BPF: report better error message for BTF_TYPE_ID_REMOTE relo failure

[yonghong-song]

Summary

Matteo Croce reported a bpf backend fatal error in
https://github.com/llvm/llvm-project/issues/52779

A simplified case looks like:

$ cat bug.c
extern int do_smth(int);
int test() {
  return __builtin_btf_type_id(*(typeof(do_smth) *)do_smth, 1); 
}
$ clang -target bpf -O2 -g -c bug.c
fatal error: error in backend: Empty type name for BTF_TYPE_ID_REMOTE reloc
... 

The reason for the fatal error is that the relocation is against
a DISubroutineType like type 13 below:

!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!11 = !{} 
!12 = !DILocation(line: 3, column: 10, scope: !7) 
!13 = !DISubroutineType(types: !14)
!14 = !{!10, !10}

The DISubroutineType doesn't have a name and there is no way for
downstream bpfloader/kernel to do proper relocation for it.

But we can improve error message to be more specific for this case.
The patch improved the error message to be:

fatal error: error in backend: SubroutineType not supported for BTF_TYPE_ID_REMOTE reloc

[yonghong-song]

• fix clang-format warning

[tstellar]

@ast Is this OK to backport to release/14.x ?

[ast]

In D116063#3350010, @tstellar wrote:

@ast Is this OK to backport to release/14.x ?

Yes. please

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

if (FlagValue == BPFCoreSharedInfo::BTF_TYPE_ID_LOCAL_RELOC) {
       while (auto *DTy = dyn_cast<DIDerivedType>(Ty)) {
         unsigned Tag = DTy->getTag();
         if (Tag != dwarf::DW_TAG_const_type &&
             Tag != dwarf::DW_TAG_volatile_type)
           break;
         Ty = DTy->getBaseType();
       }
 
-      if (Ty->getName().empty())
+      if (Ty->getName().empty()) {
+        if (isa<DISubroutineType>(Ty))
+          report_fatal_error(
+              "SubroutineType not supported for BTF_TYPE_ID_REMOTE reloc");
+        else
           report_fatal_error("Empty type name for BTF_TYPE_ID_REMOTE reloc");
+      }
       MD = Ty;
     }
 
     BasicBlock *BB = Call->getParent();
     IntegerType *VarType = Type::getInt64Ty(BB->getContext());
     std::string GVName = BaseName + std::to_string(Count) + "$" +
         std::to_string(Reloc);
     GlobalVariable *GV = new GlobalVariable(