[PATCH] D19730: [BasicAA] Treat llvm.assume as not accessing memory in getModRefBehavior(Function)

[Hal Finkel via llvm-commits]

----- Original Message -----

> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Geoff Berry" <gberry at codeaurora.org>, "Chandler Carruth"
> <chandlerc at gmail.com>, "Philip Reames" <listmail at philipreames.com>,
> "Sanjoy Das" <sanjoy at playingwithpointers.com>,
> mcrosier at codeaurora.org, "llvm-commits"
> <llvm-commits at lists.llvm.org>, "Junbum Lim"
> <junbuml at codeaurora.org>,
> reviews+D19730+public+66d50dddcf8372b8 at reviews.llvm.org
> Sent: Friday, April 29, 2016 1:49:34 PM
> Subject: Re: [PATCH] D19730: [BasicAA] Treat llvm.assume as not
> accessing memory in getModRefBehavior(Function)

> On Fri, Apr 29, 2016 at 10:57 AM, Hal Finkel < hfinkel at anl.gov >
> wrote:

> > > From: "Daniel Berlin" < dberlin at dberlin.org >
> > 
> 
> > > To: "Hal Finkel" < hfinkel at anl.gov >
> > 
> 
> > > Cc: "Geoff Berry" < gberry at codeaurora.org >, "Chandler Carruth" <
> > > chandlerc at gmail.com >, "Philip Reames" <
> > > listmail at philipreames.com
> > > >, "Sanjoy Das" < sanjoy at playingwithpointers.com >,
> > > mcrosier at codeaurora.org , "llvm-commits" <
> > > llvm-commits at lists.llvm.org >, "Junbum Lim" <
> > > junbuml at codeaurora.org
> > > >, reviews+D19730+public+66d50dddcf8372b8 at reviews.llvm.org
> > 
> 
> > > Sent: Friday, April 29, 2016 12:31:28 PM
> > 
> 
> > > Subject: Re: [PATCH] D19730: [BasicAA] Treat llvm.assume as not
> > > accessing memory in getModRefBehavior(Function)
> > 
> 

> > > > I'm not sure this is a good idea. As the comment says, they
> > > > "never
> > > > aliases any particular memory location". This, however, is not
> > > > comparing against any particular location.
> > > 
> > 
> 
> > > > It is asking for general behavior.
> > > 
> > 
> 
> > > Yes, and it neither mod nor refs anything :)
> > 
> 

> > > > AA should probably be constant with the function attributes
> > > > here.
> > > 
> > 
> 

> > > BasicAA already was working around this.
> > 
> 
> > I know, I'm responsible for the workarounds.
> 
> ;)
> > > This just puts this knowledge in one place instead of two
> > > (basicaa
> > > and memoryssa).
> > 
> 

> > > In any case, if we are going to do that, we'd have to actually
> > > fix
> > > things properly. Saying assume has side-effects and writes things
> > > was wrong in the first place compared to actually modeling
> > > control
> > > dependencies for things like assume properly.
> > 
> 

> > > It simply does not have side-effects or write anything. Period.
> > > Claiming otherwise is a lie to try to avoid fixing other
> > > infrastructure to handle this kind of intrinsic properly.
> > 
> 

> > > Until we go and have the wherewithal to fix that infrastructure,
> > > we
> > > are going to have to accept that we are going to have things like
> > > this, where we are making up for it.
> > 
> 
> > I'm well aware of the state of things, and I agree that we should
> > have a better way of modeling control dependencies than via memory.
> 

> > > The alternative, which was currently done, where basicaa and
> > > memoryssa and whoever else has to know that assume is magic and
> > > special, seems much worse.
> > 
> 

> > I understand. However, without our current system, this still
> > obviously needs to be correct. What is to prevent a pass from
> > (correctly) seeing the call to assume, using AA to conclude it does
> > not actually alter memory, seeing it has no return values,
> > intrinsics don't throw, and so removing it as dead?
> 
> Well, it will still return true to mayHaveSideEffects :)
True, but this does not mean arbitrary side effects. At the IR level, our model contains only a fixed set of possible classes of side effects. We model memory access, unwinding (or other unknown control-flow effects), and (possibly-control-flow-dependent) UB (including, but not limited to, trapping). Unless I'm forgetting something ;), our model simply does not contain anything else. In fact, mayHaveSideEffects is not even an independent property, it is defined like this: 

bool mayHaveSideEffects() const { 
return mayWriteToMemory() || mayThrow() || !mayReturn(); 
} 

To do what we want, we need something else... 

> But your question is exactly one of "how do we fix the
> infrastructure".
> I don't think changing AA, which is an optional part and supposed to
> give the best answers it can (regardless of function attributes or
> anything else), is going to affect that much.

> As an aside, note that this is already affecting things like
> GlobalsModRef (which also mixes AA and function attributes), which
> will now assume that every function in a callchain with an assume in
> it modref things (which is making much worse information :( ).

> It too would have to special case assume to tell it otherwise.

> This patch should also fix part of this without breaking assume (the
> other part it will not due to the mixing of AA and function
> attribute calls).

> However, I'm also happy to put this knowledge in the 4 places for now
> if you really believe that is a better solution.
It is not really a question of best right now, more like least bad. I think that we can leave the change as is -- there's no good reason to move farther away from the solution we want instead of closer, but the comment needs to be updated to better reflect the situation. And we need to, sooner rather than later, introduce some new property or properties that do what we need here. I think it is fairly dangerous to have, essentially, "AA says readnone" have different semantics than "the call has the readnone attribute" in subtle ways. 

The good news is that adding such a thing might not be that bad. Most code checks mayHaveSideEffects() in relevant places, and adding another thing that causes that to be true is not hard. Also, we do now have control-flow-dependent callsite properties (e.g. isConvergent), and between the two, we can hopefully find without too much difficultly most of the places that need to be updated. 

-Hal 

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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