Dynamic backend tests failure in IndexExpr

[mikeessen]

Backend dynamic tests for Pad (test_constant_pad_cpu, test_edge_pad_cpu, and test_reflect_pad_cpu) fail when the dynamic shape is -1:-1 for testing all inputs.

Assertion failed: (indexExprObj), function getObjPtr, file IndexExpr.cpp, line 439.

The tests pass if src/Dialect/Mlir/IndexExprBuilder.cpp line 200 the following condition is removed:

  if (size == ShapedType::kDynamic || i >= (uint64_t)size) {
    return UndefinedIndexExpr();
  }

These tests also pass if the dynamic shape is 0:-1. PR #2754 was delivered to to update the dynamic shape to 0:-1.

[mikeessen]

@AlexandreEichenberger is the test condition in IndexExprBuilder.cpp noted above appropriate given the dynamic tensor sizes?

[AlexandreEichenberger]

Typically not, unless it is very controlled.

Basically, the code is trying to get a value from an array for an index I when we don't know the array size. Say array is defined as a:[?xi64] and we are asking to get a[10]. The compiler doesn't know if the array has 11 values. So up to now, we only allowed a call to this method when the array was of known size.

But if you were to look at the code, and it is really like this (aka careful code)

%dim = dim %a: type<?xi64>
for %I =0; %I<%dim, %I++) {
  %v = "getValFromArray(%a, %i)

or you know that another parameter to the operation that you are lowering, say %axis must be by definition in the range 0 ... %dim even if that dim is only known at runtime.

then maybe the code might work, but I would have to look at the generated code much more carefully.

So if you are in that later situation, you could try to add a flag dynamicArraySizeAllowed, enable it for this case, and see what the output is.

In addition, if the current "user" of that call expected a constant value, then the code will also not work. Aka if the algorithm expected to know %a[10] == 3 and then use that value 3 explicitly, now having an IndexExpr = "runtime value" would not work. Namely, the algorithm using that value would have to be rewritten to accommodate a value only known at runtime.