[llvm-dev] Question about a AA result and its use in Dependence Analysis

Wed Jun 5 10:02:06 PDT 2019

Oh. So it wasn't just the memory location size that was incorrect on those aliasing checks.

I took a look at the tests and it appears that the alias call also catches some cases of restrict/noalias attributes, along with the tbaa info. My first idea of constructing an alias call with no Ptr's but valid tbaa info won't catch that at least.

I will create a bug report and try and see if I can come up with anything more sensible.

Dave

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Finkel, Hal J. via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 03 June 2019 22:01
To: Eli Friedman; Doerfert, Johannes; De Azevedo Piovezan, Felipe; llvm-dev
Subject: Re: [llvm-dev] Question about a AA result and its use in Dependence Analysis

On 6/3/19 1:45 PM, Eli Friedman via llvm-dev wrote:

Alias analysis is figuring out the relationship between two pointer expressions, at some location in the program. At a given point in the program, do two expressions always refer to the same location?  At a given point in the program, do two expressions never refer to the same location?

AliasAnalysis::alias() doesn't explicitly take a "point" in the program because we don't implement any alias analysis that cares about the precise point in the program; the answer applies to any point in the program dominated by the definitions of both values.  This produces useful results because LLVM uses SSA form.  Working from that definition, it's easy to see that the logic used in aliasSelect is valid, and that aliasPHI is returning the correct result for the given example.  Maybe it's worth calling this out in the alias analysis documentation.

+1

The flaw here seems to be in the way that DA is using AA. DependenceAnalysis's underlyingObjectsAlias is doing this:

  // Check the original locations (minus size) for noalias, which can happen for
  // tbaa, incompatible underlying object locations, etc.
  MemoryLocation LocAS(LocA.Ptr, LocationSize::unknown(), LocA.AATags);
  MemoryLocation LocBS(LocB.Ptr, LocationSize::unknown(), LocB.AATags);
  if (AA->alias(LocAS, LocBS) == NoAlias)
    return NoAlias;

  // Check the underlying objects are the same
  const Value *AObj = GetUnderlyingObject(LocA.Ptr, DL);
  const Value *BObj = GetUnderlyingObject(LocB.Ptr, DL);

  // If the underlying objects are the same, they must alias
  if (AObj == BObj)
    return MustAlias;

  // We may have hit the recursion limit for underlying objects, or have
  // underlying objects where we don't know they will alias.
  if (!isIdentifiedObject(AObj) || !isIdentifiedObject(BObj))
    return MayAlias;

  // Otherwise we know the objects are different and both identified objects so
  // must not alias.
  return NoAlias;

and I recall suggesting this use of AA to pick up the metadata-based AA results. We shouldn't need both the AA check here and the underlying-object check also, as the AA check includes an underlying-object check (and not much else, aside from checks on metadata, when both sizes are unknown). However, as this example shows, the fact that the two SSA values in question refer to different, identified underlying objects, doesn't prevent them from having a cross-iteration dependence as the identify of those underlying objects can change across different iterations (and, thus, the underlying object of one variable in one iteration might be the underlying object used by another variable in another iteration).

I'm open to suggestions on how to best resolve this situation. I'm leaning toward suggesting that we have a metadata-only AA query, and use that here, instead of calling something that will invoke BasicAA's logic. My metadata-only AA query, I specifically mean a fundamental dependency-analysis query, likely implemented by the AA subsystem for code-reuse reasons, which answers the question: is there some reason to believe that any instances of these two values never take the same value along any dynamic path through the CFG (or something similar).

Thanks again,

Hal

For information across loop iterations, the appropriate analysis is probably LoopAccessAnalysis.

-Eli

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Doerfert,
Johannes via llvm-dev
Sent: Monday, June 3, 2019 9:04 AM
To: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] Question about a AA result and its use in
Dependence Analysis

Thanks for investigating this further.

In BasicAAResult::aliasSelect the "same" logic is used:

    // If the values are Selects with the same condition, we can do a more precise
    // check: just check for aliases between the values on corresponding arms.

That is what happens for PHI nodes as well. So, if either is broken both are.
Now it remains to definitively decide if this is broken (which I still think).

---------------------------------------
Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov

________________________________________
From: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>
Sent: Monday, June 3, 2019 11:00
To: Doerfert, Johannes
Cc: llvm-dev at lists.llvm.org; Finkel, Hal J.
Subject: RE: [llvm-dev] Question about a AA result and its use in Dependence
Analysis

It seems the same bug is there if we do pointer swapping with selects. Do you
agree? (see example below)

define void @f() {
entry:
  %a1 = alloca float, align 4
  %a2 = alloca float, align 4
  br label %loop

end:
  ret void

loop:
  %phi = phi i32 [ 0, %entry ], [ 1, %loop ]
  %select_cond = icmp eq i32 %phi, 0
  %ptr1 = select i1 %select_cond, float* %a1, float* %a2
  %ptr2 = select i1 %select_cond, float* %a2, float* %a1
  store float 0.000000e+00, float* %ptr1, align 4
  store float 1.000000e+00, float* %ptr2, align 4
  br i1 %select_cond, label %end, label %loop
}

Alias Set Tracker: 2 alias sets for 2 pointer values.
  AliasSet[0x55bd786b8300, 1] must alias, Mod       Pointers: (float* %ptr1,
LocationSize::precise(4))
  AliasSet[0x55bd786b96b0, 1] must alias, Mod       Pointers: (float* %ptr2,
LocationSize::precise(4))

-----Original Message-----
From: Doerfert, Johannes [mailto:jdoerfert at anl.gov]
Sent: Saturday, June 1, 2019 2:29 PM
To: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>
Cc: llvm-dev at lists.llvm.org; Finkel, Hal J. <hfinkel at anl.gov>
Subject: Re: [llvm-dev] Question about a AA result and its use in Dependence
Analysis

Hi Felipe,

(+ Hal)

I think the reasoning in `BasicAAResult::aliasPHI(...)` is flawed but I want
someone else to take a look as well.

As far as I can tell, the reasoning there is as follows:
  - Two phi nodes in the same block do not alias by default.
  - They do alias if the pair of inputs from the same predecessor alias.
  - If all inputs are pair-wise alias free, they do not alias.

Now in the case below, %g and %h do clearly not alias, which is what the query
for %entry will determine. Since %p and %q are initially assumed to be not
aliasing, the query for the incoming values from %for.body will return NoAlias.

Cheers,
  Johannes

On 06/01, De Azevedo Piovezan, Felipe wrote:

Hi Johannes,

I followed your advice and got the same result: NoAlias and No dependence.

Would you say AA is faulty for saying NoAlias or DA is faulty for saying no
dependence? Or both?

(revised example below)

Thanks!

define float @f() {
entry:
  %g = alloca float, align 4
  %h = alloca float, align 4
  br label %for.body

for.cond.cleanup:                                 ; preds = %for.body
  ret float undef

for.body:                                         ; preds = %for.body, %entry
  %p = phi float* [ %g, %entry ], [ %q, %for.body ]
  %q = phi float* [ %h, %entry ], [ %p, %for.body ]
  %0 = load float, float* %p, align 4
  store float undef, float* %q, align 4
  %branch_cond = fcmp ugt float %0, 0.0
  br i1 %branch_cond, label %for.cond.cleanup, label %for.body }

Alias Set Tracker: 2 alias sets for 2 pointer values.
  AliasSet[0x83e1fe0, 1] must alias, Ref       Pointers: (float* %p,

LocationSize::precise(4))

  AliasSet[0x83e3390, 1] must alias, Mod       Pointers: (float* %q,

LocationSize::precise(4))

da analyze -
   %0 = load float, float* %p, align 4
  store float undef, float* %q, align 4 none!

-----Original Message-----
From: Doerfert, Johannes [mailto:jdoerfert at anl.gov]
Sent: Friday, May 31, 2019 9:07 PM
To: De Azevedo Piovezan, Felipe <felipe.de.azevedo.piovezan at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Question about a AA result and its use in
Dependence Analysis

Can you try it without the undef branch condition. If you get still the same

result I'd argue its a bug. In the program as it is, I'd say it is not because the back
edge is never taken.

On 05/31, De Azevedo Piovezan, Felipe via llvm-dev wrote:

Hello llvm-dev,

I would appreciate your feedback on the following problem. We're trying to

determine whether this is a bug in LLVM or not.

In the IR snippet below, we have two pointers (p and q) which initially point

to two completely non-overlapping locations. Then, on every iteration of a loop,
we swap the pointers and load from the first, followed by a store to the second.

1) AA says the two pointers are NoAlias, even though they do point to the

same location if we consider them in distinct loop iterations. Is this right?

2) Dependence Analysis says there is no dependence between the load and

the store. Is this right?

define float @f() {
entry:
  %g = alloca float, align 4
  %h = alloca float, align 4
  br label %for.body

for.cond.cleanup:
  ret float undef

for.body:
  %p = phi float* [ %g, %entry ], [ %q, %for.body ]
  %q = phi float* [ %h, %entry ], [ %p, %for.body ]
  %0 = load float, float* %p, align 4
  store float undef, float* %q, align 4
  br i1 undef, label %for.cond.cleanup, label %for.body }

AliasSet[0x872d800, 1] must alias, Ref       Pointers: (float* %p,

LocationSize::precise(4))

AliasSet[0x872d8b0, 1] must alias, Mod       Pointers: (float* %q,

LocationSize::precise(4))

da analyze -
   %0 = load float, float* %p, align 4   ; I added these two debug statements,

DA doesn't print the values it is looking at...

  store float undef, float* %q, align 4 none!

--
Felipe

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--

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov

--

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory