On Tue, Nov 28, 2023 at 12:12 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> 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.
>
Ok, I now see what you've meant. Then, indeed, it looks like a bug to
me (that fortunately didn't trigger?).
Since I'll have to refactor my change around REG_DEAD anyway, I can
make sure to fix this issue too.
I'll also re-try to create some test case that triggers this bug, if possible.
Thanks,
Manolis
> 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
