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>]

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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.

Richard.