the cludge part: cselib croaks if you just
- return at this point. There are two reasons for this:
-
- 1) Cselib has an idea of how many pseudos there are and
- that does not include the new one we just added.
-
- 2) Cselib does not know about the move insn we added
- above the store_info, and there is no way to tell it
- about it, because it has "moved on".
-
- So we are just going to have to lie. The move insn is
- not really an issue, cselib did not see it. But the use
- of the new pseudo read_insn is a real problem. The way
- that we solve this problem is that we are just going to
- put the mem back keep a table of mems to get rid of. At
- the end of the basic block we can put it back. */
-
- *loc = read_info->mem;
- deferred_change->next = deferred_change_list;
- deferred_change_list = deferred_change;
- deferred_change->loc = loc;
- deferred_change->reg = new_reg;
-
- /* Get rid of the read_info, from the point of view of the
- rest of dse, play like this read never happened. */
- read_insn->read_rec = read_info->next;
- pool_free (read_info_pool, read_info);
- return true;
}
- else
+ else if (shift)
+ insns = shift_seq;
+ else
{
+ /* The modes are different but the lsb are in the same
+ place, we need to extract the value in the right from the
+ rhs of the store. */
+ start_sequence ();
+ emit_move_insn (read_reg, gen_lowpart (read_mode, store_info->rhs));
+
if (dump_file)
- fprintf (dump_file, " -- validation failure\n");
- return false;
+ fprintf (dump_file, " -- adding extract insn r%d:%s = r%d:%s\n",
+ REGNO (read_reg), GET_MODE_NAME (read_mode),
+ REGNO (store_info->rhs), GET_MODE_NAME (store_mode));
+ insns = get_insns ();
+ end_sequence ();
}
+
+ /* Insert this right before the store insn where it will be safe
+ from later insns that might change it before the read. */
+ emit_insn_before (insns, store_insn->insn);
+
+ /* And now for the kludge part: cselib croaks if you just
+ return at this point. There are two reasons for this:
+
+ 1) Cselib has an idea of how many pseudos there are and
+ that does not include the new ones we just added.
+
+ 2) Cselib does not know about the move insn we added
+ above the store_info, and there is no way to tell it
+ about it, because it has "moved on".
+
+ Problem (1) is fixable with a certain amount of engineering.
+ Problem (2) is requires starting the bb from scratch. This
+ could be expensive.
+
+ So we are just going to have to lie. The move/extraction
+ insns are not really an issue, cselib did not see them. But
+ the use of the new pseudo read_insn is a real problem because
+ cselib has not scanned this insn. The way that we solve this
+ problem is that we are just going to put the mem back for now
+ and when we are finished with the block, we undo this. We
+ keep a table of mems to get rid of. At the end of the basic
+ block we can put them back. */
+
+ *loc = read_info->mem;
+ deferred_change->next = deferred_change_list;
+ deferred_change_list = deferred_change;
+ deferred_change->loc = loc;
+ deferred_change->reg = read_reg;
+
+ /* Get rid of the read_info, from the point of view of the
+ rest of dse, play like this read never happened. */
+ read_insn->read_rec = read_info->next;
+ pool_free (read_info_pool, read_info);
+ return true;
}
- else
+ else
{
- /* Someone with excellent rtl skills needs to fill this in. You
- are guaranteed that the read is of the same size or smaller
- than the store, and that the read does not hang off one of
- the ends of the store. But the offsets of each must be
- checked because the read does not have to line up on either
- end of the store so the begin fields need to be examined in
- both the store_info and read_info. */
if (dump_file)
- fprintf (dump_file, " -- complex load, currently unsupported.\n");
+ fprintf (dump_file, " -- validation failure\n");
return false;
}
}
-
/* A for_each_rtx callback in which DATA is the bb_info. Check to see
if LOC is a mem and if it is look at the address and kill any
appropriate stores that may be active. */
TODO_ggc_collect, /* todo_flags_finish */
'w' /* letter */
};
-
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