[llvm] r268452 - PM: Port LoopRotation to the new loop pass manager

Tue May 10 16:43:34 PDT 2016

On Tue, May 10, 2016 at 5:07 PM Sean Silva via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> On Tue, May 10, 2016 at 2:59 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>>
>>
>> ------------------------------
>>
>> *From: *"Sean Silva via llvm-commits" <llvm-commits at lists.llvm.org>
>> *To: *"Justin Bogner" <mail at justinbogner.com>
>> *Cc: *"llvm-commits" <llvm-commits at lists.llvm.org>
>> *Sent: *Tuesday, May 10, 2016 4:55:08 PM
>> *Subject: *Re: [llvm] r268452 - PM: Port LoopRotation to the new loop
>> pass manager
>>
>>
>>
>>
>> On Tue, May 10, 2016 at 12:46 PM, Justin Bogner <mail at justinbogner.com>
>> wrote:
>>
>>> Sean Silva <chisophugis at gmail.com> writes:
>>> > On Tue, May 10, 2016 at 11:27 AM, Justin Bogner <mail at justinbogner.com
>>> >
>>> > wrote:
>>> >
>>> >> Sean Silva <chisophugis at gmail.com> writes:
>>> >> > On Tue, May 3, 2016 at 3:02 PM, Justin Bogner via llvm-commits <
>>> >> > llvm-commits at lists.llvm.org> wrote:
>>> >> >
>>> >> >> Author: bogner
>>> >> >> Date: Tue May  3 17:02:31 2016
>>> >> >> New Revision: 268452
>>> >> >>
>>> >> >> URL: http://llvm.org/viewvc/llvm-project?rev=268452&view=rev
>>> >> >> Log:
>>> >> >> PM: Port LoopRotation to the new loop pass manager
>>> >>  ...
>>> >> >> --- llvm/trunk/test/Transforms/LoopRotate/basic.ll (original)
>>> >> >> +++ llvm/trunk/test/Transforms/LoopRotate/basic.ll Tue May  3
>>> 17:02:31
>>> >> 2016
>>> >> >> @@ -1,4 +1,6 @@
>>> >> >>  ; RUN: opt -S -loop-rotate < %s | FileCheck %s
>>> >> >> +; RUN: opt -S
>>> >> >>
>>> >>
>>> -passes='require<loops>,require<targetir>,require<assumptions>,loop(rotate)'
>>> >> >> < %s | FileCheck %s
>>> >> >>
>>> >> >
>>> >> > Sorry if this is a stupid question, but why do we need to explicitly
>>> >> > "require" the passes when the loop-rotate already declares the
>>> >> dependency?
>>> >> > (I feel like there's some part of the bigger picture that I'm
>>> missing
>>> >> here)
>>> >>
>>> >> To be clear, there are no dependencies in the new pass manager -
>>> passes
>>> >> request analysis results from an analysis manager and they're
>>> calculated
>>> >> or returned from the cache as appropriate. Naturally, the next
>>> question
>>> >> one would ask is "okay then, why aren't these calculated on demand
>>> when
>>> >> loop-rotate asks for them?".
>>> >>
>>> >
>>> > Ah, yeah. The whole "schedule vs. cache" is one of the main
>>> differences of
>>> > the new PM.
>>> >
>>> >
>>> >>
>>> >> The mechanical answer is that loop-rotate uses the `getCachedResult`
>>> >> API, not the `getResult` one - the difference being that
>>> >> `getCachedResult` only returns results if they're already calculated,
>>> >> and null otherwise. Hence, without the require<> in the test, there
>>> are
>>> >> no results.
>>> >
>>> >
>>> > Ah, makes perfect sense!
>>> >
>>> >
>>> >> This of course leads to "why does loop-rotate use
>>> >> getCachedResult?"
>>> >>
>>> >> It has to. These analyses are function analyses, and a loop pass isn't
>>> >> allowed to cause a function analysis to run, much like a function pass
>>> >> isn't allowed to cause a module analysis to be run. They have to stick
>>> >> to their own level. This enforces correct layering and acts as a
>>> >> safeguard against accidentally doing extra work.
>>> >>
>>> >> That said, the "require<loops>" is redundant, since a LoopPassManager
>>> >> can't operate without having calculated that. There may also be an
>>> >> argument to be made that the LPM should implicitly calculate some
>>> other
>>> >> analyses, but we'd have to be careful not to do too much.
>>> >>
>>> >
>>> > Makes sense. But then `+  assert((LI && TTI && AC) && "Analyses for
>>> loop
>>> > rotation not available");` is not really appropriate, right? Can't we
>>> hit
>>> > that assert just by passing the right arguments to opt? (by dropping
>>> the
>>> > `require<*>` analyses from the passes= specification?)
>>> >
>>> > (sorry, I'm away from the office and don't have an easy way to test
>>> locally)
>>>
>>> Yes it does. The other option is to just bail on the transform in that
>>> case, which would be a better option for things like fuzzers and
>>> bisection scripts, but makes it harder to ensure we're doing everything
>>> correctly while we're bringing this up. For now I think the assert is
>>> best, but we may want to revisit this at some point.
>>>
>>
>> I'm uncomfortable with the assert because it won't be there in Release
>> and I don't want to be running into UB in my release build from a mistyped
>> command line. I think report_fatal_error is more appropriate.
>>
>> +1
>>
>> Also, we might want to consider moving the error-reporting logic into the
>> pass manager itself. I could see having a 'Required' parameter to
>> getCachedResult such that it would be a fatal error should no cached result
>> be available.
>>
>
> Using a function called "getCachedResult" for this purpose seems a bit off
> in the first place. I think it would make sense to call it
> `getRequiredResult` or something like that so that it expresses the
> semantic intent ("as a loop pass, I can't force this to be computed, but I
> require it so it better be there").
>

I think "cached" is still really important -- it signifies that unlike
other query paths it won't go and do work all of a sudden. =]

That said, I agree that the pattern of requiring a cached result from an
outer pass manager is very common and we should streamline this.

The reason I hadn't actually done it is that I'm actually *really* torn
about whether it would be better to have passes just skip their logic or
fail hard. I almost *every* case I prefer the assert or fatal error...
except when it comes down to bisecting a failing pass or similar techniques.

-Chandler

>
> -- Sean Silva
>
>
>>
>>  -Hal
>>
>>
>>
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>>
>>
>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
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