[all-commits] [llvm/llvm-project] 386f2c: Allow multi-member variants in DWARF (#139300)

[Tom Tromey via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 386f2ca03bdbcf5d08e5ec417bdad6c3de01acb6
      https://github.com/llvm/llvm-project/commit/386f2ca03bdbcf5d08e5ec417bdad6c3de01acb6
  Author: Tom Tromey <tromey at adacore.com>
  Date:   2025-05-12 (Mon, 12 May 2025)

  Changed paths:
    M llvm/docs/LangRef.rst
    M llvm/include/llvm/IR/DIBuilder.h
    M llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
    M llvm/lib/IR/DIBuilder.cpp
    M llvm/lib/IR/Verifier.cpp
    A llvm/test/DebugInfo/Generic/multi-variant.ll

  Log Message:
  -----------
  Allow multi-member variants in DWARF (#139300)

Currently, each variant in the variant part of a structure type can only
contain a single member. This was sufficient for Rust, where each
variant is represented as its own type.

However, this isn't really enough for Ada, where a variant can have
multiple members.

This patch adds support for this scenario. This is done by allowing the
use of DW_TAG_variant by DICompositeType, and then changing the DWARF
generator to recognize when a DIDerivedType representing a variant holds
one of these. In this case, the fields from the DW_TAG_variant are
inlined into the variant, like so:
```
 <4><7d>: Abbrev Number: 9 (DW_TAG_variant)
    <7e>   DW_AT_discr_value : 74
 <5><7f>: Abbrev Number: 7 (DW_TAG_member)
    <80>   DW_AT_name        : (indirect string, offset: 0x43): field0
    <84>   DW_AT_type        : <0xa7>
    <88>   DW_AT_alignment   : 8
    <89>   DW_AT_data_member_location: 0
 <5><8a>: Abbrev Number: 7 (DW_TAG_member)
    <8b>   DW_AT_name        : (indirect string, offset: 0x4a): field1
    <8f>   DW_AT_type        : <0xa7>
    <93>   DW_AT_alignment   : 8
    <94>   DW_AT_data_member_location: 8
```
Note that the intermediate DIDerivedType is still needed in this
situation, because that is where the discriminants are stored.



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