[BPF] Support for DW_TAG_variant_part in BTF generation

Variant part, represented by `DW_TAG_variant_part` is a structure with a
discriminant and different variants, from which only one can be active
and valid at the same time. The discriminant is the main difference
between variant parts and unions represented by `DW_TAG_union` type.

Variant parts are used by Rust enums, which look like:

```rust
pub enum MyEnum {
    First { a: u32, b: i32 },
    Second(u32),
}
```

This type's debug info is the following `DICompositeType` with
`DW_TAG_structure_type` tag:

```llvm
!4 = !DICompositeType(tag: DW_TAG_structure_type, name: "MyEnum",
     scope: !2, file: !5, size: 96, align: 32, flags: DIFlagPublic,
     elements: !6, templateParams: !16,
     identifier: "faba668fd9f71e9b7cf3b9ac5e8b93cb")
```

With one element being also a `DICompositeType`, but with
`DW_TAG_variant_part` tag:

```llvm
!6 = !{!7}
!7 = !DICompositeType(tag: DW_TAG_variant_part, scope: !4, file: !5,
     size: 96, align: 32, elements: !8, templateParams: !16,
     identifier: "e4aee046fc86d111657622fdcb8c42f7", discriminator: !21)
```

Which has a discriminator:

```llvm
!21 = !DIDerivedType(tag: DW_TAG_member, scope: !4, file: !5,
      baseType: !13, size: 32, align: 32, flags: DIFlagArtificial)
```

Which then holds different variants as `DIDerivedType` elements with
`DW_TAG_member` tag:

```llvm
!8 = !{!9, !17}
!9 = !DIDerivedType(tag: DW_TAG_member, name: "First", scope: !7,
     file: !5, baseType: !10, size: 96, align: 32, extraData: i32 0)
!10 = !DICompositeType(tag: DW_TAG_structure_type, name: "First",
      scope: !4, file: !5, size: 96, align: 32, flags: DIFlagPublic,
      elements: !11, templateParams: !16,
      identifier: "cc7748c842e275452db4205b190c8ff7")
!11 = !{!12, !14}
!12 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !10,
      file: !5, baseType: !13, size: 32, align: 32, offset: 32,
      flags: DIFlagPublic)
!13 = !DIBasicType(name: "u32", size: 32, encoding: DW_ATE_unsigned)
!14 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !10,
      file: !5, baseType: !15, size: 32, align: 32, offset: 64,
      flags: DIFlagPublic)
!15 = !DIBasicType(name: "i32", size: 32, encoding: DW_ATE_signed)
!16 = !{}
!17 = !DIDerivedType(tag: DW_TAG_member, name: "Second", scope: !7,
      file: !5, baseType: !18, size: 96, align: 32, extraData: i32 1)
!18 = !DICompositeType(tag: DW_TAG_structure_type, name: "Second",
      scope: !4, file: !5, size: 96, align: 32, flags: DIFlagPublic,
      elements: !19, templateParams: !16,
      identifier: "a2094b1381f3082d504fbd0903aa7c06")
!19 = !{!20}
!20 = !DIDerivedType(tag: DW_TAG_member, name: "__0", scope: !18,
      file: !5, baseType: !13, size: 32, align: 32, offset: 32,
      flags: DIFlagPublic)
```

BPF backend was assuming that all the elements of any `DICompositeType`
have tag `DW_TAG_member` and are instances of `DIDerivedType`. However,
the single element of the outer composite type `!4` has tag
`DW_TAG_variant_part` and is an instance of `DICompositeType`. The
unconditional call of `cast<DIDerivedType>` on all elements was causing
an assertion failure when any Rust code with enums was compiled to the
BPF target.

Fix that by:

* Handling `DW_TAG_variant_part` in `visitStructType`.
* Replacing unconditional call of `cast<DIDerivedType>` over
  `DICompositeType` elements with a `switch` statement, handling
  both `DW_TAG_member` and `DW_TAG_variant_part` and casting the element
  to an appropriate type (`DIDerivedType` or `DICompositeType`).

To keep BTF simple and make BTF relocations correct, represent the
discriminator as the first element and apply an offset to all elements.

Fixes: #155778

Author: vadorovsky

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -14,6 +14,7 @@
 #include "BPF.h"
 #include "BPFCORE.h"
 #include "MCTargetDesc/BPFMCTargetDesc.h"
+#include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/CodeGen/AsmPrinter.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
@@ -23,6 +24,7 @@
 #include "llvm/MC/MCObjectFileInfo.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/LineIterator.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
@@ -301,21 +303,61 @@ void BTFTypeStruct::completeType(BTFDebug &BDebug) {
 
   BTFType.NameOff = BDebug.addString(STy->getName());
 
+  uint64_t InitialOffset = 0;
+  if (STy->getTag() == dwarf::DW_TAG_variant_part) {
+    // Variant parts have a discriminator, which has its own memory location at
+    // the beginning, and variants, which share the memory location afterwards.
+    // LLVM DI doesn't consider discriminator as an element and instead keeps
+    // it as a separate reference.
+    // To keep BTF simple, let's represent the structure as an union with
+    // discriminator as the first element and apply offsets to the other
+    // elements.
+    struct BTF::BTFMember Discriminator;
+    const auto *DDTy = STy->getDiscriminator();
+
+    InitialOffset += DDTy->getOffsetInBits() + DDTy->getSizeInBits();
+
+    Discriminator.NameOff = BDebug.addString(DDTy->getName());
+    Discriminator.Offset = DDTy->getOffsetInBits();
+    const auto *BaseTy = DDTy->getBaseType();
+    Discriminator.Type = BDebug.getTypeId(BaseTy);
+
+    Members.push_back(Discriminator);
+  }
+
   // Add struct/union members.
   const DINodeArray Elements = STy->getElements();
   for (const auto *Element : Elements) {
     struct BTF::BTFMember BTFMember;
-    const auto *DDTy = cast<DIDerivedType>(Element);
 
-    BTFMember.NameOff = BDebug.addString(DDTy->getName());
-    if (HasBitField) {
-      uint8_t BitFieldSize = DDTy->isBitField() ? DDTy->getSizeInBits() : 0;
-      BTFMember.Offset = BitFieldSize << 24 | DDTy->getOffsetInBits();
-    } else {
-      BTFMember.Offset = DDTy->getOffsetInBits();
+    switch (Element->getTag()) {
+    case dwarf::DW_TAG_member: {
+      const auto *DDTy = cast<DIDerivedType>(Element);
+
+      BTFMember.NameOff = BDebug.addString(DDTy->getName());
+      uint64_t Offset;
+      if (HasBitField) {
+        uint8_t BitFieldSize = DDTy->isBitField() ? DDTy->getSizeInBits() : 0;
+        Offset = BitFieldSize << 24 | DDTy->getOffsetInBits();
+      } else {
+        Offset = DDTy->getOffsetInBits();
+      }
+      BTFMember.Offset = InitialOffset + Offset;
+      const auto *BaseTy = tryRemoveAtomicType(DDTy->getBaseType());
+      BTFMember.Type = BDebug.getTypeId(BaseTy);
+      break;
+    }
+    case dwarf::DW_TAG_variant_part: {
+      const auto *DCTy = dyn_cast<DICompositeType>(Element);
+
+      BTFMember.NameOff = BDebug.addString(DCTy->getName());
+      BTFMember.Offset = InitialOffset + DCTy->getOffsetInBits();
+      BTFMember.Type = BDebug.getTypeId(DCTy);
+      break;
+    }
+    default:
+      llvm_unreachable("Unexpected DI tag of a struct/union element");
     }
-    const auto *BaseTy = tryRemoveAtomicType(DDTy->getBaseType());
-    BTFMember.Type = BDebug.getTypeId(BaseTy);
     Members.push_back(BTFMember);
   }
 }
@@ -672,16 +714,23 @@ void BTFDebug::visitStructType(const DICompositeType *CTy, bool IsStruct,
                                uint32_t &TypeId) {
   const DINodeArray Elements = CTy->getElements();
   uint32_t VLen = Elements.size();
+  // Variant parts have a discriminator. LLVM DI doesn't consider it as an
+  // element and instead keeps it as a separate reference. But we represent it
+  // as an element in BTF.
+  if (CTy->getTag() == dwarf::DW_TAG_variant_part)
+    VLen++;
   if (VLen > BTF::MAX_VLEN)
     return;
 
   // Check whether we have any bitfield members or not
   bool HasBitField = false;
   for (const auto *Element : Elements) {
-    auto E = cast<DIDerivedType>(Element);
-    if (E->isBitField()) {
-      HasBitField = true;
-      break;
+    if (Element->getTag() == dwarf::DW_TAG_member) {
+      auto E = cast<DIDerivedType>(Element);
+      if (E->isBitField()) {
+        HasBitField = true;
+        break;
+      }
     }
   }
 
@@ -696,9 +745,22 @@ void BTFDebug::visitStructType(const DICompositeType *CTy, bool IsStruct,
   // Visit all struct members.
   int FieldNo = 0;
   for (const auto *Element : Elements) {
-    const auto Elem = cast<DIDerivedType>(Element);
-    visitTypeEntry(Elem);
-    processDeclAnnotations(Elem->getAnnotations(), TypeId, FieldNo);
+    switch (Element->getTag()) {
+    case dwarf::DW_TAG_member: {
+      const auto Elem = cast<DIDerivedType>(Element);
+      visitTypeEntry(Elem);
+      processDeclAnnotations(Elem->getAnnotations(), TypeId, FieldNo);
+      break;
+    }
+    case dwarf::DW_TAG_variant_part: {
+      const auto Elem = cast<DICompositeType>(Element);
+      visitTypeEntry(Elem);
+      processDeclAnnotations(Elem->getAnnotations(), TypeId, FieldNo);
+      break;
+    }
+    default:
+      llvm_unreachable("Unexpected DI tag of a struct/union element");
+    }
     FieldNo++;
   }
 }
@@ -781,16 +843,25 @@ void BTFDebug::visitFwdDeclType(const DICompositeType *CTy, bool IsUnion,
 void BTFDebug::visitCompositeType(const DICompositeType *CTy,
                                   uint32_t &TypeId) {
   auto Tag = CTy->getTag();
-  if (Tag == dwarf::DW_TAG_structure_type || Tag == dwarf::DW_TAG_union_type) {
+  switch (CTy->getTag()) {
+  case dwarf::DW_TAG_structure_type:
+  case dwarf::DW_TAG_union_type:
+  case dwarf::DW_TAG_variant_part:
     // Handle forward declaration differently as it does not have members.
     if (CTy->isForwardDecl())
       visitFwdDeclType(CTy, Tag == dwarf::DW_TAG_union_type, TypeId);
     else
       visitStructType(CTy, Tag == dwarf::DW_TAG_structure_type, TypeId);
-  } else if (Tag == dwarf::DW_TAG_array_type)
+    break;
+  case dwarf::DW_TAG_array_type:
     visitArrayType(CTy, TypeId);
-  else if (Tag == dwarf::DW_TAG_enumeration_type)
+    break;
+  case dwarf::DW_TAG_enumeration_type:
     visitEnumType(CTy, TypeId);
+    break;
+  default:
+    llvm_unreachable("Unexpected DI tag of a composite type");
+  }
 }
 
 bool BTFDebug::IsForwardDeclCandidate(const DIType *Base) {

llvm/test/CodeGen/BPF/BTF/variant-part.ll

@@ -0,0 +1,87 @@
+; RUN: llc -mtriple=bpfel -filetype=obj -o %t1 %s
+; RUN: llvm-objcopy --dump-section='.BTF'=%t2 %t1
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF %s
+; RUN: llc -mtriple=bpfeb -filetype=obj -o %t1 %s
+; RUN: llvm-objcopy --dump-section='.BTF'=%t2 %t1
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF %s
+;
+; Source:
+;   #![no_std]
+;   #![no_main]
+;
+;   pub enum MyEnum {
+;       First { a: u32, b: i32 },
+;       Second(u32),
+;   }
+;
+;   #[unsafe(no_mangle)]
+;   pub static X: MyEnum = MyEnum::First { a: 54, b: -23 };
+;
+;   #[cfg(not(test))]
+;   #[panic_handler]
+;   fn panic(_info: &core::panic::PanicInfo) -> ! {
+;       loop {}
+;   }
+; Compilation flag:
+;   cargo +nightly rustc -Zbuild-std=core --target=bpfel-unknown-none -- --emit=llvm-bc
+;   llvm-extract --glob=X $(find target/ -name "*.bc" | head -n 1) -o variant-part.bc
+;   llvm-dis variant-part.bc -o variant-part.ll
+
+; ModuleID = 'variant-part.bc'
+source_filename = "c0znihgkvro8hs0n88fgrtg6x"
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "bpfel"
+
+@X = constant [12 x i8] c"\00\00\00\006\00\00\00\E9\FF\FF\FF", align 4, !dbg !0
+
+!llvm.module.flags = !{!22, !23, !24, !25}
+!llvm.ident = !{!26}
+!llvm.dbg.cu = !{!27}
+
+; CHECK-BTF:      [1] STRUCT 'MyEnum' size=12 vlen=1
+; CHECK-BTF-NEXT:         '(anon)' type_id=2 bits_offset=0
+; CHECK-BTF-NEXT: [2] UNION '(anon)' size=12 vlen=3
+; CHECK-BTF-NEXT:         '(anon)' type_id=4 bits_offset=0
+; CHECK-BTF-NEXT:         'First' type_id=3 bits_offset=32
+; CHECK-BTF-NEXT:         'Second' type_id=6 bits_offset=32
+; CHECK-BTF-NEXT: [3] STRUCT 'First' size=12 vlen=2
+; CHECK-BTF-NEXT:         'a' type_id=4 bits_offset=32
+; CHECK-BTF-NEXT:         'b' type_id=5 bits_offset=64
+; CHECK-BTF-NEXT: [4] INT 'u32' size=4 bits_offset=0 nr_bits=32 encoding=(none)
+; CHECK-BTF-NEXT: [5] INT 'i32' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
+; CHECK-BTF-NEXT: [6] STRUCT 'Second' size=12 vlen=1
+; CHECK-BTF-NEXT:         '__0' type_id=4 bits_offset=32
+; CHECK-BTF-NEXT: [7] VAR 'X' type_id=1, linkage=global
+; CHECK-BTF-NEXT: [8] DATASEC '.rodata' size=0 vlen=1
+; CHECK-BTF-NEXT:         type_id=7 offset=0 size=1
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "X", scope: !2, file: !3, line: 10, type: !4, isLocal: false, isDefinition: true, align: 32)
+!2 = !DINamespace(name: "variant_part", scope: null)
+!3 = !DIFile(filename: "variant-part/src/main.rs", directory: "/tmp/variant-part", checksumkind: CSK_MD5, checksum: "b94cd53886ea8f14cbc116b36bc7dd36")
+!4 = !DICompositeType(tag: DW_TAG_structure_type, name: "MyEnum", scope: !2, file: !5, size: 96, align: 32, flags: DIFlagPublic, elements: !6, templateParams: !16, identifier: "faba668fd9f71e9b7cf3b9ac5e8b93cb")
+!5 = !DIFile(filename: "<unknown>", directory: "")
+!6 = !{!7}
+!7 = !DICompositeType(tag: DW_TAG_variant_part, scope: !4, file: !5, size: 96, align: 32, elements: !8, templateParams: !16, identifier: "e4aee046fc86d111657622fdcb8c42f7", discriminator: !21)
+!8 = !{!9, !17}
+!9 = !DIDerivedType(tag: DW_TAG_member, name: "First", scope: !7, file: !5, baseType: !10, size: 96, align: 32, extraData: i32 0)
+!10 = !DICompositeType(tag: DW_TAG_structure_type, name: "First", scope: !4, file: !5, size: 96, align: 32, flags: DIFlagPublic, elements: !11, templateParams: !16, identifier: "cc7748c842e275452db4205b190c8ff7")
+!11 = !{!12, !14}
+!12 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !10, file: !5, baseType: !13, size: 32, align: 32, offset: 32, flags: DIFlagPublic)
+!13 = !DIBasicType(name: "u32", size: 32, encoding: DW_ATE_unsigned)
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !10, file: !5, baseType: !15, size: 32, align: 32, offset: 64, flags: DIFlagPublic)
+!15 = !DIBasicType(name: "i32", size: 32, encoding: DW_ATE_signed)
+!16 = !{}
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "Second", scope: !7, file: !5, baseType: !18, size: 96, align: 32, extraData: i32 1)
+!18 = !DICompositeType(tag: DW_TAG_structure_type, name: "Second", scope: !4, file: !5, size: 96, align: 32, flags: DIFlagPublic, elements: !19, templateParams: !16, identifier: "a2094b1381f3082d504fbd0903aa7c06")
+!19 = !{!20}
+!20 = !DIDerivedType(tag: DW_TAG_member, name: "__0", scope: !18, file: !5, baseType: !13, size: 32, align: 32, offset: 32, flags: DIFlagPublic)
+!21 = !DIDerivedType(tag: DW_TAG_member, scope: !4, file: !5, baseType: !13, size: 32, align: 32, flags: DIFlagArtificial)
+!22 = !{i32 8, !"PIC Level", i32 2}
+!23 = !{i32 7, !"PIE Level", i32 2}
+!24 = !{i32 7, !"Dwarf Version", i32 4}
+!25 = !{i32 2, !"Debug Info Version", i32 3}
+!26 = !{!"rustc version 1.91.0-nightly (160e7623e 2025-08-26)"}
+!27 = distinct !DICompileUnit(language: DW_LANG_Rust, file: !28, producer: "clang LLVM (rustc version 1.91.0-nightly (160e7623e 2025-08-26))", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !29, splitDebugInlining: false, nameTableKind: None)
+!28 = !DIFile(filename: "variant-part/src/main.rs/@/c0znihgkvro8hs0n88fgrtg6x", directory: "/tmp/variant-part")
+!29 = !{!0}