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ifcvt: Check if cmovs are needed.

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When if-converting multiple SETs and we encounter a swap-style idiom

  if (a > b)
    {
      tmp = c;   // [1]
      c = d;
      d = tmp;
    }

ifcvt should not generate a conditional move for the instruction at
[1].

In order to achieve that, this patch goes through all relevant SETs
and marks the relevant instructions.  This helps to evaluate costs.

On top, only generate temporaries if the current cmov is going to
overwrite one of the comparands of the initial compare.

gcc/ChangeLog:

	* ifcvt.cc (need_cmov_or_rewire): New function.
	(noce_convert_multiple_sets): Call it.
This commit is contained in:
Robin Dapp 2022-01-19 17:36:27 +01:00
parent d5148d4faa
commit 4c46f5a471
1 changed files with 150 additions and 24 deletions
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158
gcc/ifcvt.cc
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@ -98,6 +98,8 @@ static int dead_or_predicable (basic_block, basic_block, basic_block,
edge, int);
static void noce_emit_move_insn (rtx, rtx);
static rtx_insn *block_has_only_trap (basic_block);
static void need_cmov_or_rewire (basic_block, hash_set<rtx_insn *> *,
hash_map<rtx_insn *, int> *);
/* Count the number of non-jump active insns in BB. */
@ -3209,6 +3211,11 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
auto_vec<rtx_insn *> unmodified_insns;
int count = 0;
hash_set<rtx_insn *> need_no_cmov;
hash_map<rtx_insn *, int> rewired_src;
need_cmov_or_rewire (then_bb, &need_no_cmov, &rewired_src);
FOR_BB_INSNS (then_bb, insn)
{
/* Skip over non-insns. */
@ -3219,26 +3226,49 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
gcc_checking_assert (set);
rtx target = SET_DEST (set);
rtx temp = gen_reg_rtx (GET_MODE (target));
rtx temp;
rtx new_val = SET_SRC (set);
if (int *ii = rewired_src.get (insn))
new_val = simplify_replace_rtx (new_val, targets[*ii],
temporaries[*ii]);
rtx old_val = target;
/* If we were supposed to read from an earlier write in this block,
we've changed the register allocation. Rewire the read. While
we are looking, also try to catch a swap idiom. */
for (int i = count - 1; i >= 0; --i)
if (reg_overlap_mentioned_p (new_val, targets[i]))
/* As we are transforming
if (x > y)
{
/* Catch a "swap" style idiom. */
if (find_reg_note (insn, REG_DEAD, new_val) != NULL_RTX)
/* The write to targets[i] is only live until the read
here. As the condition codes match, we can propagate
the set to here. */
new_val = SET_SRC (single_set (unmodified_insns[i]));
else
new_val = temporaries[i];
break;
a = b;
c = d;
}
into
a = (x > y) ...
c = (x > y) ...
we potentially check x > y before every set.
Even though the check might be removed by subsequent passes, this means
that we cannot transform
if (x > y)
{
x = y;
...
}
into
x = (x > y) ...
...
since this would invalidate x and the following to-be-removed checks.
Therefore we introduce a temporary every time we are about to
overwrite a variable used in the check. Costing of a sequence with
these is going to be inaccurate so only use temporaries when
needed. */
if (reg_overlap_mentioned_p (target, cond))
temp = gen_reg_rtx (GET_MODE (target));
else
temp = target;
/* We have identified swap-style idioms before. A normal
set will need to be a cmov while the first instruction of a swap-style
idiom can be a regular move. This helps with costing. */
bool need_cmov = !need_no_cmov.contains (insn);
/* If we had a non-canonical conditional jump (i.e. one where
the fallthrough is to the "else" case) we need to reverse
@ -3281,8 +3311,12 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
old_val = lowpart_subreg (dst_mode, old_val, src_mode);
}
rtx temp_dest = NULL_RTX;
if (need_cmov)
{
/* Actually emit the conditional move. */
rtx temp_dest = noce_emit_cmove (if_info, temp, cond_code,
temp_dest = noce_emit_cmove (if_info, temp, cond_code,
x, y, new_val, old_val);
/* If we failed to expand the conditional move, drop out and don't
@ -3292,6 +3326,15 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
end_sequence ();
return FALSE;
}
}
else
{
if (if_info->then_else_reversed)
noce_emit_move_insn (temp, old_val);
else
noce_emit_move_insn (temp, new_val);
temp_dest = temp;
}
/* Bookkeeping. */
count++;
@ -3814,6 +3857,89 @@ check_cond_move_block (basic_block bb,
return TRUE;
}
/* Find local swap-style idioms in BB and mark the first insn (1)
that is only a temporary as not needing a conditional move as
it is going to be dead afterwards anyway.
(1) int tmp = a;
a = b;
b = tmp;
ifcvt
-->
tmp = a;
a = cond ? b : a_old;
b = cond ? tmp : b_old;
Additionally, store the index of insns like (2) when a subsequent
SET reads from their destination.
(2) int c = a;
int d = c;
ifcvt
-->
c = cond ? a : c_old;
d = cond ? d : c; // Need to use c rather than c_old here.
*/
static void
need_cmov_or_rewire (basic_block bb,
hash_set<rtx_insn *> *need_no_cmov,
hash_map<rtx_insn *, int> *rewired_src)
{
rtx_insn *insn;
int count = 0;
auto_vec<rtx_insn *> insns;
auto_vec<rtx> dests;
/* Iterate over all SETs, storing the destinations
in DEST.
- If we hit a SET that reads from a destination
that we have seen before and the corresponding register
is dead afterwards, the register does not need to be
moved conditionally.
- If we encounter a previously changed register,
rewire the read to the original source. */
FOR_BB_INSNS (bb, insn)
{
rtx set, src, dest;
if (!active_insn_p (insn))
continue;
set = single_set (insn);
if (set == NULL_RTX)
continue;
src = SET_SRC (set);
if (SUBREG_P (src))
src = SUBREG_REG (src);
dest = SET_DEST (set);
/* Check if the current SET's source is the same
as any previously seen destination.
This is quadratic but the number of insns in BB
is bounded by PARAM_MAX_RTL_IF_CONVERSION_INSNS. */
if (REG_P (src))
for (int i = count - 1; i >= 0; --i)
if (reg_overlap_mentioned_p (src, dests[i]))
{
if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
need_no_cmov->add (insns[i]);
else
rewired_src->put (insn, i);
}
insns.safe_push (insn);
dests.safe_push (dest);
count++;
}
}
/* Given a basic block BB suitable for conditional move conversion,
a condition COND, and pointer maps THEN_VALS and ELSE_VALS containing
the register values depending on COND, emit the insns in the block as