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 9/28/22 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.


jeff