[BPF] Fix a BTF gen bug related to a pointer struct member

[yonghong-song]

Summary

Currently, BTF generation stops at pointer struct members
if the pointee type is a struct. This is to avoid bloating
generated BTF size. The following is the process to
correctly record types for these pointee struct types.

• During type traversal stage, when a struct member, which is a pointer to another struct, is encountered, the pointee struct type, keyed with its name, is remembered in a Fixup map.
• Later, when all type traversal is done, the Fixup map is scanned, based on struct name matching, to either resolve as pointing to a real already generated type or as a forward declaration.

Andrii discovered a bug if the struct member pointee struct
is anonymous. In this case, a struct with empty name is
recorded in Fixup map, and later it happens another anonymous
struct with empty name is defined in BTF. So wrong type
resolution happens.

To fix the problem, if the struct member pointee struct
is anonymous, pointee struct type will be generated in
stead of being put in Fixup map.

llvm/lib/Target/BPF/BTFDebug.cpp

void BTFDebug::visitDerivedType(const DIDerivedType *DTy, uint32_t &TypeId,
 
   if (CheckPointer && SeenPointer) {
     const DIType *Base = DTy->getBaseType();
     if (Base) {
       if (const auto *CTy = dyn_cast<DICompositeType>(Base)) {
         auto CTag = CTy->getTag();
         if ((CTag == dwarf::DW_TAG_structure_type ||
              CTag == dwarf::DW_TAG_union_type) &&
-            !CTy->isForwardDecl()) {
+            !CTy->getName().empty() && !CTy->isForwardDecl()) {
           /// Find a candidate, generate a fixup. Later on the struct/union
           /// pointee type will be replaced with either a real type or
           /// a forward declaration.
           auto TypeEntry = std::make_unique<BTFTypeDerived>(DTy, Tag, true);
           auto &Fixup = FixupDerivedTypes[CTy->getName()];
           Fixup.first = CTag == dwarf::DW_TAG_union_type;
           Fixup.second.push_back(TypeEntry.get());
           TypeId = addType(std::move(TypeEntry), DTy);

llvm/test/CodeGen/BPF/BTF/struct-anon-2.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+;
+; Source:
+;   struct s1 {
+;     struct { int A1; } a1;
+;     struct { long B1; } *b1;
+;   };
+;   int f1(struct s1 *s1) { return 0; }
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm t.c
+
+%struct.s1 = type { %struct.anon, %struct.anon.0* }
+%struct.anon = type { i32 }
+%struct.anon.0 = type { i64 }
+
+; Function Attrs: norecurse nounwind readnone
+define dso_local i32 @f1(%struct.s1* nocapture readnone %s1) local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.s1* %s1, metadata !25, metadata !DIExpression()), !dbg !26
+  ret i32 0, !dbg !27
+}
+
+; CHECK:             .long   0                       # BTF_KIND_PTR(id = 1)
+; CHECK-NEXT:        .long   33554432                # 0x2000000
+; CHECK-NEXT:        .long   2
+; CHECK-NEXT:        .long   1                       # BTF_KIND_STRUCT(id = 2)
+; CHECK-NEXT:        .long   67108866                # 0x4000002
+; CHECK-NEXT:        .long   16
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   3
+; CHECK-NEXT:        .long   0                       # 0x0
+; CHECK-NEXT:        .long   7
+; CHECK-NEXT:        .long   5
+; CHECK-NEXT:        .long   64                      # 0x40
+; CHECK-NEXT:        .long   0                       # BTF_KIND_STRUCT(id = 3)
+; CHECK-NEXT:        .long   67108865                # 0x4000001
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   10
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   0                       # 0x0
+; CHECK-NEXT:        .long   13                      # BTF_KIND_INT(id = 4)
+; CHECK-NEXT:        .long   16777216                # 0x1000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   16777248                # 0x1000020
+; CHECK-NEXT:        .long   0                       # BTF_KIND_PTR(id = 5)
+; CHECK-NEXT:        .long   33554432                # 0x2000000
+; CHECK-NEXT:        .long   6
+; CHECK-NEXT:        .long   0                       # BTF_KIND_STRUCT(id = 6)
+; CHECK-NEXT:        .long   67108865                # 0x4000001
+; CHECK-NEXT:        .long   8
+; CHECK-NEXT:        .long   17
+; CHECK-NEXT:        .long   7
+; CHECK-NEXT:        .long   0                       # 0x0
+; CHECK-NEXT:        .long   20                      # BTF_KIND_INT(id = 7)
+; CHECK-NEXT:        .long   16777216                # 0x1000000
+; CHECK-NEXT:        .long   8
+; CHECK-NEXT:        .long   16777280                # 0x1000040
+
+; CHECK:             .ascii  "s1"                    # string offset=1
+; CHECK:             .ascii  "a1"                    # string offset=4
+; CHECK:             .ascii  "b1"                    # string offset=7
+; CHECK:             .ascii  "A1"                    # string offset=10
+; CHECK:             .ascii  "int"                   # string offset=13
+; CHECK:             .ascii  "B1"                    # string offset=17
+; CHECK:             .ascii  "long int"              # string offset=20
+
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { norecurse nounwind readnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0 (https://github.com/llvm/llvm-project.git ef36f5143d83897cc6f59ff918769d29ad5a0612)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "t.c", directory: "/tmp/home/yhs/btf/tmp")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git ef36f5143d83897cc6f59ff918769d29ad5a0612)"}
+!7 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !24)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !11}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s1", file: !1, line: 1, size: 128, elements: !13)
+!13 = !{!14, !18}
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "a1", scope: !12, file: !1, line: 2, baseType: !15, size: 32)
+!15 = distinct !DICompositeType(tag: DW_TAG_structure_type, scope: !12, file: !1, line: 2, size: 32, elements: !16)
+!16 = !{!17}
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "A1", scope: !15, file: !1, line: 2, baseType: !10, size: 32)
+!18 = !DIDerivedType(tag: DW_TAG_member, name: "b1", scope: !12, file: !1, line: 3, baseType: !19, size: 64, offset: 64)
+!19 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 64)
+!20 = distinct !DICompositeType(tag: DW_TAG_structure_type, scope: !12, file: !1, line: 3, size: 64, elements: !21)
+!21 = !{!22}
+!22 = !DIDerivedType(tag: DW_TAG_member, name: "B1", scope: !20, file: !1, line: 3, baseType: !23, size: 64)
+!23 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!24 = !{!25}
+!25 = !DILocalVariable(name: "s1", arg: 1, scope: !7, file: !1, line: 5, type: !11)
+!26 = !DILocation(line: 0, scope: !7)
+!27 = !DILocation(line: 5, column: 25, scope: !7)