[PATCH] Add a callback to FunctionPass to enable skipping execution on a per-function basis

[Eric Christopher]

It's a real case, check out the ARM port for it.

On Wed, Apr 8, 2015, 8:02 PM Chandler Carruth <chandlerc at google.com> wrote:

Is this a real use case or a hypothetical one? Because it seems somewhat
contrived to me...

If there really is some predicate that necessitates really radically
different pass pipelines, I feel like they should be, well, two separate
pass pipelines.


 On Wed, Apr 8, 2015 at 7:54 PM Eric Christopher <echristo at gmail.com> wrote:

Optnone, IMO, needs to be replaced by something less terrible.

I'm not sure how this is going to work with the "I want to run the first
cfgcleanup unconditionally, but not the second" without tying the
subtargets to things like shouldRunCfgCleanup2().

On Wed, Apr 8, 2015, 7:38 PM Sean Silva <chisophugis at gmail.com> wrote:

 Adding Paul as this seems related to optnone.

On Wed, Apr 8, 2015 at 7:31 PM, Chandler Carruth <chandlerc at gmail.com>
wrote:

 I feel like we could do something much simpler than this. This feeling is
predicated on one primary theory: most passes will run for most subtargets.
Put another way, there will only be a small number of passes that we
actually want to opt out of on a per-subtarget basis.

If we think that's likely to be the case, here is an alternative suggestion:

- Add bool-returning predicates for each pass to the subtarget base class
(eg, "isIfConversionProfitable()") with the expected default ("true").
- Override these for the subtargets that want to opt out.
- Change the pass to directly get the subtarget, query it, and bail without
doing anything if it gets "false".

>From looking at and thinking about if-conversion at least, this seems nicer
to me. It makes someone working on the pass aware that there are subtarget
profitability concerns, and it makes it very clear that we are *running*
all of the passes, just that some have no effect on certain subtargets.

This also matches how an optimization pass should query the function for
the 'noopt' attribute and bail.

Thoughts?


   http://reviews.llvm.org/D8717

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