[BPF] Fix a BTF type pruning bug

In BPF backend, BTF type generation may skip
some debuginfo types if they are the pointee
type of a struct member. For example,
  struct task_struct {
    ...
    struct mm_struct                *mm;
    ...
  };
BPF backend may generate a forward decl for
'struct mm_struct' instead of full type if
there are no other usage of 'struct mm_struct'.
The reason is to avoid bringing too much unneeded types
in BTF.

Alexei found a pruning bug where we may miss
some full type generation. The following is an illustrating
example:
   struct t1 { ... }
   struct t2 { struct t1 *p; };
   struct t2 g;
   void foo(struct t1 *arg) { ... }
In the above case, we will have partial debuginfo chain like below:
   struct t2 -> member p
                        \ -> ptr -> struct t1
                        /
     foo -> argument arg
During traversing
   struct t2 -> member p -> ptr -> struct t1
The corresponding BTF types are generated except 'struct t1' which
will be in FixUp stage. Later, when traversing
   foo -> argument arg -> ptr -> struct t1
The 'ptr' BTF type has been generated and currently implementation
ignores 'pointer' type hence 'struct t1' is not generated.

This patch fixed the issue not just for the above case, but for
general case with multiple derived types, e.g.,
   struct t2 -> member p
                        \ -> const -> ptr -> volatile -> struct t1
                        /
     foo -> argument arg

Differential Revision: https://reviews.llvm.org/D119986

Author: yonghong-song

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -773,15 +773,31 @@ void BTFDebug::visitTypeEntry(const DIType *Ty, uint32_t &TypeId,
     // already defined, we should keep moving to eventually
     // bring in types for "struct t". Otherwise, the "struct s2"
     // definition won't be correct.
+    //
+    // In the above, we have following debuginfo:
+    //  {ptr, struct_member} ->  typedef -> struct
+    // and BTF type for 'typedef' is generated while 'struct' may
+    // be in FixUp. But let us generalize the above to handle
+    //  {different types} -> [various derived types]+ -> another type.
+    // For example,
+    //  {func_param, struct_member} -> const -> ptr -> volatile -> struct
+    // We will traverse const/ptr/volatile which already have corresponding
+    // BTF types and generate type for 'struct' which might be in Fixup
+    // state.
     if (Ty && (!CheckPointer || !SeenPointer)) {
       if (const auto *DTy = dyn_cast<DIDerivedType>(Ty)) {
-        unsigned Tag = DTy->getTag();
-        if (Tag == dwarf::DW_TAG_typedef || Tag == dwarf::DW_TAG_const_type ||
-            Tag == dwarf::DW_TAG_volatile_type ||
-            Tag == dwarf::DW_TAG_restrict_type) {
-          uint32_t TmpTypeId;
-          visitTypeEntry(DTy->getBaseType(), TmpTypeId, CheckPointer,
-                         SeenPointer);
+        while (DTy) {
+          const DIType *BaseTy = DTy->getBaseType();
+          if (!BaseTy)
+            break;
+
+          if (DIToIdMap.find(BaseTy) != DIToIdMap.end()) {
+            DTy = dyn_cast<DIDerivedType>(BaseTy);
+          } else {
+            uint32_t TmpTypeId;
+            visitTypeEntry(BaseTy, TmpTypeId, CheckPointer, SeenPointer);
+            break;
+          }
         }
       }
     }

llvm/test/CodeGen/BPF/BTF/pruning-multi-derived-type.ll

@@ -0,0 +1,87 @@
+; 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 t1 {
+;     int a;
+;   };
+;   struct t2 {
+;     const struct t1 * const a;
+;   };
+;   int foo(struct t2 *arg) { return 0; }
+;   int bar(const struct t1 * const arg) { return 0; }
+; Compilation flags:
+;   clang -target bpf -O2 -g -S -emit-llvm t.c
+
+%struct.t2 = type { %struct.t1* }
+%struct.t1 = type { i32 }
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn
+define dso_local i32 @foo(%struct.t2* nocapture noundef readnone %arg) local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.t2* %arg, metadata !22, metadata !DIExpression()), !dbg !23
+  ret i32 0, !dbg !24
+}
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn
+define dso_local i32 @bar(%struct.t1* nocapture noundef readnone %arg) local_unnamed_addr #0 !dbg !25 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.t1* %arg, metadata !29, metadata !DIExpression()), !dbg !30
+  ret i32 0, !dbg !31
+}
+
+; CHECK:             .long   10                              # BTF_KIND_INT(id = 7)
+; CHECK-NEXT:        .long   16777216                        # 0x1000000
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   16777248                        # 0x1000020
+
+; CHECK:             .long   69                              # BTF_KIND_STRUCT(id = 9)
+; CHECK-NEXT:        .long   67108865                        # 0x4000001
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   4
+; CHECK-NEXT:        .long   7
+
+; CHECK:             .byte   97                              # string offset=4
+; CHECK:             .ascii  "t1"                            # string offset=69
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { mustprogress nofree norecurse nosync nounwind readnone willreturn "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #1 = { nofree nosync nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 15.0.0 (https://github.com/llvm/llvm-project.git c34c8afcb85ae9142d0f783bb899c464e8bd2356)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/llvm/btf_ptr", checksumkind: CSK_MD5, checksum: "d43a0541e830263021772349589e47a5")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 7, !"frame-pointer", i32 2}
+!6 = !{!"clang version 15.0.0 (https://github.com/llvm/llvm-project.git c34c8afcb85ae9142d0f783bb899c464e8bd2356)"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 7, type: !8, scopeLine: 7, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !21)
+!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: "t2", file: !1, line: 4, size: 64, elements: !13)
+!13 = !{!14}
+!14 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !12, file: !1, line: 5, baseType: !15, size: 64)
+!15 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !16)
+!16 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !17, size: 64)
+!17 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !18)
+!18 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", file: !1, line: 1, size: 32, elements: !19)
+!19 = !{!20}
+!20 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !18, file: !1, line: 2, baseType: !10, size: 32)
+!21 = !{!22}
+!22 = !DILocalVariable(name: "arg", arg: 1, scope: !7, file: !1, line: 7, type: !11)
+!23 = !DILocation(line: 0, scope: !7)
+!24 = !DILocation(line: 7, column: 27, scope: !7)
+!25 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 8, type: !26, scopeLine: 8, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !28)
+!26 = !DISubroutineType(types: !27)
+!27 = !{!10, !15}
+!28 = !{!29}
+!29 = !DILocalVariable(name: "arg", arg: 1, scope: !25, file: !1, line: 8, type: !15)
+!30 = !DILocation(line: 0, scope: !25)
+!31 = !DILocation(line: 8, column: 40, scope: !25)