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From: Tamar Christina <Tamar.Christina@arm.com>
To: Richard Biener <rguenther@suse.de>
Cc: Richard Sandiford <Richard.Sandiford@arm.com>,
	Tamar Christina via Gcc-patches <gcc-patches@gcc.gnu.org>,
	nd <nd@arm.com>, Jeff Law <jeffreyalaw@gmail.com>
Subject: RE: [PATCH 1/2]middle-end: RFC: On expansion of conditional branches, give hint if argument is a truth type to backend
Date: Fri, 30 Sep 2022 11:11:35 +0000	[thread overview]
Message-ID: <VI1PR08MB5325FD8DD50DC89A6D958672FF569@VI1PR08MB5325.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <nycvar.YFH.7.77.849.2209301011580.18337@jbgna.fhfr.qr>

> -----Original Message-----
> From: Richard Biener <rguenther@suse.de>
> Sent: Friday, September 30, 2022 11:17 AM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: Richard Sandiford <Richard.Sandiford@arm.com>; Tamar Christina via
> Gcc-patches <gcc-patches@gcc.gnu.org>; nd <nd@arm.com>; Jeff Law
> <jeffreyalaw@gmail.com>
> Subject: RE: [PATCH 1/2]middle-end: RFC: On expansion of conditional
> branches, give hint if argument is a truth type to backend
> 
> On Fri, 30 Sep 2022, Tamar Christina wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: Richard Sandiford <richard.sandiford@arm.com>
> > > Sent: Friday, September 30, 2022 9:49 AM
> > > To: Tamar Christina <Tamar.Christina@arm.com>
> > > Cc: Richard Biener <rguenther@suse.de>; Tamar Christina via
> > > Gcc-patches <gcc-patches@gcc.gnu.org>; nd <nd@arm.com>; Jeff Law
> > > <jeffreyalaw@gmail.com>
> > > Subject: Re: [PATCH 1/2]middle-end: RFC: On expansion of conditional
> > > branches, give hint if argument is a truth type to backend
> > >
> > > Tamar Christina <Tamar.Christina@arm.com> writes:
> > > >> -----Original Message-----
> > > >> From: Richard Sandiford <richard.sandiford@arm.com>
> > > >> Sent: Friday, September 30, 2022 9:29 AM
> > > >> To: Tamar Christina <Tamar.Christina@arm.com>
> > > >> Cc: Richard Biener <rguenther@suse.de>; Tamar Christina via
> > > >> Gcc-patches <gcc-patches@gcc.gnu.org>; nd <nd@arm.com>; Jeff
> Law
> > > >> <jeffreyalaw@gmail.com>
> > > >> Subject: Re: [PATCH 1/2]middle-end: RFC: On expansion of
> > > >> conditional branches, give hint if argument is a truth type to
> > > >> backend
> > > >>
> > > >> Tamar Christina <Tamar.Christina@arm.com> writes:
> > > >> >> -----Original Message-----
> > > >> >> From: Gcc-patches <gcc-patches-
> > > >> >> bounces+tamar.christina=arm.com@gcc.gnu.org> On Behalf Of
> > > Richard
> > > >> >> Biener via Gcc-patches
> > > >> >> Sent: Thursday, September 29, 2022 12:09 PM
> > > >> >> To: Tamar Christina via Gcc-patches <gcc-patches@gcc.gnu.org>
> > > >> >> Cc: Richard Sandiford <Richard.Sandiford@arm.com>; nd
> > > <nd@arm.com>
> > > >> >> Subject: Re: [PATCH 1/2]middle-end: RFC: On expansion of
> > > >> >> conditional branches, give hint if argument is a truth type to
> > > >> >> backend
> > > >> >>
> > > >> >>
> > > >> >>
> > > >> >> > Am 29.09.2022 um 12:23 schrieb Tamar Christina via
> > > >> >> > Gcc-patches
> > > >> >> > <gcc-
> > > >> >> patches@gcc.gnu.org>:
> > > >> >> >
> > > >> >> >
> > > >> >> >>
> > > >> >> >> -----Original Message-----
> > > >> >> >> From: Richard Biener <rguenther@suse.de>
> > > >> >> >> Sent: Thursday, September 29, 2022 10:41 AM
> > > >> >> >> To: Richard Sandiford <Richard.Sandiford@arm.com>
> > > >> >> >> Cc: Jeff Law <jeffreyalaw@gmail.com>; Tamar Christina
> > > >> >> >> <Tamar.Christina@arm.com>; gcc-patches@gcc.gnu.org; nd
> > > >> >> <nd@arm.com>
> > > >> >> >> Subject: Re: [PATCH 1/2]middle-end: RFC: On expansion of
> > > >> >> >> conditional branches, give hint if argument is a truth type
> > > >> >> >> to backend
> > > >> >> >>
> > > >> >> >>> On Thu, 29 Sep 2022, Richard Sandiford wrote:
> > > >> >> >>>
> > > >> >> >>> Jeff Law <jeffreyalaw@gmail.com> writes:
> > > >> >> >>>> On 9/28/22 09:04, Richard Sandiford wrote:
> > > >> >> >>>>> Tamar Christina <Tamar.Christina@arm.com> writes:
> > > >> >> >>>>>>> Maybe the target could use (subreg:SI (reg:BI ...)) as
> > > argument.
> > > >> >> Heh.
> > > >> >> >>>>>> But then I'd still need to change the expansion code. I
> > > >> >> >>>>>> suppose this could prevent the issue with changes to
> > > >> >> >>>>>> code on
> > > >> other targets.
> > > >> >> >>>>>>
> > > >> >> >>>>>>>>> We have undocumented addcc, negcc, etc. patterns,
> > > should
> > > >> we
> > > >> >> >>>>>>>>> have aandcc
> > > >> >> >>>>>> pattern for this indicating support for andcc + jump as
> > > >> >> >>>>>> opposedto
> > > >> >> >> cmpcc + jump?
> > > >> >> >>>>>>>> This could work yeah. I didn't know these existed.
> > > >> >> >>>>>>> Ah, so they are conditional add, not add setting CC,
> > > >> >> >>>>>>> so andcc wouldn't be appropriate.
> > > >> >> >>>>>>> So I'm not sure how we'd handle such situation - maybe
> > > >> >> >>>>>>> looking at REG_DECL and recognizing a _Bool PARM_DECL
> > > >> >> >>>>>>> is
> > > OK?
> > > >> >> >>>>>> I have a slight suspicion that Richard Sandiford would
> > > >> >> >>>>>> likely reject this though..
> > > >> >> >>>>> Good guess :-P  We shouldn't rely on something like that
> > > >> >> >>>>> for
> > > >> >> >> correctness.
> > > >> >> >>>>>
> > > >> >> >>>>> Would it help if we promoted the test-and-branch
> > > >> >> >>>>> instructions to optabs, alongside cbranch?  The jump
> > > >> >> >>>>> expanders could then target it
> > > >> >> >> directly.
> > > >> >> >>>>>
> > > >> >> >>>>> IMO that'd be a reasonable thing to do if it does help.
> > > >> >> >>>>> It's a relatively common operation, especially on CISCy
> targets.
> > > >> >> >>>>
> > > >> >> >>>> But don't we represent these single bit tests using
> > > >> >> >>>> zero_extract as the condition of the branch?  I guess if
> > > >> >> >>>> we can generate them directly rather than waiting for
> > > >> >> >>>> combine to deduce that we're dealing with a single bit
> > > >> >> >>>> test and constructing the zero_extract form would be an
> > > >> >> >>>> improvement and might help aarch at the same
> > > >> time.
> > > >> >> >>>
> > > >> >> >>> Do you mean that the promote_mode stuff should use ext(z)v
> > > >> >> >>> rather than zero_extend to promote a bool, where available?
> > > >> >> >>> If so, I agree that might help.  But it sounds like it
> > > >> >> >>> would have downsides
> > > >> too.
> > > >> >> >>> Currently a bool memory can be zero-extended on the fly
> > > >> >> >>> using a load, but if we used the zero_extract form
> > > >> >> >>> instead, we'd have to extract the bit after the load.  And
> > > >> >> >>> (as an
> > > >> >> >>> alternative) choosing different behaviour based on whether
> > > >> >> >>> expand sees a REG or a MEM sounds like it could still
> > > >> >> >>> cause problems, since REGs could be replaced by MEMs (or
> > > >> >> >>> vice versa)
> > > later in the RTL passes.
> > > >> >> >>>
> > > >> >> >>> ISTM that the original patch was inserting an extra
> > > >> >> >>> operation in the branch expansion in order to target a specific
> instruction.
> > > >> >> >>> Targeting the instruction in expand seems good, but IMO we
> > > >> >> >>> should do it directly, based on knowledge of whether the
> > > >> >> >>> instruction actually
> > > >> >> exists.
> > > >> >> >>
> > > >> >> >> Yes, I think a compare-and-branch pattern is the best fit here.
> > > >> >> >> Note on GIMPLE we'd rely on the fact this is a BOOLEAN_TYPE
> > > >> >> >> (so even 8 bit precision bools only have 1 and 0 as meaningful
> values).
> > > >> >> >> So the 'compare-' bit in compare-and-branch would be
> > > >> >> >> interpreting a BOOLEAN_TYPE, not so much a general compare.
> > > >> >> >
> > > >> >> > Oh, I was thinking of adding a constant argument
> > > >> >> > representing the precision that is relevant for the compare
> > > >> >> > in order to make this a bit more
> > > >> >> general/future proof.
> > > >> >> >
> > > >> >> > Are you thinking I should instead just make the optab
> > > >> >> > implicitly only work for 1-bit precision comparisons?
> > > >> >>
> > > >> >> What?s the optab you propose (cite also the documentation part)?
> > > >> >
> > > >> > tbranchmode5
> > > >> >   Conditional branch instruction combined with a bit test instruction.
> > > >> Operand 0 is a comparison operator.
> > > >> >   Operand 1 and Operand 2 are the first and second operands of
> > > >> > the
> > > >> comparison, respectively.
> > > >> >   Operand 3 is the number of low-order bits that are relevant
> > > >> > for the
> > > >> comparison.
> > > >> >   Operand 4 is the code_label to jump to.
> > > >>
> > > >> For the TB instructions (and for other similar instructions that
> > > >> I've seen on other architectures) it would be more useful to have
> > > >> a single-bit test, with operand 4 specifying the bit position.
> > > >> Arguably it might then be better to have separate eq and ne
> > > >> optabs, to avoid the awkward doubling of the operands (operand 1
> > > >> contains
> > > operands 2 and 3).
> > > >>
> > > >> I guess a more general way of achieving the same thing would be
> > > >> to make operand 4 in the optab above a mask rather than a bit count.
> > > >> But that might be overly general, if there are no known
> > > >> architectures that have such an instruction.
> > > >
> > > > One of the reasons I wanted a range rather than a single bit is
> > > > that I can the use this to generate cbz/cbnz early on as well.
> > >
> > > We already have the opportunity to do that via cbranch<mode>4.
> > > But at the moment aarch64.md always forces the separate comparison
> > > instead.  (Not sure why TBH.  Does it enable more ifcvt
> > > opportunities?)
> > >
> > > If we change the body of cbranch<mode>4 to:
> > >
> > >   if ((GET_CODE (operands[0]) != EQ && GET_CODE (operands[0]) != NE)
> > >       || operands[2] != const0_rtx)
> > >     {
> > >       operands[1] = aarch64_gen_compare_reg (GET_CODE (operands[0]),
> > > 					     operands[1], operands[2]);
> > >       operands[2] = const0_rtx;
> > >     }
> > >
> > > then we generate the cbz/cbnz directly.
> > >
> >
> > Ah ok, then if Richi agrees, bitpos it is then instead of bit count.
> 
> Somehow I understood that cbranch<>4 is already fully capable of the
> optimization?
> 
> On your earlier proposal I'd have commented that if it wouldn't make sense
> to instead have a CCmode setter instead of an expander with a branch label?
> That would be a bit test, like {sign,zero}_extract compared against zero which
> can then be combined with a branch?
>

I missed that part, that could work too.

> Of course if the ISA is really bit-test-and-branch then cbranch<>4 itself or a
> variant of it might be more useful.  Maybe
> cbranchbi4 would be "abused" for this?

The instruction is an actual bit-test-and-branch with any arbitrary bitpos.
Yes we can abuse cbranchbi4 for this, but then it also means we can't e.g.
use this to optimize a < 0 where a is a signed value.  With the new optab
this would just be a bit-test-and-branch of the sign bit.

But also I'm not entirely convinced that using `BImode` and assuming a single
bit is safe here. What happens if I compile my source with -std=c89?

So I personally think the new optab makes more sense here. The CC setter would work too.

I guess my question is, between you folks, which approach would you like. It seems that Richi
You'd like a CC setter. Richard do you have a preference of one over the other?

Tamar

> 
> > Tamar
> >
> >
> > > Thanks,
> > > Richard
> > >
> > >
> > > > This would mean we could use my earlier patch that tried to drop
> > > > the QI/HI promotions without needing the any_extend additional
> > > > pass if we wanted to.
> > > >
> > > > We'd also no longer need to rely on seeing a paradoxical subreg for a
> tst.
> > > >
> > > > Tamar.
> > > >
> > > >>
> > > >> Thanks,
> > > >> Richard
> > > >>
> > > >> > Specifically this representation would allow us to emit all our
> > > >> > different conditional branching instructions without needing to
> > > >> > rely on combine.  We have some cases that happen during
> > > >> > optimization that sometimes prevent the optimal sequence from
> > > >> > being generated. This
> > > >> would also solve that as we would expand to what we want to start
> with.
> > > >> >
> > > >> > Tamar.
> > > >> >
> > > >> >>
> > > >> >> >
> > > >> >> > Thanks,
> > > >> >> > Tamar
> > > >> >> >
> > > >> >> >>
> > > >> >> >> Richard.
> >
> 
> --
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461
> Nuernberg, Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald,
> Boudien Moerman; HRB 36809 (AG Nuernberg)

  reply	other threads:[~2022-09-30 11:11 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-23  9:24 Tamar Christina
2022-09-23  9:25 ` [PATCH 2/2]AArch64 Extend tbz pattern to allow SI to SI extensions Tamar Christina
2022-09-23  9:42   ` Richard Sandiford
2022-09-23  9:48     ` Tamar Christina
2022-09-26 10:35 ` [PATCH 1/2]middle-end: RFC: On expansion of conditional branches, give hint if argument is a truth type to backend Richard Biener
2022-09-26 11:05   ` Tamar Christina
2022-09-26 11:32     ` Richard Biener
2022-09-26 11:46       ` Tamar Christina
2022-09-26 12:34         ` Richard Biener
2022-09-26 12:43           ` Richard Biener
2022-09-26 14:02             ` Tamar Christina
2022-09-28 15:04         ` Richard Sandiford
2022-09-28 17:23           ` Jeff Law
2022-09-29  9:37             ` Richard Sandiford
2022-09-29  9:40               ` Richard Biener
2022-09-29 10:21                 ` Tamar Christina
2022-09-29 11:09                   ` Richard Biener
2022-09-30  8:00                     ` Tamar Christina
2022-09-30  8:28                       ` Richard Sandiford
2022-09-30  8:38                         ` Tamar Christina
2022-09-30  8:48                           ` Richard Sandiford
2022-09-30  9:15                             ` Tamar Christina
2022-09-30 10:16                               ` Richard Biener
2022-09-30 11:11                                 ` Tamar Christina [this message]
2022-09-30 11:52                                   ` Richard Biener
2022-09-30 12:48                                     ` Tamar Christina
2022-09-30 14:28                                       ` Richard Sandiford
2022-09-30 14:33                                         ` Richard Biener
2022-09-29 20:49               ` Jeff Law
2022-10-27  3:22 ` Andrew Pinski

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