llvm dev - Nov 2011 - [LLVMdev] How to make Polly ignore some non-affine memory accesses

On 11/14/2011 01:24 AM, Marcello Maggioni wrote:
> Hi Tobias.
>
> I worked on enabling Polly accepting non affine memory accesses and I
> produced a patch.
Great.
> I saw that there were a lot of updates in Polly recently, so I had to
> redo a lot of the work I did and that slowed me quite a bit.
Ups, sorry! However, I believe without these changes detecting 
non-affine memory access functions would have been a lot more complicated.
> I tested the patch on some programs and it seems to work and not to
> break anything, but you know the topic much better than me, so if you
> find something wrong please tell me! (at least it shouldn't produce a
> crash I believe ... I tested the patch quite a lot from that point of
> view ...). The patch is in the attachment to this mail and was last
> tested on LLVM SVN revision 144502 .
OK. Review is inlined in the patch.
> I also found a bug in polly or ISL/GMP libraries. If polly encounters
> an array access of type:
>
> A[i][i]
>
> it enters an infinite loop (I believe it is an infinite loop because I
> waited 5 minutes and it didn't terminate compiling a code that was
> like 15 lines long).  I tried to track down the problem with gdb , but
> it seems to be in some gmp function called by ISL of which I'm not a
> very expert of.
[remove backtrace & code]
> I actually don't know if it is an ISL bug or an improper use by polly.
> To compile isl I use gmp-5.0.2
I will look into this.
> --- ./include/polly/ScopDetection.h	2011-11-13 02:37:59.000000000 +0100
> +++ ../llvm2/tools/polly/./include/polly/ScopDetection.h	2011-11-13
01:11:17.000000000 +0100
> @@ -145,6 +145,8 @@
>     /// @return True if the call instruction is valid, false otherwise.
>     static bool isValidCallInst(CallInst&CI);
>
> +  const Value *GetBaseValue(const SCEV *S)  const;
Why is this function needed? For me it looks like it implements
the same as

SCEVValue *Operand = dyn_cast<SCEVValue>(getPointerOperand())

if (!Operand)
   return invalid Value;

return Operand->getValue();
>     /// @brief Check if a memory access can be part of a Scop.
>     ///
>     /// @param Inst The instruction accessing the memory.
> --- ./include/polly/TempScopInfo.h	2011-11-13 02:37:59.000000000 +0100
> +++ ../llvm2/tools/polly/./include/polly/TempScopInfo.h	2011-11-13
02:34:47.000000000 +0100
> @@ -45,12 +45,13 @@
>   private:
>     unsigned ElemBytes;
>     TypeKind Type;
> +  bool is_affine;
I think IsAffine matches more the LLVM coding conventions.
>
>   public:
>     explicit IRAccess (TypeKind Type, const Value *BaseAddress,
> -                     const SCEV *Offset, unsigned elemBytes)
> +                     const SCEV *Offset, unsigned elemBytes, bool affine)
'affine' should start with an uppercase letter. Polly itself has some 
inconsistent naming, but we started to follow the LLVM coding 
conventions since a while.
>       : BaseAddress(BaseAddress), Offset(Offset),
> -      ElemBytes(elemBytes), Type(Type) {}
> +      ElemBytes(elemBytes), Type(Type), is_affine(affine) {}
>
>     enum TypeKind getType() const { return Type; }
>
> @@ -60,7 +61,10 @@
>
>     unsigned getElemSizeInBytes() const { return ElemBytes; }
>
> +  bool isAffine() const { return is_affine; }
> +
>     bool isRead() const { return Type == READ; }
> +
>   };
>
>   class Comparison {
> --- ./lib/Analysis/ScopDetection.cpp	2011-11-13 02:38:06.000000000 +0100
> +++ ../llvm2/tools/polly/./lib/Analysis/ScopDetection.cpp	2011-11-13
01:28:00.000000000 +0100
> @@ -226,6 +226,24 @@
>     return false;
>   }
>
> +const Value *ScopDetection::GetBaseValue(const SCEV *S)  const {
> +  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
> +    // In an addrec, assume that the base will be in the start, rather
> +    // than the step.
> +    return GetBaseValue(AR->getStart());
> +  } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
> +    // If there's a pointer operand, it'll be sorted at the end of
the list.
> +    const SCEV *Last = A->getOperand(A->getNumOperands()-1);
> +    if (Last->getType()->isPointerTy())
> +      return GetBaseValue(Last);
> +  } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
> +    // This is a leaf node.
> +    return U->getValue();
> +  }
> +  // No Identified object found.
> +  return 0;
> +}
As remarked earlier, I believ this can be replaced by getPointerOperand().
>   bool ScopDetection::isValidMemoryAccess(Instruction&Inst,
>                                           DetectionContext&Context)
const {
>     Value *Ptr = getPointerOperand(Inst);
> @@ -236,8 +254,12 @@
>     BasePointer =
dyn_cast<SCEVUnknown>(SE->getPointerBase(AccessFunction));
>
>     if (!BasePointer)
> -    INVALID(AffFunc, "No base pointer");
> -
> +  {
> +//    INVALID(AffFunc, "No base pointer");
> +    BaseValue = (Value*) GetBaseValue(AccessFunction);
> +  }
> +  else
> +  {
>     BaseValue = BasePointer->getValue();
I don't see a need for any change here. Both functions get the base 
pointer and we should fail, if we cannot derive a base pointer. Do you 
have a test case where getPointerBase() does not yield a valid base 
pointer, but GetBaseValue(AccessFunction) does?
>     if (isa<UndefValue>(BaseValue))
> @@ -245,8 +267,9 @@
>
>     AccessFunction = SE->getMinusSCEV(AccessFunction, BasePointer);
>
> -  if (!isAffineExpr(&Context.CurRegion, AccessFunction, *SE,
BaseValue))
> -    INVALID(AffFunc, "Bad memory address "<< 
*AccessFunction);
> +    //if (!isAffineExpr(&Context.CurRegion, AccessFunction, *SE,
BaseValue))
> +    //  INVALID(AffFunc, "Bad memory address "<< 
*AccessFunction);
Can you leave this and add a command line option
'-polly-allow-nonaffine' to allow non affine access functions.
> +  }
>
>     // FIXME: Alias Analysis thinks IntToPtrInst aliases with alloca
instructions
>     // created by IndependentBlocks Pass.
> --- ./lib/Analysis/ScopInfo.cpp	2011-11-13 02:38:06.000000000 +0100
> +++ ../llvm2/tools/polly/./lib/Analysis/ScopInfo.cpp	2011-11-13
02:36:12.000000000 +0100
> @@ -310,9 +310,13 @@
>     Type = Access.isRead() ? Read : Write;
>     statement = Statement;
>
> -  isl_pw_aff *Affine = SCEVAffinator::getPwAff(Statement,
Access.getOffset());
>     BaseAddr = Access.getBase();
>
> +  if (Access.isAffine())
> +  {
This should be

if (Access.isAffine()) {

> +
> +  isl_pw_aff *Affine = SCEVAffinator::getPwAff(Statement,
Access.getOffset());
> +
>     setBaseName();
>
>     // Devide the access function by the size of the elements in the array.
> @@ -334,6 +338,12 @@
>     AccessRelation = isl_map_set_tuple_name(AccessRelation, isl_dim_out,
>                                             getBaseName().c_str());
>   }
> +  else
> +  {
this should be
     } else {
> +    Type = MayWrite;
You are right, we should use may-accesses here. But why always setting 
the type to may-write? A read should remain a read (as there is no 
difference between read and may-read).
> +    AccessRelation =
isl_map_from_basic_map(createBasicAccessMap(Statement));
> +  }
> +}
>
>   void MemoryAccess::realignParams() {
>     isl_space *ParamSpace = statement->getParent()->getParamSpace();
> --- ./lib/Analysis/TempScopInfo.cpp	2011-11-13 02:38:06.000000000 +0100
> +++ ../llvm2/tools/polly/./lib/Analysis/TempScopInfo.cpp	2011-11-13
02:35:22.000000000 +0100
> @@ -98,13 +98,24 @@
>          
dyn_cast<SCEVUnknown>(SE->getPointerBase(AccessFunction));
>
>         assert(BasePointer&&  "Could not find base
pointer");
> -
>         AccessFunction = SE->getMinusSCEV(AccessFunction, BasePointer);
> +
> +      if (isAffineExpr(&R, AccessFunction, *SE,
BasePointer->getValue()))
> +      {
Put the '{' in the previous line.
> +
>         Functions.push_back(std::make_pair(IRAccess(Type,
>                                                    
BasePointer->getValue(),
> -                                                  AccessFunction, Size),
> +                                                  AccessFunction, Size,
true),
>                                            &Inst));
>       }
> +      else
> +      {
This should be

} else {
> +         Functions.push_back(std::make_pair(IRAccess(Type,
> +                                                 
BasePointer->getValue(),
> +                                                  AccessFunction, Size,
false),
> +&Inst));
> +      }
> +    }
>     }
>
>     if (Functions.empty())
Besides the comments above, the patch looks great. It is a nice 
improvement to Polly. Can you repost it after the comments are 
addressed? Also could you include a minimal test case in the patch, that 
verifies this features is working correctly.

Thanks
Tobi

P.S.: Please post patches to the mailing lists.

On 11/14/2011 02:45 PM, Marcello Maggioni wrote:
> 2011/11/14 Tobias Grosser<tobias at grosser.es>:
>> On 11/14/2011 01:24 AM, Marcello Maggioni wrote:
>>>
>>> Hi Tobias.
>>>
>>> I worked on enabling Polly accepting non affine memory accesses and
I
>>> produced a patch.
>>
>> Great.
>>
>>> I saw that there were a lot of updates in Polly recently, so I had
to
>>> redo a lot of the work I did and that slowed me quite a bit.
>>
>> Ups, sorry! However, I believe without these changes detecting
non-affine
>> memory access functions would have been a lot more complicated.
>
> Indeed! The patch almost shrinked to half the code it was before those
> changes! Great work.
Thanks. (You motivated me to fix this, when you asked about non-affine 
access functions)
>> this should be
>>     } else {
>>
>>> +    Type = MayWrite;
>>
>> You are right, we should use may-accesses here. But why always setting
the
>> type to may-write? A read should remain a read (as there is no
difference
>> between read and may-read).
>
> You are right, in the patch for the old version that was handled
> correctly, I don't know how this reverted back to this way in the
> patch for the new version , I'll get it fixed.
Wonderful. Very cool that you realized I added MayWrite exactly for this 
case.
>>> +         Functions.push_back(std::make_pair(IRAccess(Type,
>>> +
>>>   BasePointer->getValue(),
>>> +                                                  AccessFunction,
Size,
>>> false),
>>> +&Inst));
>>> +      }
>>> +    }
>>>     }
>>>
>>>     if (Functions.empty())
>
> Yeah, there are quite a few stylistic problems actually, my bad!! I'll
> get all the style problems fixed as fast as possible! Sorry for that.
Alright. I failed on this myself, which is why Polly is not everywhere 
perfect. Still, we should try to follow the LLVM style guide.

BTW, if you find any problems in patches I commit or in Polly itself or 
if your find anything that you don't understand, please go ahead and let 
me know. Even if it is correct and you just did not get it, I think this 
is a documentation bug and should be fixed.
>> Besides the comments above, the patch looks great. It is a nice
improvement
>> to Polly. Can you repost it after the comments are addressed? Also
could you
>> include a minimal test case in the patch, that verifies this features
is
>> working correctly.
>
> Thank you, for your time Tobias explaining all the issues with the
> patch so throughly and in a clear manner .
You did a good job working on the patch. It is great if people go ahead, 
work for themselves and send in a patch that works. Even better if the 
right approach was taken and just some smaller remarks are necessary.
> Should I make a specific subdirectory for the test case?
No. I think you can put one test case in test/ScopInfo that runs 
-polly-scops -analyze and one in test/CodeGeneration that ensures that 
the correct code is generated.
> >> Thanks
>> Tobi
>>
>> P.S.: Please post patches to the mailing lists.
> What do you mean with post patches to the mailing lists? Do you mean
> in llvm-dev or llvm-commit?
> The previous patch has been posted on llvm-dev, was that wrong?
Normally patches should either be posted on llvm-commit or 
polly-dev at googlegroups.com.

In this case it seems you sent two separate emails, one to me and 
another to Sebastian and llvmdev. I just saw the one to me and did not 
see that another one was sent to llvmdev. Just send next time one email 
with all of us copied.

Cheers
Tobi