Lunderberg commented on PR #15577:
URL: https://github.com/apache/tvm/pull/15577#issuecomment-1708887108
@Hzfengsy Thank you, and I do think the value should be a `tir.PrimExpr` and
not just a `tir.Var` for a few reasons.
* API consistency when applying `BindSymbolicVars`. When a function is
updated using `BindSymbolicVars`, the allowed values for each parameter are
restricted, but the calling convention is otherwise kept identical. For
example, binding `{n: 16}` would reduce the set of allowed values for
`R.Tensor([n, n])` (the set of all square tensors) to `R.Tensor([16, 16])` (the
set of all tensors of shape `[16,16]`), and remains part of the function
signature. Similarly, a PrimValue parameter would be reduced from
`R.Prim(dtype='int64', value=n)` (the set of all 64-bit integers) to
`R.Prim(dtype='int64', value=16)` (the set containing only the integer 16), and
remains part of the function signature.
If `R.Prim` were instead restricted to a `tir.Var`, it wouldn't be
possible to write `R.Prim(dtype='int64', value=16)`, and the parameter would
need to be removed entirely. As a result, the calling scope would need to be
updated (e.g. from `my_func(tensor, 16)` to `my_func(tensor)`).
* API consistency with `R.Shape`. Similar to a `R.Prim`, a `R.Shape` can
contain only static parameters. However, for similar reasons, parameters of
type `R.Shape([n, n])` are specialized to `R.Shape([16,16])` and retained in
the function signature, even though they only have a single legal value of
`ShapeTuple([16,16])`.
* Usefulness as constituents in other StructInfo objects. Currently, many
types of StructInfo may internally contain a `PrimExpr` (e.g.
`TensorStructInfo`, `ShapeStructInfo`), and so visitors that inspect the struct
info for `PrimExpr` instances must handle each case independently, performing
the handoff from `StructInfoFunctor` to `tir::ExprFunctor`. If these were
instead expressed in terms of `PrimStructInfo`, this would only need to occur
in a single location. Because `TensorStructInfo` and `ShapeStructInfo` may
contain either static values or expressions in terms of other symbolic
variables, this potential usage would require `PrimStructInfo` to also contain
these values.
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