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

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/29/22 03:37, 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?

No, just that if we're doing a single bit test that the way to handle 
that is with a zero_extract and the earlier we can generate that form, 
the better.


Jeff