[Comb] Fix assertion failure in extractConcatToConcatExtract pattern (Fixes #9573)

[m2kar]

Summary

Fixes #9573

The extractConcatToConcatExtract canonicalization pattern crashed with an assertion failure when attempting to replace an ExtractOp with a single value using replaceOpAndCopyNamehint().

Root Cause: When reverseConcatArgs.size() == 1, the code created a new ExtractOp and immediately tried to replace the original op with it. This led to an inconsistent IR state because replaceOpAndCopyNamehint() calls rewriter.replaceOp() which expects the operation to be in a valid state for erasure.

Fix: Remove the special case for single-element results and always use replaceOpWithNewOpAndCopyNamehint<ConcatOp>, which performs atomic operation creation and replacement. Single-operand ConcatOps are automatically optimized away by the ConcatOp folder (line 1726-1727 returns getOperand(0) for single operand).

Test plan

• Existing test extractCatOnSinglePartialElement in test/Dialect/Comb/canonicalization.mlir:460-472 covers the crash scenario
• All Comb dialect tests pass
• All HW dialect tests pass

[uenoku]

Could you add a test? I'm not certain why first one causes an issue.

[m2kar]

Could you add a test? I'm not certain why first one causes an issue.

Here is a test case. Here are details you should look at: #9573.

module test_module(
  input  logic [1:0] in,
  output logic [3:0] out
);

  // Continuous assignments to output bits
  assign out[0] = in[0] ^ in[1];
  assign out[3] = 1'h0;

  // Combinational always block with implicit sensitivity list
  always @* begin
    out[1] = in[0] & in[1];
    out[2] = in[0] | in[1];
  end

endmodule

[m2kar]

I must test it completely and make sure that all the issues have been resolved. This PR is therefore in draft status until the check is approved.

[uenoku]

Sorry I meant to add a test to test/Dialect/Comb/canonicalization.mlir. You can extract IR before the crashing canonicalizer with --mlir-print-ir-after-all.

[m2kar]

Sorry I meant to add a test to test/Dialect/Comb/canonicalization.mlir. You can extract IR before the crashing canonicalizer with --mlir-print-ir-after-all.

OK, I will add it.