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Re: [PATCH][RFC][x86] Fix PR91154, add SImode smax, allow SImode add in SSE regs


On Mon, Aug 5, 2019 at 11:13 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Uros Bizjak <ubizjak@gmail.com> writes:
> > On Sat, Aug 3, 2019 at 7:26 PM Richard Biener <rguenther@suse.de> wrote:
> >>
> >> On Thu, 1 Aug 2019, Uros Bizjak wrote:
> >>
> >> > On Thu, Aug 1, 2019 at 11:28 AM Richard Biener <rguenther@suse.de> wrote:
> >> >
> >> >>>> So you unconditionally add a smaxdi3 pattern - indeed this looks
> >> >>>> necessary even when going the STV route.  The actual regression
> >> >>>> for the testcase could also be solved by turing the smaxsi3
> >> >>>> back into a compare and jump rather than a conditional move sequence.
> >> >>>> So I wonder how you'd do that given that there's pass_if_after_reload
> >> >>>> after pass_split_after_reload and I'm not sure we can split
> >> >>>> as late as pass_split_before_sched2 (there's also a split _after_
> >> >>>> sched2 on x86 it seems).
> >> >>>>
> >> >>>> So how would you go implement {s,u}{min,max}{si,di}3 for the
> >> >>>> case STV doesn't end up doing any transform?
> >> >>>
> >> >>> If STV doesn't transform the insn, then a pre-reload splitter splits
> >> >>> the insn back to compare+cmove.
> >> >>
> >> >> OK, that would work.  But there's no way to force a jumpy sequence then
> >> >> which we know is faster than compare+cmove because later RTL
> >> >> if-conversion passes happily re-discover the smax (or conditional move)
> >> >> sequence.
> >> >>
> >> >>> However, considering the SImode move
> >> >>> from/to int/xmm register is relatively cheap, the cost function should
> >> >>> be tuned so that STV always converts smaxsi3 pattern.
> >> >>
> >> >> Note that on both Zen and even more so bdverN the int/xmm transition
> >> >> makes it no longer profitable but a _lot_ slower than the cmp/cmov
> >> >> sequence... (for the loop in hmmer which is the only one I see
> >> >> any effect of any of my patches).  So identifying chains that
> >> >> start/end in memory is important for cost reasons.
> >> >
> >> > Please note that the cost function also considers the cost of move
> >> > from/to xmm. So, the cost of the whole chain would disable the
> >> > transformation.
> >> >
> >> >> So I think the splitting has to happen after the last if-conversion
> >> >> pass (and thus we may need to allocate a scratch register for this
> >> >> purpose?)
> >> >
> >> > I really hope that the underlying issue will be solved by a machine
> >> > dependant pass inserted somewhere after the pre-reload split. This
> >> > way, we can split unconverted smax to the cmove, and this later pass
> >> > would handle jcc and cmove instructions. Until then... yes your
> >> > proposed approach is one of the ways to avoid unwanted if-conversion,
> >> > although sometimes we would like to split to cmove instead.
> >>
> >> So the following makes STV also consider SImode chains, re-using the
> >> DImode chain code.  I've kept a simple incomplete smaxsi3 pattern
> >> and also did not alter the {SI,DI}mode chain cost function - it's
> >> quite off for TARGET_64BIT.  With this I get the expected conversion
> >> for the testcase derived from hmmer.
> >>
> >> No further testing sofar.
> >>
> >> Is it OK to re-use the DImode chain code this way?  I'll clean things
> >> up some more of course.
> >
> > Yes, the approach looks OK to me. It makes chain building mode
> > agnostic, and the chain building can be used for
> > a) DImode x86_32 (as is now), but maybe 64bit minmax operation can be added.
> > b) SImode x86_32 and x86_64 (this will be mainly used for SImode
> > minmax and surrounding SImode operations)
> > c) DImode x86_64 (also, mainly used for DImode minmax and surrounding
> > DImode operations)
> >
> >> Still need help with the actual patterns for minmax and how the splitters
> >> should look like.
> >
> > Please look at the attached patch. Maybe we can add memory_operand as
> > operand 1 and operand 2 predicate, but let's keep things simple for
> > now.
> >
> > Uros.
> >
> > Index: i386.md
> > ===================================================================
> > --- i386.md   (revision 274008)
> > +++ i386.md   (working copy)
> > @@ -17721,6 +17721,27 @@
> >      std::swap (operands[4], operands[5]);
> >  })
> >
> > +;; min/max patterns
> > +
> > +(define_code_attr smaxmin_rel [(smax "ge") (smin "le")])
> > +
> > +(define_insn_and_split "<code><mode>3"
> > +  [(set (match_operand:SWI48 0 "register_operand")
> > +     (smaxmin:SWI48 (match_operand:SWI48 1 "register_operand")
> > +                    (match_operand:SWI48 2 "register_operand")))
> > +   (clobber (reg:CC FLAGS_REG))]
> > +  "TARGET_STV && TARGET_SSE4_1
> > +   && can_create_pseudo_p ()"
> > +  "#"
> > +  "&& 1"
> > +  [(set (reg:CCGC FLAGS_REG)
> > +     (compare:CCGC (match_dup 1)(match_dup 2)))
> > +   (set (match_dup 0)
> > +     (if_then_else:SWI48
> > +       (<smaxmin_rel> (reg:CCGC FLAGS_REG)(const_int 0))
> > +       (match_dup 1)
> > +       (match_dup 2)))])
> > +
>
> The pattern could in theory be matched after the last pre-RA split pass
> has run, so I think the pattern still needs to have constraints and be
> matchable even without can_create_pseudo_p.  It looks like the split
> above should work post-RA.
>
> A bit pedantic, because the pattern's probably fine in practice...

Currently, all unmatched STV patterns split before reload, and there
were no problems. If the pattern matches after last pre-RA split, then
the post-reload splitter will fail, since can_create_pseudo_p also
applies to the part that splits the insn. In any case, thanks for the
heads-up, hopefully we didn't assume something that doesn't hold.

Thanks,
Uros.

> Thanks,
> Richard
>
> >  ;; Conditional addition patterns
> >  (define_expand "add<mode>cc"
> >    [(match_operand:SWI 0 "register_operand")


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