The `OneDimMultiReductionToTwoDim` pattern had some issues. For the
input program:
```mlir
func.func @rank1_multi_reduction(%arg0: vector<8xf32>, %acc: f32) -> f32 {
%0 = vector.multi_reduction <add>, %arg0, %acc [0] : vector<8xf32> to f32
return %0 : f32
}
```
* when lowering using the inner-parallel strategy, the compiler would
essentially produce scalar code:
```mlir
func.func @rank1_multi_reduction(%arg0: vector<8xf32>, %arg1: f32) -> f32 {
%0 = vector.shape_cast %arg0 : vector<8xf32> to vector<1x8xf32>
%1 = vector.broadcast %arg1 : f32 to vector<1xf32>
%2 = vector.transpose %0, [1, 0] : vector<1x8xf32> to vector<8x1xf32>
%3 = vector.extract %2[0] : vector<1xf32> from vector<8x1xf32>
%4 = arith.addf %3, %1 : vector<1xf32>
%5 = vector.extract %2[1] : vector<1xf32> from vector<8x1xf32>
%6 = arith.addf %5, %4 : vector<1xf32>
... (repeats for all 8 elements) ...
%17 = vector.extract %2[7] : vector<1xf32> from vector<8x1xf32>
%18 = arith.addf %17, %16 : vector<1xf32>
%19 = vector.extract %18[0] : f32 from vector<1xf32>
return %19 : f32
}
```
* when lowering using the inner-reduction strategy, the compiler would
first unnecessarily transform it into a 2-D multi_reduction operation
<1x8xf32> and then extract an <8xf32> vector and apply reduction. The
canonicalization and folding would lead to the following final result:
```mlir
func.func @rank1_multi_reduction(%arg0: vector<8xf32>, %arg1: f32) -> f32 {
%0 = vector.reduction <add>, %arg0, %arg1 : vector<8xf32> into f32
return %0 : f32
}
```
Now, after this change:
* when lowering the compiler now produces for both strategies in one
step.
```
func.func @rank1_multi_reduction(%arg0: vector<8xf32>, %arg1: f32) -> f32 {
%0 = vector.reduction <add>, %arg0, %arg1 : vector<8xf32> into f32
return %0 : f32
}
```
This pattern is also useful for an ongoing refactoring that is happening
in the multi_reduction patterns. It is the only pattern that increases
multi_reduction in rank and would lead to an infinite loop when
attempting to reach a fixed point once we generalize other unrolling
patterns.
Assisted-by: Claude
613a5c555e