[mlir][vector] Avoid setting padding by default to `0` in `vector.transfer_read` prefer `ub.poison` (#146088)

Context:
`vector.transfer_read` always requires a padding value. Most of its
builders take no `padding` value and assume the safe value of `0`.
However, this should be a conscious choice by the API user, as it makes
it easy to introduce bugs.
For example, I found several occasions while making this patch that the
padding value was not getting propagated (`vector.transfer_read` was
transformed into another `vector.transfer_read`). These bugs, were
always caused because of constructors that don't require specifying
padding.

Additionally, using `ub.poison` as a possible default value is better,
as it indicates the user "doesn't care" about the actual padding value,
forcing users to specify the actual padding semantics they want.

With that in mind, this patch changes the builders in
`vector.transfer_read` to always having a `std::optional<Value> padding`
argument. This argument is never optional, but for convenience users can
pass `std::nullopt`, padding the transfer read with `ub.poison`.

---------

Signed-off-by: Fabian Mora <fabian.mora-cordero@amd.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/mlir/lib/Dialect/Vector/Transforms/VectorTransferOpTransforms.cpp b/mlir/lib/Dialect/Vector/Transforms/VectorTransferOpTransforms.cpp
index 785a8aaf3f0a..efdae93e730b 100644
--- a/mlir/lib/Dialect/Vector/Transforms/VectorTransferOpTransforms.cpp
+++ b/mlir/lib/Dialect/Vector/Transforms/VectorTransferOpTransforms.cpp
@@ -660,7 +660,8 @@ public:
     VectorType flatVectorType = VectorType::get({vectorType.getNumElements()},
                                                 vectorType.getElementType());
     vector::TransferReadOp flatRead = rewriter.create<vector::TransferReadOp>(
-        loc, flatVectorType, collapsedSource, collapsedIndices, collapsedMap);
+        loc, flatVectorType, collapsedSource, collapsedIndices,
+        transferReadOp.getPadding(), collapsedMap);
     flatRead.setInBoundsAttr(rewriter.getBoolArrayAttr({true}));
 
     // 4. Replace the old transfer_read with the new one reading from the