[circt-verilog][hw] --ir-hw timeout in canonicalize after llhd-sig2reg on cyclic hw.struct_inject/hw.bitcast

[m2kar]

Title

[circt-verilog][hw] --ir-hw timeout in canonicalize after llhd-sig2reg on cyclic hw.struct_inject/hw.bitcast

Summary

circt-verilog --ir-hw hangs (timeout) on a minimal valid SystemVerilog testcase.

The timeout is inside circt-verilog LLHD->core lowering (not in arcilator).

Reproducer

module M(output logic O);
  typedef struct packed { logic a; logic b; } S;
  S s;
  always_comb s.b = 0;
  assign O = s.a;
endmodule

Commands

# Hangs
timeout 20s circt-verilog --ir-hw bug.sv

# Control: succeeds
timeout 20s circt-verilog --ir-llhd bug.sv

Actual vs Expected

• Actual: --ir-hw times out (exit code 124, no diagnostics).
• Expected: --ir-hw should terminate and emit HW/core IR (or emit a proper diagnostic), but not hang.

Where It Hangs

Pass bisection of the LLHD->core sequence shows first timeout at:

• canonicalize{...} right after llhd-sig2reg, cse

Evidence (IR before failing canonicalize)

%1 = hw.bitcast %3 : (!hw.struct<a: i1, b: i1>) -> i2
%2 = hw.bitcast %1 : (i2) -> !hw.struct<a: i1, b: i1>
%3 = hw.struct_inject %2["b"], %false : !hw.struct<a: i1, b: i1>
%a = hw.struct_extract %2["a"] : !hw.struct<a: i1, b: i1>

This creates a cycle: %3 -> %1 -> %2 -> %3.

Suspected Root Cause

Non-converging canonicalization on cyclic aggregate update pattern involving hw.struct_inject + hw.bitcast produced after llhd-sig2reg.

Likely relevant code:

• tools/circt-verilog/circt-verilog.cpp
• lib/Dialect/HW/HWOps.cpp (StructInjectOp::canonicalize, BitcastOp::canonicalize, StructExtractOp::canonicalize)

Environment

• CIRCT: firtool-1.139.0
• LLVM: 22.0.0git
• slang: 9.1.0+0
• OS: Linux x86_64