Manolis Tsamis <manolis.tsa...@vrull.eu> writes:
> On Thu, Nov 23, 2023 at 11:01 PM Richard Sandiford
> <richard.sandif...@arm.com> wrote:
>>
>> Manolis Tsamis <manolis.tsa...@vrull.eu> writes:
>> > The existing implementation of need_cmov_or_rewire and
>> > noce_convert_multiple_sets_1 assumes that sets are either REG or SUBREG.
>> > This commit enchances them so they can handle/rewire arbitrary set
>> > statements.
>> >
>> > To do that a new helper struct noce_multiple_sets_info is introduced which
>> > is
>> > used by noce_convert_multiple_sets and its helper functions. This results
>> > in
>> > cleaner function signatures, improved efficientcy (a number of vecs and
>> > hash
>> > set/map are replaced with a single vec of struct) and simplicity.
>> >
>> > gcc/ChangeLog:
>> >
>> > * ifcvt.cc (need_cmov_or_rewire): Renamed
>> > init_noce_multiple_sets_info.
>> > (init_noce_multiple_sets_info): Initialize noce_multiple_sets_info.
>> > (noce_convert_multiple_sets_1): Use noce_multiple_sets_info and
>> > handle
>> > rewiring of multiple registers.
>> > (noce_convert_multiple_sets): Updated to use noce_multiple_sets_info.
>> > * ifcvt.h (struct noce_multiple_sets_info): Introduce new struct
>> > noce_multiple_sets_info to store info for noce_convert_multiple_sets.
>> >
>> > Signed-off-by: Manolis Tsamis <manolis.tsa...@vrull.eu>
>> > ---
>>
>> Thanks, this looks like a really nice clean-up. One comment below:
>>
> Thanks!
>
>> >
>> > Changes in v2:
>> > - Made standalone patch.
>> > - Better comments, some more checks.
>> >
>> > gcc/ifcvt.cc | 252 +++++++++++++++++++++++----------------------------
>> > gcc/ifcvt.h | 16 ++++
>> > 2 files changed, 129 insertions(+), 139 deletions(-)
>> >
>> > diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
>> > index a0af553b9ff..9486d54de34 100644
>> > --- a/gcc/ifcvt.cc
>> > +++ b/gcc/ifcvt.cc
>> > @@ -98,14 +98,10 @@ static bool dead_or_predicable (basic_block,
>> > basic_block, basic_block,
>> > edge, bool);
>> > 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> *);
>> > +static void init_noce_multiple_sets_info (basic_block,
>> > + auto_delete_vec<noce_multiple_sets_info> &);
>> > static bool noce_convert_multiple_sets_1 (struct noce_if_info *,
>> > - hash_set<rtx_insn *> *,
>> > - hash_map<rtx_insn *, int> *,
>> > - auto_vec<rtx> *,
>> > - auto_vec<rtx> *,
>> > - auto_vec<rtx_insn *> *, int *);
>> > + auto_delete_vec<noce_multiple_sets_info> &, int *);
>> >
>> > /* Count the number of non-jump active insns in BB. */
>> >
>> > @@ -3270,24 +3266,13 @@ noce_convert_multiple_sets (struct noce_if_info
>> > *if_info)
>> > rtx x = XEXP (cond, 0);
>> > rtx y = XEXP (cond, 1);
>> >
>> > - /* The true targets for a conditional move. */
>> > - auto_vec<rtx> targets;
>> > - /* The temporaries introduced to allow us to not consider register
>> > - overlap. */
>> > - auto_vec<rtx> temporaries;
>> > - /* The insns we've emitted. */
>> > - auto_vec<rtx_insn *> unmodified_insns;
>> > -
>> > - 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);
>> > + auto_delete_vec<noce_multiple_sets_info> insn_info;
>> > + init_noce_multiple_sets_info (then_bb, insn_info);
>> >
>> > int last_needs_comparison = -1;
>> >
>> > bool ok = noce_convert_multiple_sets_1
>> > - (if_info, &need_no_cmov, &rewired_src, &targets, &temporaries,
>> > - &unmodified_insns, &last_needs_comparison);
>> > + (if_info, insn_info, &last_needs_comparison);
>> > if (!ok)
>> > return false;
>> >
>> > @@ -3302,8 +3287,7 @@ noce_convert_multiple_sets (struct noce_if_info
>> > *if_info)
>> > end_sequence ();
>> > start_sequence ();
>> > ok = noce_convert_multiple_sets_1
>> > - (if_info, &need_no_cmov, &rewired_src, &targets, &temporaries,
>> > - &unmodified_insns, &last_needs_comparison);
>> > + (if_info, insn_info, &last_needs_comparison);
>> > /* Actually we should not fail anymore if we reached here,
>> > but better still check. */
>> > if (!ok)
>> > @@ -3312,12 +3296,12 @@ noce_convert_multiple_sets (struct noce_if_info
>> > *if_info)
>> >
>> > /* We must have seen some sort of insn to insert, otherwise we were
>> > given an empty BB to convert, and we can't handle that. */
>> > - gcc_assert (!unmodified_insns.is_empty ());
>> > + gcc_assert (!insn_info.is_empty ());
>> >
>> > /* Now fixup the assignments. */
>> > - for (unsigned i = 0; i < targets.length (); i++)
>> > - if (targets[i] != temporaries[i])
>> > - noce_emit_move_insn (targets[i], temporaries[i]);
>> > + for (unsigned i = 0; i < insn_info.length (); i++)
>> > + if (insn_info[i]->target != insn_info[i]->temporary)
>> > + noce_emit_move_insn (insn_info[i]->target, insn_info[i]->temporary);
>> >
>> > /* Actually emit the sequence if it isn't too expensive. */
>> > rtx_insn *seq = get_insns ();
>> > @@ -3332,10 +3316,10 @@ noce_convert_multiple_sets (struct noce_if_info
>> > *if_info)
>> > set_used_flags (insn);
>> >
>> > /* Mark all our temporaries and targets as used. */
>> > - for (unsigned i = 0; i < targets.length (); i++)
>> > + for (unsigned i = 0; i < insn_info.length (); i++)
>> > {
>> > - set_used_flags (temporaries[i]);
>> > - set_used_flags (targets[i]);
>> > + set_used_flags (insn_info[i]->temporary);
>> > + set_used_flags (insn_info[i]->target);
>> > }
>> >
>> > set_used_flags (cond);
>> > @@ -3354,7 +3338,7 @@ noce_convert_multiple_sets (struct noce_if_info
>> > *if_info)
>> > return false;
>> >
>> > emit_insn_before_setloc (seq, if_info->jump,
>> > - INSN_LOCATION (unmodified_insns.last ()));
>> > + INSN_LOCATION (insn_info.last
>> > ()->unmodified_insn));
>> >
>> > /* Clean up THEN_BB and the edges in and out of it. */
>> > remove_edge (find_edge (test_bb, join_bb));
>> > @@ -3389,21 +3373,13 @@ check_for_cc_cmp_clobbers (rtx dest, const_rtx,
>> > void *p0)
>> > p[0] = NULL_RTX;
>> > }
>> >
>> > -/* This goes through all relevant insns of IF_INFO->then_bb and tries to
>> > - create conditional moves. In case a simple move sufficis the insn
>> > - should be listed in NEED_NO_CMOV. The rewired-src cases should be
>> > - specified via REWIRED_SRC. TARGETS, TEMPORARIES and UNMODIFIED_INSNS
>> > - are specified and used in noce_convert_multiple_sets and should be
>> > passed
>> > - to this function.. */
>> > +/* This goes through all relevant insns of IF_INFO->then_bb and tries to
>> > create
>> > + conditional moves. Information for the insns is kept in INSN_INFO. */
>> >
>> > static bool
>> > noce_convert_multiple_sets_1 (struct noce_if_info *if_info,
>> > - hash_set<rtx_insn *> *need_no_cmov,
>> > - hash_map<rtx_insn *, int> *rewired_src,
>> > - auto_vec<rtx> *targets,
>> > - auto_vec<rtx> *temporaries,
>> > - auto_vec<rtx_insn *> *unmodified_insns,
>> > - int *last_needs_comparison)
>> > + auto_delete_vec<noce_multiple_sets_info> &insn_info,
>> > + int *last_needs_comparison)
>> > {
>> > basic_block then_bb = if_info->then_bb;
>> > rtx_insn *jump = if_info->jump;
>> > @@ -3421,11 +3397,6 @@ noce_convert_multiple_sets_1 (struct noce_if_info
>> > *if_info,
>> >
>> > rtx_insn *insn;
>> > int count = 0;
>> > -
>> > - targets->truncate (0);
>> > - temporaries->truncate (0);
>> > - unmodified_insns->truncate (0);
>> > -
>> > bool second_try = *last_needs_comparison != -1;
>> >
>> > FOR_BB_INSNS (then_bb, insn)
>> > @@ -3434,6 +3405,8 @@ noce_convert_multiple_sets_1 (struct noce_if_info
>> > *if_info,
>> > if (!active_insn_p (insn))
>> > continue;
>> >
>> > + noce_multiple_sets_info *info = insn_info[count];
>> > +
>> > rtx set = single_set (insn);
>> > gcc_checking_assert (set);
>> >
>> > @@ -3441,9 +3414,12 @@ noce_convert_multiple_sets_1 (struct noce_if_info
>> > *if_info,
>> > 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]);
>> > +
>> > + int i, ii;
>> > + FOR_EACH_VEC_ELT (info->rewired_src, i, ii)
>> > + new_val = simplify_replace_rtx (new_val, insn_info[ii]->target,
>> > + insn_info[ii]->temporary);
>> > +
>> > rtx old_val = target;
>> >
>> > /* As we are transforming
>> > @@ -3484,7 +3460,7 @@ noce_convert_multiple_sets_1 (struct noce_if_info
>> > *if_info,
>> > /* 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);
>> > + bool need_cmov = info->need_cmov;
>> >
>> > /* If we had a non-canonical conditional jump (i.e. one where
>> > the fallthrough is to the "else" case) we need to reverse
>> > @@ -3632,21 +3608,102 @@ noce_convert_multiple_sets_1 (struct noce_if_info
>> > *if_info,
>> >
>> > /* Bookkeeping. */
>> > count++;
>> > - targets->safe_push (target);
>> > - temporaries->safe_push (temp_dest);
>> > - unmodified_insns->safe_push (insn);
>> > +
>> > + info->target = target;
>> > + info->temporary = temp_dest;
>> > + info->unmodified_insn = insn;
>> > }
>> >
>> > + /* The number of processed insns must match
>> > init_noce_multiple_sets_info. */
>> > + gcc_assert (count == (int) insn_info.length ());
>> > +
>> > /* Even if we did not actually need the comparison, we want to make sure
>> > to try a second time in order to get rid of the temporaries. */
>> > if (*last_needs_comparison == -1)
>> > *last_needs_comparison = 0;
>> >
>> > -
>> > 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
>> > +init_noce_multiple_sets_info (basic_block bb,
>> > + auto_delete_vec<noce_multiple_sets_info> &insn_info)
>> > +{
>> > + rtx_insn *insn;
>> > + int count = 0;
>> > + 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)
>> > + {
>> > + if (!active_insn_p (insn))
>> > + continue;
>> > +
>> > + noce_multiple_sets_info *info = new noce_multiple_sets_info;
>> > + info->target = NULL_RTX;
>> > + info->temporary = NULL_RTX;
>> > + info->unmodified_insn = NULL;
>> > + info->need_cmov = true;
>> > + insn_info.safe_push (info);
>> > +
>> > + rtx set = single_set (insn);
>> > + gcc_checking_assert (set);
>> > +
>> > + rtx src = SET_SRC (set);
>> > + rtx 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. */
>> > + for (int i = count - 1; i >= 0; --i)
>> > + if (reg_mentioned_p (dests[i], src))
>> > + {
>> > + if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
>>
>> This is pre-existing, but is a REG_DEAD note enough? I think we also
>> need to check that the register isn't live on exit from the condition
>> block (or, alternatively, isn't live on entry to the join block).
>>
>> E.g. for:
>>
>> if (cond)
>> {
>> tmp = a;
>> a = b;
>> b = tmp;
>> tmp = ...;
>> }
>> ...tmp...;
>>
>> we need to preserve the original value of tmp when !cond, even though
>> tmp is temporarily dead in the "then" block.
>>
>> Thanks,
>> Richard
>>
> Good question. I played around with some tests and looked at the
> original code again and I believe it's fine.
> In your example, when insn is at `b = tmp;` then `tmp = a;` will be
> marked as need_no_cmov because tmp will have a REG_DEAD at insn. `tmp
> = ...;` will be emitted as a conditional set and the use outside the
> conditional will be just fine. I don't see any issue.
But I think we end up with:
tmp = a;
a = cond ? b : a;
b = cond ? tmp : b;
tmp = cond ? ... : tmp;
So the net effect for !cond will be to set tmp to a, rather than
preserve the original value of tmp.
Thanks,
Richard
>
> But it made me realize that my updated code doesn't really make sense
> on that part. Since I allow src to be an arbitrary expression and not
> a single reg, the code
>
> if (find_reg_note (insn, REG_DEAD, src) != NULL_RTX)
>
> is nonsensical. Apart from the fact that the check is loop invariant
> it also doesn't properly handle src anyway. I'll have to think again
> about how to refactor this check now that src is not just a reg.
>
> Thanks,
> Manolis
