[mlir][Tensor] Canonicalize dynamic tensor.pad ops with constant inputs

[gpetters94]

Summary

This patch adds a canonicalization pattern for tensor.pad which changes the output type to static at each dimension where the input shape is static and the high and low operands are constants. This corrects an issue arising in Torch-MLIR where pad ops would sometimes introduce dynamic shapes unnecessarily.

[raikonenfnu]

In general looks good to me, although a couple NITS as well as this patterns needs a test.
Please add a lit test to test this around here https://github.com/llvm/llvm-project/blob/cb05c2ffc79eefe74c569263e27fcb5fad167ba3/mlir/test/Dialect/Tensor/canonicalize.mlir#L1111

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

2842

Please add a LIT test to ensure this pattern is always working somewhere here https://github.com/llvm/llvm-project/blob/cb05c2ffc79eefe74c569263e27fcb5fad167ba3/mlir/test/Dialect/Tensor/canonicalize.mlir#L1111

2853

NIT: You missed a full stop after the end of sentence.
Extract the static info from the high and low operands.

2856

NIT: Can we do for (auto operand : padTensorOp.getOperands()) { ...

2861

NIT: Can we do ShapedType::kDynamic instead of -1.

2872

NIT: You missed a full stop after the end of sentence.

2882

NIT: You missed a full stop after the end of sentence.

2886

NIT: Can we do ShapedType::kDynamic instead of -1. for better readability.

2891

NIT: You missed a full stop after the end of sentence.

[raikonenfnu]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

2842

I'd also add a test where a padOp actually needs to output dynamic, stays dynamic.

[gpetters94]

Fixed the failing tests.

[raikonenfnu]

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

2914

For some reason the comment on test dissapeared. Please add two lit test:

1. test where a padOp actually needs to output dynamic, stays dynamic
2. test where a padOp is outputting dynamic result but can actually be static.

Should be added somewhere here: https://github.com/llvm/llvm-project/blob/cb05c2ffc79eefe74c569263e27fcb5fad167ba3/mlir/test/Dialect/Tensor/canonicalize.mlir#L1111

[raikonenfnu]

Style NIT: Please update the commit name to have [mlir][Tensor] prefix.

[gpetters94]

Added a test, fixed bug in the code. Should be good to go now.

[raikonenfnu]

LGTM!

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

   }
 };
 
+struct FoldStaticPadding : public OpRewritePattern<PadOp> {
+  using OpRewritePattern<PadOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(PadOp padTensorOp,
+                                PatternRewriter &rewriter) const override {
+    Value input = padTensorOp.getSource();
+    if (!input.getType().isa<RankedTensorType>())
+      return failure();
+    auto inputDims = input.getType().cast<RankedTensorType>().getShape();
+    auto inputRank = inputDims.size();
+
+    // Extract the static info from the high and low operands
+    auto operands = padTensorOp.getOperands();
+    SmallVector<int64_t> constOperands;
+    for (auto op : operands) {
+      if (op.getType().isa<TensorType>())
+        continue;
+      APSInt intOp;
+      if (!matchPattern(op, m_ConstantInt(&intOp))) {
+        constOperands.push_back(-1);
+        continue;
+      }
+      constOperands.push_back(intOp.getExtValue());
+    }
+
+    if (!padTensorOp.getResult().getType().isa<RankedTensorType>())
+      return failure();
+    auto outputDims =
+        padTensorOp.getResult().getType().cast<RankedTensorType>().getShape();
+
+    // Verify the op is well-formed
+    if (inputDims.size() != outputDims.size() ||
+        inputDims.size() != (constOperands.size() / 2))
+      return failure();
+
+    auto staticLow = ArrayRef<int64_t>(constOperands.begin(),
+                                       constOperands.begin() + inputRank);
+    auto staticHigh = ArrayRef<int64_t>(constOperands.begin() + inputRank,
+                                        constOperands.end());
+
+    // Calculate the output sizes with the static information
+    SmallVector<int64_t> newOutDims;
+    for (size_t i = 0; i < inputRank; i++)
+      newOutDims.push_back((staticLow[i] < 0 || staticHigh[i] < 0
+                                ? -1
+                                : inputDims[i] + staticLow[i] + staticHigh[i]));
+    if (SmallVector<int64_t>(outputDims) == newOutDims)
+      return failure();
+
+    // Rewrite the op using the new static type
+    auto newResultType = RankedTensorType::get(
+        newOutDims, padTensorOp.getType().getElementType());
+    auto newOp = rewriter.create<PadOp>(
+        padTensorOp->getLoc(), newResultType, input, padTensorOp.getLow(),
+        padTensorOp.getHigh(), staticLow, staticHigh, padTensorOp.getNofold());
+
+    IRMapping mapper;
+    padTensorOp.getRegion().cloneInto(&newOp.getRegion(), mapper);
+    rewriter.replaceOpWithNewOp<tensor::CastOp>(
+        padTensorOp, padTensorOp.getResultType(), newOp);
+
+    return success();
+  }
+};
+
 } // namespace
 
 void PadOp::getCanonicalizationPatterns(RewritePatternSet &results,
                                         MLIRContext *context) {
   results.add<FoldStaticZeroPadding, FoldSourceTensorCast, FoldTargetTensorCast,
-              FoldOrthogonalPaddings>(context);
+              FoldOrthogonalPaddings, FoldStaticPadding>(context);
 }
 
 /// Return the padding value of the PadOp if it constant. In this context,