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

[Uros Bizjak <ubizjak at gmail dot com>]

On Mon, Aug 5, 2019 at 12:12 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Uros Bizjak <ubizjak@gmail.com> writes:
> > 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.
>
> But what I meant was: you should be able to remove the
> can_create_pseudo_p () and add constraints.  (You'd have to remove
> can_create_pseudo_p () with constraints anyway, since the insn
> wouldn't match after RA otherwise.)

I was under impression that it is better to split pseudo->pseudo, so
reload has some more freedom on what register to choose, especially
with matched and earlyclobbered DImode regs in x86_32 DImode patterns.
There were some complications with andn pattern (that needed
earlyclobber on a register to avoid clobbering registers in a memory
address), and it was necessary to clobber the whole DImode register
pair, wasting a SImode register. We can avoid all these complications
by splitting before the RA, where also a pseudo can be allocated.

Uros.