llvm dev - Jul 2017 - PartialAlias: different start addresses

Thank you all for your replies.
So here seems to be an agreement that the documentation for PartialAlias is 
incorrect.

Daniel: now you got me wondering about MustAlias. This is what the docs say:
"The MustAlias response may only be returned if the two memory objects are 
*guaranteed to always start at exactly the same location*"

This statement is regardless of the access sizes.  For example, in SCEV AA:
// If they evaluate to the same expression, it's a MustAlias.
if (AS == BS)
  return MustAlias;

AS/BS are scev expressions for the pointers. So no check for the access 
size.

So, does must needs to check for access sizes?  If so, SCEV AA is buggy and 
the documentation needs tweaking.

Thanks,
Nuno


-----Original Message----- 
From: Daniel Berlin
Sent: Friday, July 14, 2017 9:25 PM
To: Hal Finkel
Cc: Davide Italiano ; Nuno Lopes ; LLVMdev
Subject: Re: [llvm-dev] PartialAlias: different start addresses


Yeah, we definitely don't implement this, and i'm not sure why we would.
The best guess i can come up with is that it's really attempting to say 
something like

1 = *A+4 (size 4)
2 = *A (size 8)

load from 2 PartialAlias 1 (not MustAlias)

Which is conservative but correct, and meant to ensure alias(1, 2) == 
alias(2, 1)

That would match with our definition of MustAlias, which would not allow 2 
MustAlias 1 either.

There isn't really a good set of consistent answers between most compilers 
when the sizes are different but memory addresses overlap.

IE it seems reasonable to say that a load of 2 MustAlias 1 but a load of 1 
PartialAlias 2. It also seems reasonable to declare these both PartialAlias.

In any case, our current implementation, from what i remember (i didn't 
trace *every* path), says:

if MemoryLocations are same size:
may if no idea
must if the same
partial if we can prove overlap

If MemoryLocations are different sizes:
may if no idea
partial if overlap
never must




On Fri, Jul 14, 2017 at 1:06 PM, Hal Finkel via llvm-dev 
<llvm-dev at lists.llvm.org> wrote:


On 07/14/2017 03:00 PM, Davide Italiano via llvm-dev wrote:
On Fri, Jul 14, 2017 at 12:50 PM, Nuno Lopes via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
Hi,

I going through the alias analysis documentation
(http://llvm.org/docs/AliasAnalysis.html) and noticed the following in the
definition of PartialAlias:
"
The PartialAlias response is used when the two memory objects are known to
be overlapping in some way, but *do not start at the same address*.
"

Is it really required that the objects do no start at the same address?  if
that's the case the AA algorithm would need to prove that.
I'm asking this because:
1) This condition seems very strict and I don't think it's met in a few
places I found by manual inspection
If I read the definition correctly, at least our Andersens' AA
implementation violates it.
see:

AliasResult CFLAndersAAResult::alias(const MemoryLocation &LocA,
                                      const MemoryLocation &LocB) {
   if (LocA.Ptr == LocB.Ptr)
     return LocA.Size == LocB.Size ? MustAlias : PartialAlias;


(i.e. the two objects are overlapping here *and* start at the same address).

I've never noticed that part of the definition myself. I'm fairly
certain
that's not what we actually implement.

-Hal

On 07/14/2017 04:37 PM, Nuno Lopes wrote:
> Thank you all for your replies.
> So here seems to be an agreement that the documentation for 
> PartialAlias is incorrect.
>
> Daniel: now you got me wondering about MustAlias. This is what the 
> docs say:
> "The MustAlias response may only be returned if the two memory objects
> are *guaranteed to always start at exactly the same location*"
>
> This statement is regardless of the access sizes.  For example, in 
> SCEV AA:
> // If they evaluate to the same expression, it's a MustAlias.
> if (AS == BS)
>  return MustAlias;
>
> AS/BS are scev expressions for the pointers. So no check for the 
> access size.
>
> So, does must needs to check for access sizes?  If so, SCEV AA is 
> buggy and the documentation needs tweaking.
I'm under the impression that there is code that depends on the size 
check, but I don't trust my recollection in this regard. SCEV AA is 
known to cause miscompiles, IIRC, maybe you just found out why ;)

  -Hal
>
> Thanks,
> Nuno
>
>
> -----Original Message----- From: Daniel Berlin
> Sent: Friday, July 14, 2017 9:25 PM
> To: Hal Finkel
> Cc: Davide Italiano ; Nuno Lopes ; LLVMdev
> Subject: Re: [llvm-dev] PartialAlias: different start addresses
>
>
> Yeah, we definitely don't implement this, and i'm not sure why we
would.
> The best guess i can come up with is that it's really attempting to 
> say something like
>
> 1 = *A+4 (size 4)
> 2 = *A (size 8)
>
> load from 2 PartialAlias 1 (not MustAlias)
>
> Which is conservative but correct, and meant to ensure alias(1, 2) == 
> alias(2, 1)
>
> That would match with our definition of MustAlias, which would not 
> allow 2 MustAlias 1 either.
>
> There isn't really a good set of consistent answers between most 
> compilers when the sizes are different but memory addresses overlap.
>
> IE it seems reasonable to say that a load of 2 MustAlias 1 but a load 
> of 1 PartialAlias 2. It also seems reasonable to declare these both 
> PartialAlias.
>
> In any case, our current implementation, from what i remember (i 
> didn't trace *every* path), says:
>
> if MemoryLocations are same size:
> may if no idea
> must if the same
> partial if we can prove overlap
>
> If MemoryLocations are different sizes:
> may if no idea
> partial if overlap
> never must
>
>
>
>
> On Fri, Jul 14, 2017 at 1:06 PM, Hal Finkel via llvm-dev 
> <llvm-dev at lists.llvm.org> wrote:
>
>
> On 07/14/2017 03:00 PM, Davide Italiano via llvm-dev wrote:
> On Fri, Jul 14, 2017 at 12:50 PM, Nuno Lopes via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I going through the alias analysis documentation
> (http://llvm.org/docs/AliasAnalysis.html) and noticed the following in 
> the
> definition of PartialAlias:
> "
> The PartialAlias response is used when the two memory objects are 
> known to
> be overlapping in some way, but *do not start at the same address*.
> "
>
> Is it really required that the objects do no start at the same 
> address?  if
> that's the case the AA algorithm would need to prove that.
> I'm asking this because:
> 1) This condition seems very strict and I don't think it's met in a
few
> places I found by manual inspection
> If I read the definition correctly, at least our Andersens' AA
> implementation violates it.
> see:
>
> AliasResult CFLAndersAAResult::alias(const MemoryLocation &LocA,
>                                      const MemoryLocation &LocB) {
>   if (LocA.Ptr == LocB.Ptr)
>     return LocA.Size == LocB.Size ? MustAlias : PartialAlias;
>
>
> (i.e. the two objects are overlapping here *and* start at the same 
> address).
>
> I've never noticed that part of the definition myself. I'm fairly 
> certain that's not what we actually implement.
>
> -Hal
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

> On 07/14/2017 04:37 PM, Nuno Lopes wrote:
>> Thank you all for your replies.
>> So here seems to be an agreement that the documentation for
PartialAlias
>> is incorrect.
>>
>> Daniel: now you got me wondering about MustAlias. This is what the docs
>> say:
>> "The MustAlias response may only be returned if the two memory
objects
>> are *guaranteed to always start at exactly the same location*"
>>
>> This statement is regardless of the access sizes.  For example, in SCEV
>> AA:
>> // If they evaluate to the same expression, it's a MustAlias.
>> if (AS == BS)
>>  return MustAlias;
>>
>> AS/BS are scev expressions for the pointers. So no check for the access
>> size.
>>
>> So, does must needs to check for access sizes?  If so, SCEV AA is buggy
>> and the documentation needs tweaking.
>
> I'm under the impression that there is code that depends on the size 
> check, but I don't trust my recollection in this regard. SCEV AA is
known
> to cause miscompiles, IIRC, maybe you just found out why ;)
It's true that the CFL AAs have this code:
if (LocA.Ptr == LocB.Ptr)
  return LocA.Size == LocB.Size ? MustAlias : PartialAlias;


I grepped for clients of MustAlias:
~/llvm/lib/Transforms $ grep -Rl MustAlias .
./ObjCARC/ObjCARCOpts.cpp
./ObjCARC/ProvenanceAnalysis.cpp
./Scalar/DeadStoreElimination.cpp
./Scalar/GVN.cpp
./Scalar/LICM.cpp
./Scalar/LoopVersioningLICM.cpp
./Scalar/MemCpyOptimizer.cpp
./Scalar/MergedLoadStoreMotion.cpp
./Scalar/NewGVN.cpp
./Utils/VNCoercion.cpp

I glanced over all the uses in these files and I couldn't find any usage 
that requires sizes to match.  Actually most clients check access sizes 
themselves. Most don't need equal sizes, just need one to be smaller than 
the other.

BTW, Basic AA doesn't check for size either:
if (isValueEqualInPotentialCycles(V1, V2))
  return MustAlias;

bool BasicAAResult::isValueEqualInPotentialCycles(const Value *V,
                                                  const Value *V2) {
  if (V != V2)
    return false;

  const Instruction *Inst = dyn_cast<Instruction>(V);
  if (!Inst)
    return true;
(...)
}

So if V1==V2, BasicAA will yield MustAlias.

I couldn't find any evidence that access sizes must be equal for 2 pointers 
to be MustAlias.  Unless someone can prove that statement wrong, we have to 
conclude that CFL* AAs are being too conservative by yielding PartialMatch 
when sizes don't match.

Nuno

P.S.: This is not to say that SCEV AA hasn't bugs: I think I found one by 
inspection; I've asked a colleague to review and will report it soon.  Not 
sure there's an easy fix for it, though.