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

[Richard Biener <rguenther@suse.de>]

> 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)?

> 
> Thanks,
> Tamar
> 
>> 
>> Richard.