[MooreToCore] Add UnionType conversion support (Fixes #9570)

[m2kar]

Summary

This PR adds complete support for SystemVerilog union type conversion in the MooreToCore pass, fixing a crash when processing union types with circt-verilog.

Fixes #9570

Problem

When processing SystemVerilog code containing union types, circt-verilog --ir-hw would crash with an assertion failure because the MooreToCore conversion pass lacked support for union type conversion.

Example crash (from #9570):

$ circt-verilog --ir-hw bug.sv
circt-verilog: assertion failed

Solution

This PR implements:

1. Type Converters: Added converters for UnionType and UnpackedUnionType to hw::UnionType
2. Operation Converters: Added conversion patterns for:
   • UnionCreateOp → hw::UnionCreateOp
   • UnionExtractOp → hw::UnionExtractOp
   • UnionExtractRefOp → llhd::SigStructExtractOp
3. Bug Fix: Fixed UnionCreateOp::verify() which was incorrectly comparing member.type with resultType instead of inputType
4. LLHD Support: Extended SigStructExtractOp to support both struct and union types
5. Test Coverage: Added comprehensive union conversion test in basic.mlir

Changes

• include/circt/Dialect/LLHD/LLHDOps.td: Updated type constraint to accept both structs and unions
• lib/Conversion/MooreToCore/MooreToCore.cpp: Added union type converters and operation patterns
• lib/Dialect/LLHD/IR/LLHDOps.cpp: Extended SigStructExtractOp for union support
• lib/Dialect/Moore/MooreOps.cpp: Fixed verifier bug in UnionCreateOp
• test/Conversion/MooreToCore/basic.mlir: Added union conversion test

Testing

New Tests

• Added @Union test module covering all union operations
• Tests union_create, union_extract, and union_extract_ref
• Includes both value and reference operations

Verification

# All existing tests pass
$ ninja -C build check-circt
✅ All tests pass

# Original bug is fixed
$ ./build/bin/circt-verilog --ir-hw bug.sv
✅ Successfully converts without crash

Design Decisions

1. Mirrored Struct Patterns: Union converters follow the same structure as existing struct converters for consistency
2. Reused LLHD Ops: SigStructExtractOp now handles both structs and unions rather than creating a separate union-specific op
3. Zero Offset: Packed unions use offset=0 for all fields (standard union semantics)

Backward Compatibility

✅ All changes are backward compatible:

• Existing struct operations unchanged
• LLHD ops gracefully handle both types
• No API changes

Related Issues

Fixes #9570 - Crash when processing SystemVerilog union types in MooreToCore conversion.

---

Checklist

• Added comprehensive test coverage
• All existing tests pass
• Code follows CIRCT conventions
• Code formatted with clang-format
• No debug output or unnecessary changes
• Backward compatible

[tobiasgrosser]

LGTM, except https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements.

This requires more input from @fabianschuiki @maerhart @TaoBi22 @KavyaChopra04 or @fzi-hielscher.

[m2kar]

LGTM, except https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements.

This requires more input from @fabianschuiki @maerhart @TaoBi22 @KavyaChopra04 or @fzi-hielscher.

Thank you for the corrections. I am still familiarizing myself with the LLVM project and appreciate the support.

[fzi-hielscher]

Is this conversion necessary? populateHWModuleLikeTypeConversionPattern should already take care of module signatures.

[m2kar]

I appreciate your correction. You're right, the new conversion isn't needed, so I've removed it.
dad8b19

[fzi-hielscher]

Thanks