llvm dev - Feb 2021 - [LAI] Question about getDependences

Hi,
> On 18 Feb 2021, at 18:17, Dangeti Tharun kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi,
> 
> I am using LAI->getDependences(), It returns a vector of Dependence
objects in a Loop.
> If the loop is un-analyzable or parallel, an empty list is returned.
> 
> for example:
> 
> Loop A:
> for (int i = 0; i < N; i ++) {
>         a[i] = c[i] + d[i];
>         b[i] = c[i] * d[i];
> }
> Loop B:
> for (int i = 0; i < N; i++) {
>      a[b[i]] = c[i] + d[i];
>      c[i] = c[i  - 1] + 1; 
> }
> 
> Loop A is parallel and Loop B has non-simple dependences(LAI bails out) for
both the cases we get an empty set of dependences.
> 
> Is there an API that can tell if LAI was successful in dependence
computation?
I think canVectorizeMemory should do what you want (possibly in combination with
getMaxSafeDepDistBytes

Cheers,
Florian
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Hi Florian,

Thanks for the reply.

We have tried a condition like this:

*if (LAI.getMaxSafeDepDistBytes() == -1ULL && !LAI.canVectorizeMemory())
{*
*  print("Unknown dependences in loop");*
*  return false;*
*}*

Looks like *LAI.getMaxSafeDepDistBytes() *is *not always *capturing the
distance for non-vectorizable loops.

This example from TSVC, is not vectorizable but dependences are analyzable.

for (int i = 1; i < LEN_1D - 1; i++) {
      a[i] = b[i - 1] + c[i] * d[i];
      b[i] = b[i + 1] - e[i] * d[i];
 }


* LAI.getMaxSafeDepDistBytes() is : (ULL)-1*
* LAI.canVectroizeMemory is : false*

Any more suggestions would be appreciated. :)




On Thu, Feb 18, 2021 at 11:54 PM Florian Hahn <florian_hahn at apple.com>
wrote:
> Hi,
>
> On 18 Feb 2021, at 18:17, Dangeti Tharun kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> I am using LAI->getDependences(), It returns a vector of Dependence
> objects in a Loop.
> If the loop is un-analyzable or parallel, an empty list is returned.
>
> *for example:*
>
> *Loop A:*
> for (int i = 0; i < N; i ++) {
>         a[i] = c[i] + d[i];
>         b[i] = c[i] * d[i];
> }
> *Loop B:*
> for (int i = 0; i < N; i++) {
>      a[b[i]] = c[i] + d[i];
>      c[i] = c[i  - 1] + 1;
> }
>
> Loop A is parallel and Loop B has non-simple dependences(LAI bails out)
> for both the cases we get an empty set of dependences.
>
> Is there an API that can tell if LAI was successful in dependence
> computation?
>
>
> I think canVectorizeMemory should do what you want (possibly in
> combination with getMaxSafeDepDistBytes
>
> Cheers,
> Florian
>

-- 
Regards,
DTharun

-- 


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