Description

mlir-opt --allow-unregistered-dialect --canonicalize crashes with an assertion failure in RewriterBase::eraseOp ("expected 'op' to have no uses") when the canonicalizer attempts to fold or erase a builtin.unrealized_conversion_cast op whose result is its own operand (a self-referential cast) inside a test.graph_region. Graph regions allow uses to precede definitions, so a self-referential SSA value is valid IR in this context. The canonicalizer's folder for unrealized_conversion_cast does not account for this case and attempts to erase the op while it still has a use (itself), triggering the assertion.

Reproducer

module {
  test.graph_region {
    %0 = builtin.unrealized_conversion_cast %0 : i32 to i32
    "test.return"() : () -> ()
  }
}

Command

mlir-opt --allow-unregistered-dialect --canonicalize reproduce.mlir

Expected behavior

The canonicalizer should detect that the unrealized_conversion_cast op is self-referential and either leave it alone or handle it gracefully without attempting to erase an op that still has uses. It should not crash.

Actual behavior

mlir-opt: mlir/lib/IR/PatternMatch.cpp:156:
  virtual void mlir::RewriterBase::eraseOp(mlir::Operation*):
  Assertion `op->use_empty() && "expected 'op' to have no uses"' failed.
Aborted (core dumped)

Call chain

--canonicalize
  → Canonicalizer::runOnOperation
    → applyPatternsGreedily
      → GreedyPatternRewriteDriver::processWorklist
        → (unrealized_conversion_cast folder/canonicalization pattern)
          → RewriterBase::eraseOp       ← assertion: op still has uses

Root cause

builtin.unrealized_conversion_cast with identical input and output types (i32 to i32) is a candidate for folding — the canonicalizer replaces it with its input operand and erases it. In a normal region this is safe. However, in a graph region the cast's result %0 is also its own operand, so after the folder replaces all uses of %0 with the input (which is %0 itself), the op still has a use through its own operand. The canonicalizer then calls eraseOp without checking use_empty(), triggering the assertion. The fix should add a self-use check in the unrealized_conversion_cast canonicalization pattern, or in eraseOp's caller, to bail out when the op's operands include its own results.

Environment

• Tool: mlir-opt
• Pass: --canonicalize (with --allow-unregistered-dialect)
• Affected file: mlir/lib/IR/PatternMatch.cpp:156 (RewriterBase::eraseOp)
• Triggered by: builtin.unrealized_conversion_cast folder in --canonicalize