Re: [PATCH 1/2]middle-end: RFC: On expansion of conditional branches, give hint if argument is a truth type to backend

[Richard Sandiford <richard.sandiford@arm.com>]

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.

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.