On Tue, 13 Jul 2021, guojiufu wrote:
> On 2021-07-12 23:53, guojiufu via Gcc-patches wrote:
> > On 2021-07-12 22:46, Richard Biener wrote:
> >> On Mon, 12 Jul 2021, guojiufu wrote:
> >>
> >>> On 2021-07-12 18:02, Richard Biener wrote:
> >>> > On Mon, 12 Jul 2021, guojiufu wrote:
> >>> >
> >>> >> On 2021-07-12 16:57, Richard Biener wrote:
> >>> >> > On Mon, 12 Jul 2021, guojiufu wrote:
> >>> >> >
> >>> >> >> On 2021-07-12 14:20, Richard Biener wrote:
> >>> >> >> > On Fri, 9 Jul 2021, Segher Boessenkool wrote:
> >>> >> >> >
> >>> >> >> >> On Fri, Jul 09, 2021 at 08:43:59AM +0200, Richard Biener wrote:
> >>> >> >> >> > I wonder if there's a way to query the target what modes the
> >>> >> >> >> > doloop
> >>> >> >> >> > pattern can handle (not being too familiar with the doloop
> >>> >> >> >> > code).
> >>> >> >> >>
> >>> >> >> >> You can look what modes are allowed for operand 0 of doloop_end,
> >>> >> >> >> perhaps? Although that is a define_expand, not a define_insn, so
> >>> >> >> >> it
> >>> >> >> >> is
> >>> >> >> >> hard to introspect.
> >>> >> >> >>
> >>> >> >> >> > Why do you need to do any checks besides the new type being
> >>> >> >> >> > able to
> >>> >> >> >> > represent all IV values? The original doloop IV will never
> >>> >> >> >> > wrap
> >>> >> >> >> > (OTOH if niter is U*_MAX then we compute niter + 1 which will
> >>> >> >> >> > become
> >>> >> >> >> > zero ... I suppose the doloop might still do the correct thing
> >>> >> >> >> > here
> >>> >> >> >> > but it also still will with a IV with larger type).
> >>> >> >>
> >>> >> >> The issue comes from U*_MAX (original short MAX), as you said: on
> >>> >> >> which
> >>> >> >> niter + 1 becomes zero. And because the step for doloop is -1;
> >>> >> >> then, on
> >>> >> >> larger type 'zero - 1' will be a very large number on larger type
> >>> >> >> (e.g. 0xff...ff); but on the original short type 'zero - 1' is a
> >>> >> >> small
> >>> >> >> value
> >>> >> >> (e.g. "0xff").
> >>> >> >
> >>> >> > But for the larger type the small type MAX + 1 fits and does not
> >>> >> > yield
> >>> >> > zero so it should still work exactly as before, no? Of course you
> >>> >> > have to compute the + 1 in the larger type.
> >>> >> >
> >>> >> You are right, if compute the "+ 1" in the larger type it is ok, as
> >>> >> below
> >>> >> code:
> >>> >> ```
> >>> >> /* Use type in word size may fast. */
> >>> >> if (TYPE_PRECISION (ntype) < BITS_PER_WORD)
> >>> >> {
> >>> >> ntype = lang_hooks.types.type_for_size (BITS_PER_WORD, 1);
> >>> >> niter = fold_convert (ntype, niter);
> >>> >> }
> >>> >>
> >>> >> tree base = fold_build2 (PLUS_EXPR, ntype, unshare_expr (niter),
> >>> >> build_int_cst (ntype, 1));
> >>> >>
> >>> >>
> >>> >> add_candidate (data, base, build_int_cst (ntype, -1), true, NULL,
> >>> >> NULL,
> >>> >> true);
> >>> >> ```
> >>> >> The issue of this is, this code generates more stmt for doloop.xxx:
> >>> >> _12 = (unsigned int) xx(D);
> >>> >> _10 = _12 + 4294967295;
> >>> >> _24 = (long unsigned int) _10;
> >>> >> doloop.6_8 = _24 + 1;
> >>> >>
> >>> >> if use previous patch, "+ 1" on original type, then the stmts will
> >>> >> looks
> >>> >> like:
> >>> >> _12 = (unsigned int) xx(D);
> >>> >> doloop.6_8 = (long unsigned int) _12;
> >>> >>
> >>> >> This is the reason for checking
> >>> >> wi::ltu_p (niter_desc->max, wi::to_widest (TYPE_MAX_VALUE (ntype)))
> >>> >
> >>> > But this then only works when there's an upper bound on the number
> >>> > of iterations. Note you should not use TYPE_MAX_VALUE here but
> >>> > you can instead use
> >>> >
> >>> > wi::ltu_p (niter_desc->max, wi::to_widest (wi::max_value
> >>> > (TYPE_PRECISION (ntype), TYPE_SIGN (ntype))));
> >>>
> >>> Ok, Thanks!
> >>> I remember you mentioned that:
> >>> widest_int::from (wi::max_value (TYPE_PRECISION (ntype), TYPE_SIGN
> >>> (ntype)),
> >>> TYPE_SIGN (ntype))
> >>> would be better than
> >>> wi::to_widest (TYPE_MAX_VALUE (ntype)).
> >>>
> >>> It seems that:
> >>> "TYPE_MAX_VALUE (ntype)" is "NUMERICAL_TYPE_CHECK
> >>> (NODE)->type_non_common.maxval"
> >>> which do a numerical-check and return the field of maxval. And then call
> >>> to
> >>> wi::to_widest
> >>>
> >>> The other code "widest_int::from (wi::max_value (..,..),..)" calls
> >>> wi::max_value
> >>> and widest_int::from.
> >>>
> >>> I'm wondering if wi::to_widest (TYPE_MAX_VALUE (ntype)) is cheaper?
> >>
> >> TYPE_MAX_VALUE can be "suprising", it does not necessarily match the
> >> underlying modes precision. At some point we've tried to eliminate
> >> most of its uses, not sure what the situation/position is right now.
> > Ok, get it, thanks.
> > I will use "widest_int::from (wi::max_value (..,..),..)".
> >
> >>
> >>> > I think the -1 above comes from number of latch iterations vs. header
> >>> > entries - it's a common source for this kind of issues. range analysis
> >>> > might be able to prove that we can still merge the two adds even with
> >>> > the intermediate extension.
> >>> Yes, as you mentioned here, it relates to number of latch iterations
> >>> For loop looks like : while (l < n) or for (i = 0; i < n; i++)
> >>> This kind of loop, the niter is used to be 'n - 1' after transformed
> >>> into 'do-while' form.
> > For this kind of loop, the max value for the number of iteration "n - 1"
> > would be "max_value_type(n) - 1" which is wi::ltu than max_value_type.
> > This kind of loop is already common, and we could use wi::ltu (max,
> > max_value_type)
> > to check.
> >
> > For loop looks like:
> > do ;
> > while (n-- > 0); /* while (n-- > low); */
> >
> > The niter_desc->max will wi::eq to max_value_type, and niter would be "n",
> > and then doloop.xx is 'n+1'.
> >
> >>> I would see how to merge these two adds safely at this point
> >>> when generating doloop iv. (maybe range info, thanks!
> >>>
>
> For some cases, it may not easy to merge -1 +1 pair:
> (from doloop-1.c)
> ```
> int __attribute__ ((noinline))
> test (unsigned char n)
> {
> do ;
> while (--n > 0);
> return n;
> }
>
> int main ()
> {
> unsigned char z = 0;
>
> return test (z);
> }
> ```
>
> For this loop, niter_desc->max is 255, and niter is 'n - 1', then
> doloop.xx is "'n - 1' + 1", 'n - 1' should be compute at uchar type
> (QImode).
> It is ok to compute '- 1' and "+ 1" under the original shorter type.
> And it is ok if computing "n - 1" under original type and computing
> "+ 1" under larger mode.
> But it is not ok to compute both "- 1" and "+ 1" in large mode.
>
> As discussed previously: when 'n - 1' is U*_MAX (original short MAX),
> '(n - 1) + 1' becomes zero. And the step for doloop is -1; then,
> on the larger type 'zero - 1' will be a very large number.
>
> It is ok: Original
> unsigned char doloop.5;
> # doloop.5_4 = PHI <n_2(D)(2), doloop.5_6(5)>
> doloop.5_6 = doloop.5_4 - 1;
>
> it would be error if:
> doloop.5_7 = (long unsigned int) n_2(D);
> # doloop.5_4 = PHI <doloop.5_7(2), doloop.5_6(5)>
> doloop.5_6 = doloop.5_4 - 1;
>
>
> >>> >
> >>> > Is this pre-loop extra add really offsetting the in-loop doloop
> >>> > improvements?
> >>> I'm not catching this question too much, sorry. I guess your concern
> >>> is if the "+1" is an offset: it may not, "+1" may be just that doloop.xx
> >>> is decreasing niter until 0 (all number >0).
> >>> If misunderstand, thanks for point out.
> >>
> >> I'm questioning the argument that not being able to eliminate the +1-1
> >> pair effects the overall cost improvement for the doloop IV type change
> >> which hopefully is _not_ just loop IV setup cost but improving
> >> performance in the loop body itself?
> > Yes, I think so. It would affect doloop IV setup cost, it may also affect
> > loop itself on some targets, if the target prefers a changed type:
> > using one jump-counter instruction for wider type, but cmp/jump instructions
> > for other types.
> > If the loop is nested in outer-loop, eliminating +1-1 would improving
> > outer-loop performance.
Yes. So we're asking targets whether using a different IV mode for
the doloop is going to improve performance for the doloop related
instructions, for other uses IVOPTs usual cost model will apply.
Now that you only add one IV candidate for doloop it's a on/off
decision here?
Still the setup cost should still make the inner loop faster, you
can keep the < TYPE_MAX check and perform the -1 adjustment in the
original type but I think it's not a reason to not perform the
doloop IV promotion if we cannot do that.
> There is a patch that could mitigate "-1 +1 pair" in rtl part.
> https://gcc.gnu.org/g:8a15faa730f99100f6f3ed12663563356ec5a2c0
>
> With that patch, one minor thing is doloop.xxx is using shorter type on some
> cases, then need subreg to access it. I have this patch is trying to use
> better
> type and avoid to use subreg.
Yes, that's understood.
Richard.
> BR,
>
> Jiufu Guo.
>
> >
> > BR,
> > Jiufu Guo.
> >
> >>
> >> Richard.
> >>
> >>> >
> >>> >> >> >>
> >>> >> >> >> doloop_valid_p guarantees it is simple and doesn't wrap.
> >>> >> >> >>
> >>> >> >> >> > I'd have expected sth like
> >>> >> >> >> >
> >>> >> >> >> > ntype = lang_hooks.types.type_for_mode (word_mode,
> >>> >> >> >> > TYPE_UNSIGNED
> >>> >> >> >> > (ntype));
> >>> >> >> >> >
> >>> >> >> >> > thus the decision made using a mode - which is also why I
> >>> >> >> >> > wonder
> >>> >> >> >> > if there's a way to query the target for this. As you say,
> >>> >> >> >> > it _may_ be fast, so better check (somehow).
> >>> >> >>
> >>> >> >>
> >>> >> >> I was also thinking of using hooks like type_for_size/type_for_mode.
> >>> >> >> /* Use type in word size may fast. */
> >>> >> >> if (TYPE_PRECISION (ntype) < BITS_PER_WORD
> >>> >> >> && Wi::ltu_p (niter_desc->max, wi::to_widest (TYPE_MAX_VALUE
> >>> >> >> (ntype))))
> >>> >> >> {
> >>> >> >> ntype = lang_hooks.types.type_for_size (BITS_PER_WORD, 1);
> >>> >> >> base = fold_convert (ntype, base);
> >>> >> >> }
> >>> >> >>
> >>> >> >> As you pointed out, this does not query the mode from targets.
> >>> >> >> As Segher pointed out "doloop_end" checks unsupported mode, while it
> >>> >> >> seems
> >>> >> >> not easy to use it in tree-ssa-loop-ivopts.c.
> >>> >> >> For implementations of doloop_end, tartgets like rs6000/aarch64/ia64
> >>> >> >> requires
> >>> >> >> Pmode/DImode; while there are other targets that work on other
> >>> >> >> 'mode'
> >>> >> >> (e.g.
> >>> >> >> SI).
> >>> >> >>
> >>> >> >>
> >>> >> >> In doloop_optimize, there is code:
> >>> >> >>
> >>> >> >> ```
> >>> >> >> mode = desc->mode;
> >>> >> >> .....
> >>> >> >> doloop_reg = gen_reg_rtx (mode);
> >>> >> >> rtx_insn *doloop_seq = targetm.gen_doloop_end (doloop_reg,
> >>> >> >> start_label);
> >>> >> >>
> >>> >> >> word_mode_size = GET_MODE_PRECISION (word_mode);
> >>> >> >> word_mode_max = (HOST_WIDE_INT_1U << (word_mode_size - 1) << 1)
> >>> >> >> -
> >>> >> >> 1;
> >>> >> >> if (! doloop_seq
> >>> >> >> && mode != word_mode
> >>> >> >> /* Before trying mode different from the one in that # of
> >>> >> >> iterations is
> >>> >> >> computed, we must be sure that the number of iterations
> >>> >> >> fits
> >>> >> >> into
> >>> >> >> the new mode. */
> >>> >> >> && (word_mode_size >= GET_MODE_PRECISION (mode)
> >>> >> >> || wi::leu_p (iterations_max, word_mode_max)))
> >>> >> >> {
> >>> >> >> if (word_mode_size > GET_MODE_PRECISION (mode))
> >>> >> >> count = simplify_gen_unary (ZERO_EXTEND, word_mode, count,
> >>> >> >> mode);
> >>> >> >> else
> >>> >> >> count = lowpart_subreg (word_mode, count, mode);
> >>> >> >> PUT_MODE (doloop_reg, word_mode);
> >>> >> >> doloop_seq = targetm.gen_doloop_end (doloop_reg,
> >>> >> >> start_label);
> >>> >> >> }
> >>> >> >> if (! doloop_seq)
> >>> >> >> {
> >>> >> >> if (dump_file)
> >>> >> >> fprintf (dump_file,
> >>> >> >> "Doloop: Target unwilling to use doloop
> >>> >> >> pattern!\n");
> >>> >> >> return false;
> >>> >> >> }
> >>> >> >> ```
> >>> >> >> The above code first tries the mode of niter_desc by call
> >>> >> >> targetm.gen_doloop_end
> >>> >> >> to see if the target can generate doloop insns, if fail, then try to
> >>> >> >> use
> >>> >> >> 'word_mode' against gen_doloop_end.
> >>> >> >>
> >>> >> >>
> >>> >> >> >>
> >>> >> >> >> Almost all targets just use Pmode, but there is no such guarantee
> >>> >> >> >> I
> >>> >> >> >> think, and esp. some targets that do not have machine insns for
> >>> >> >> >> this
> >>> >> >> >> (but want to generate different code for this anyway) can do
> >>> >> >> >> pretty
> >>> >> >> >> much
> >>> >> >> >> anything.
> >>> >> >> >>
> >>> >> >> >> Maybe using just Pmode here is good enough though?
> >>> >> >> >
> >>> >> >> > I think Pmode is a particularly bad choice and I'd prefer
> >>> >> >> > word_mode
> >>> >> >> > if we go for any hardcoded mode. s390x for example seems to
> >>> >> >> > handle
> >>> >> >> > both SImode and DImode (but names the helper gen_doloop_si64
> >>> >> >> > for SImode?!). But indeed it looks like somehow querying
> >>> >> >> > doloop_end
> >>> >> >> > is going to be difficult since the expander doesn't have any mode,
> >>> >> >> > so we'd have to actually try emit RTL here.
> >>> >> >>
> >>> >> >> Instead of using hardcode mode, maybe we could add a hook for
> >>> >> >> targets to
> >>> >> >> return
> >>> >> >> the preferred mode.
> >>> >> >
> >>> >> > That's a possiblity of course. Like the following which just shows
> >>> >> > the
> >>> >> > default implementation then (pass in current mode, return a more
> >>> >> > preferred
> >>> >> > mode or the mode itself)
> >>> >> >
> >>> >> > enum machine_mode
> >>> >> > prefered_doloop_mode (enum machine_mode mode)
> >>> >> > {
> >>> >> > return mode;
> >>> >> > }
> >>> >> >
> >>> >> Yes, thanks!
> >>> >>
> >>> >> Checking current do_loop_end in targets, in general, when do_loop_end
> >>> >> requires
> >>> >> SI mode, the target is defining Pmode as SImode and word_mode (from
> >>> >> BITS_PER_WORD
> >>> >> which defaults from UNITS_PER_WORD) is also defined to align with SI
> >>> >> mode.
> >>> >> When do_loop_end requires DI mode, the target is defining Pmode as
> >>> >> DImode
> >>> >> and word_mode/UNITS_PER_WORD is also defined to align with DI mode.
> >>> >>
> >>> >> So, if aggressively, then by default we may just return word_mode.
> >>> >
> >>> > Note we still have to check whether the prefered mode is valid
> >>> > (passing in TImode but then returning DImode would be wrong).
> >>> Yes, some code like in doloop_optimize (rtl code)
> >>> ```
> >>> mode != word_mode
> >>> && (word_mode_size >= GET_MODE_PRECISION (mode)
> >>> || wi::leu_p (iterations_max, word_mode_max))
> >>> ```
> >>>
> >>> Thanks again for your comments!
> >>>
> >>> BR,
> >>> Jiufu Guo
> >>> >
> >>> > Richard.
> >>> >
> >>> >> BR,
> >>> >>
> >>> >> Jiufu Guo.
> >>> >>
> >>> >>
> >>> >> >>
> >>> >> >> Thanks for those valuable comments!
> >>> >> >>
> >>> >> >> Jiufu Guo
> >>> >> >>
> >>> >> >>
> >>> >> >>
> >>> >> >> >
> >>> >> >> > Richard.
> >>> >> >>
> >>> >> >>
> >>> >>
> >>>
> >>>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
