This patch enforces a restriction in the Vector dialect: the non-indexed
operands of `vector.insert` and `vector.extract` must no longer be 0-D
vectors. In other words, rank-0 vector types like `vector<f32>` are
disallowed as the source or result.
EXAMPLES
--------
The following are now **illegal** (note the use of `vector<f32>`):
```mlir
%0 = vector.insert %v, %dst[0, 0] : vector<f32> into vector<2x2xf32>
%1 = vector.extract %src[0, 0] : vector<f32> from vector<2x2xf32>
```
Instead, use scalars as the source and result types:
```mlir
%0 = vector.insert %v, %dst[0, 0] : f32 into vector<2x2xf32>
%1 = vector.extract %src[0, 0] : f32 from vector<2x2xf32>
```
Note, this change serves three goals. These are summarised below.
## 1. REDUCED AMBIGUITY
By enforcing scalar-only semantics when the result (`vector.extract`)
or source (`vector.insert`) are rank-0, we eliminate ambiguity
in interpretation. Prior to this patch, both `f32` and `vector<f32>`
were accepted.
## 2. MATCH IMPLEMENTATION TO DOCUMENTATION
The current behaviour contradicts the documented intent. For example,
`vector.extract` states:
> Degenerates to an element type if n-k is zero.
This patch enforces that intent in code.
## 3. ENSURE SYMMETRY BETWEEN INSERT AND EXTRACT
With the stricter semantics in place, it’s natural and consistent to
make `vector.insert` behave symmetrically to `vector.extract`, i.e.,
degenerate the source type to a scalar when n = 0.
NOTES FOR REVIEWERS
-------------------
1. Main change is in "VectorOps.cpp", where stricter type checks are
implemented.
2. Test updates in "invalid.mlir" and "ops.mlir" are minor cleanups to
remove now-illegal examples.
2. Lowering changes in "VectorToSCF.cpp" are the main trade-off: we now
require an additional `vector.extract` when a preceding
`vector.transfer_read` generates a rank-0 vector.
RELATED RFC
-----------
*
https://discourse.llvm.org/t/rfc-should-we-restrict-the-usage-of-0-d-vectors-in-the-vector-dialect
e9e25f02e6