[mlir][vector] Add mask elimination transform (#99314)

This adds a new transform `eliminateVectorMasks()` which aims at
removing scalable `vector.create_masks` that will be all-true at
runtime. It attempts to do this by simply pattern-matching the mask
operands (similar to some canonicalizations), if that does not lead to
an answer (is all-true? yes/no), then value bounds analysis will be used
to find the lower bound of the unknown operands. If the lower bound is
>= to the corresponding mask vector type dim, then that dimension of the
mask is all true.

Note that the pattern matching prevents expensive value-bounds analysis
in cases where the mask won't be all true.

For example:
```mlir
%mask = vector.create_mask %dynamicValue, %c2 : vector<8x4xi1>
```
From looking at `%c2` we can tell this is not going to be an all-true
mask, so we don't need to run the value-bounds analysis for
`%dynamicValue` (and can exit the transform early).

Note: Eliminating create_masks here means replacing them with all-true
constants (which will then lead to the masks folding away).

1 files changed, 29 insertions, 0 deletions

diff --git a/mlir/test/lib/Dialect/Vector/TestVectorTransforms.cpp b/mlir/test/lib/Dialect/Vector/TestVectorTransforms.cpp
index 592e24af94d6..29c763b622e8 100644
--- a/mlir/test/lib/Dialect/Vector/TestVectorTransforms.cpp
+++ b/mlir/test/lib/Dialect/Vector/TestVectorTransforms.cpp
@@ -873,6 +873,33 @@ struct TestVectorLinearize final
       return signalPassFailure();
   }
 };
+
+struct TestEliminateVectorMasks
+    : public PassWrapper<TestEliminateVectorMasks,
+                         OperationPass<func::FuncOp>> {
+  MLIR_DEFINE_EXPLICIT_INTERNAL_INLINE_TYPE_ID(TestEliminateVectorMasks)
+
+  TestEliminateVectorMasks() = default;
+  TestEliminateVectorMasks(const TestEliminateVectorMasks &pass)
+      : PassWrapper(pass) {}
+
+  Option<unsigned> vscaleMin{
+      *this, "vscale-min", llvm::cl::desc("Minimum possible value of vscale."),
+      llvm::cl::init(1)};
+  Option<unsigned> vscaleMax{
+      *this, "vscale-max", llvm::cl::desc("Maximum possible value of vscale."),
+      llvm::cl::init(16)};
+
+  StringRef getArgument() const final { return "test-eliminate-vector-masks"; }
+  StringRef getDescription() const final {
+    return "Test eliminating vector masks";
+  }
+  void runOnOperation() override {
+    IRRewriter rewriter(&getContext());
+    eliminateVectorMasks(rewriter, getOperation(),
+                         VscaleRange{vscaleMin, vscaleMax});
+  }
+};
 } // namespace
 
 namespace mlir {
@@ -919,6 +946,8 @@ void registerTestVectorLowerings() {
   PassRegistration<TestVectorEmulateMaskedLoadStore>();
 
   PassRegistration<TestVectorLinearize>();
+
+  PassRegistration<TestEliminateVectorMasks>();
 }
 } // namespace test
 } // namespace mlir