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>]

> -----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.

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.