[Mlir-commits] [mlir] [MLIR][Conversion] PDLToPDLInterp: Don't shadow maximum depth position when creating constraint position. (PR #162265)

[llvmlistbot at llvm.org]

https://github.com/jumerckx created https://github.com/llvm/llvm-project/pull/162265

`getConstraintPredicates` generates an EqualToConstraint if a constraint result is used elsewhere. There is logic that decides at which position this should occur: `comparePosDepth(second, first)`
```cpp
  // Push the constraint to the furthest position.
  Position *pos = *llvm::max_element(allPositions, comparePosDepth);
  ResultRange results = op.getResults();
  PredicateBuilder::Predicate pred = builder.getConstraint(
      op.getName(), allPositions, SmallVector<Type>(results.getTypes()),
      op.getIsNegated());

  // For each result register a position so it can be used later
  for (auto [i, result] : llvm::enumerate(results)) {
    ConstraintQuestion *q = cast<ConstraintQuestion>(pred.first);
    ConstraintPosition *pos = builder.getConstraintPosition(q, i);
    auto [it, inserted] = inputs.try_emplace(result, pos);
    // If this is an input value that has been visited in the tree, add a
    // constraint to ensure that both instances refer to the same value.
    if (!inserted) {
      Position *first = pos;
      Position *second = it->second;
      if (comparePosDepth(second, first))
        std::tie(second, first) = std::make_pair(first, second);

      predList.emplace_back(second, builder.getEqualTo(first));
    }

```
I believe this is not the intended behavior since `pos` at this point is a ConstraintPosition. This means that, when `comparePosDepth` is later called, the depth will always be zero.

This pr makes it such that the original `pos` defined above is not shadowed by the new one. I believe this is the intended behavior because this way the position of the "deepest" operand to the native constraint will be considered.

>From f625ff78242e100292c12b83592b000d9e2768f9 Mon Sep 17 00:00:00 2001
From: jumerckx <31353884+jumerckx at users.noreply.github.com>
Date: Tue, 7 Oct 2025 13:31:57 +0200
Subject: [PATCH] don't shadow maximum depth position when creating constraint
 position.

---
 mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp b/mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp
index 39d4815dc73b7..f19536261f077 100644
--- a/mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp
+++ b/mlir/lib/Conversion/PDLToPDLInterp/PredicateTree.cpp
@@ -279,8 +279,8 @@ static void getConstraintPredicates(pdl::ApplyNativeConstraintOp op,
   // For each result register a position so it can be used later
   for (auto [i, result] : llvm::enumerate(results)) {
     ConstraintQuestion *q = cast<ConstraintQuestion>(pred.first);
-    ConstraintPosition *pos = builder.getConstraintPosition(q, i);
-    auto [it, inserted] = inputs.try_emplace(result, pos);
+    ConstraintPosition *constraintPos = builder.getConstraintPosition(q, i);
+    auto [it, inserted] = inputs.try_emplace(result, constraintPos);
     // If this is an input value that has been visited in the tree, add a
     // constraint to ensure that both instances refer to the same value.
     if (!inserted) {