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

Hello everyone,

I'm trying to use LLVM+Polly to obtain a polyhedral representation of
some loops to use later for passes I want to implement, but seems like
Polly will stop when reaching any statement that has non-affine access
functions of the loop bounds discarding the whole SCoP entirely.

What I would like to achieve is still getting some information for the
statements that , instead, have affine access functions, practically
"ignoring" the non-affine ones.

There is a way to do such a thing?

Thanks

Marcello

On 10/01/2011 03:26 PM, Marcello Maggioni wrote:
> Hello everyone,
>
> I'm trying to use LLVM+Polly to obtain a polyhedral representation of
> some loops to use later for passes I want to implement, but seems like
> Polly will stop when reaching any statement that has non-affine access
> functions of the loop bounds discarding the whole SCoP entirely.
>
> What I would like to achieve is still getting some information for the
> statements that , instead, have affine access functions, practically
> "ignoring" the non-affine ones.
>
> There is a way to do such a thing?
Hi Marcello,

this is not yet integrated into Polly, but a very valuable extension 
which I planned to integrate myself. In case you want to give it a try,
I am very glad to help you.

Basically, what you want is to adapt lib/Analysis/ScopDetection.cpp to
not reject non-affine constructs. In lib/Analysis/ScopInfo.cpp we would
need to adapt the translation into the polyhedral description. Instead 
of creating a map defining an affine must access (such as
Stmt[i,j] -> A[2 * i + j]), we create a map that defines a may access to 
all elements of the corresponding array. Something like 'Stmt[i,j] ->
A[x]'.

Cheers
Tobi

Hi Tobias,

thanks for the answer. I'll try to give a look to the code you pointed
me to , and I'll try to make the modification myself. I'm new to LLVM
and Polly, but the code of both seem clean and understandable, so I
hope to be able to do it myself. In case I'll ask here for support :)

Marcello

2011/10/1 Tobias Grosser <tobias at grosser.es>:
> On 10/01/2011 03:26 PM, Marcello Maggioni wrote:
>>
>> Hello everyone,
>>
>> I'm trying to use LLVM+Polly to obtain a polyhedral representation
of
>> some loops to use later for passes I want to implement, but seems like
>> Polly will stop when reaching any statement that has non-affine access
>> functions of the loop bounds discarding the whole SCoP entirely.
>>
>> What I would like to achieve is still getting some information for the
>> statements that , instead, have affine access functions, practically
>> "ignoring" the non-affine ones.
>>
>> There is a way to do such a thing?
>
> Hi Marcello,
>
> this is not yet integrated into Polly, but a very valuable extension which
I
> planned to integrate myself. In case you want to give it a try,
> I am very glad to help you.
>
> Basically, what you want is to adapt lib/Analysis/ScopDetection.cpp to
> not reject non-affine constructs. In lib/Analysis/ScopInfo.cpp we would
> need to adapt the translation into the polyhedral description. Instead of
> creating a map defining an affine must access (such as
> Stmt[i,j] -> A[2 * i + j]), we create a map that defines a may access to
all
> elements of the corresponding array. Something like 'Stmt[i,j] ->
A[x]'.
>
> Cheers
> Tobi
>
>