[LAI] Question about getDependences

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#1

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?

#2

Hi,

#3

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. :slight_smile:

#4

A gentle reminder.

#5

I think getMaxSafeDepDistBytes should only be used if canVectorizeMemory is true. It sounds like you might be asking LAI questions it isn’t really built for. If you are interested in information about dependences and do not require runtime-check generation, https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Analysis/DependenceAnalysis.h might give you the information you need.