[BPF] Make llvm-objdump disasm default cpu v4 (#102166)

Currently, with the following example,
  $ cat t.c
  void foo(int a, _Atomic int *b)
  {
   *b &= a;
  }
  $ clang --target=bpf -O2 -c -mcpu=v3 t.c
  $ llvm-objdump -d t.o
  t.o:    file format elf64-bpf

  Disassembly of section .text:

  0000000000000000 <foo>:
       0:       c3 12 00 00 51 00 00 00 <unknown>
       1:       95 00 00 00 00 00 00 00 exit

Basically, the default cpu for llvm-objdump is v1 and it won't be able
to decode insn properly.

If we add --mcpu=v3 to llvm-objdump command line, we will have
  $ llvm-objdump -d --mcpu=v3 t.o

  t.o:    file format elf64-bpf

  Disassembly of section .text:

  0000000000000000 <foo>:
0: c3 12 00 00 51 00 00 00 w1 = atomic_fetch_and((u32 *)(r2 + 0x0), w1)
       1:       95 00 00 00 00 00 00 00 exit

The atomic_fetch_and insn can be decoded properly. Using latest cpu
version --mcpu=v4 can also decode properly like the above --mcpu=v3.

To avoid the above '<unknown>' decoding with common 'llvm-objdump -d
t.o', this patch marked the default cpu for llvm-objdump with the
current highest cpu number v4 in ELFObjectFileBase::tryGetCPUName(). The
cpu number in ELFObjectFileBase::tryGetCPUName() will be adjusted in the
future if cpu number is increased e.g. v5 etc. Such an approach also
aligns with gcc-bpf as discussed in [1].

Six bpf unit tests are affected with this change. I changed test output
for three unit tests and added --mcpu=v1 for the other three unit tests,
to demonstrate the default (cpu v4) behavior and explicit --mcpu=v1
behavior.

[1]
https://lore.kernel.org/bpf/6f32c0a1-9de2-4145-92ea-be025362182f@linux.dev/T/#m0f7e63c390bc8f5a5523e7f2f0537becd4205200

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Object/ELFObjectFile.cpp b/llvm/lib/Object/ELFObjectFile.cpp
index 53c3de0..f79c233 100644
--- a/llvm/lib/Object/ELFObjectFile.cpp
+++ b/llvm/lib/Object/ELFObjectFile.cpp
@@ -441,6 +441,8 @@ std::optional<StringRef> ELFObjectFileBase::tryGetCPUName() const {
   case ELF::EM_PPC:
   case ELF::EM_PPC64:
     return StringRef("future");
+  case ELF::EM_BPF:
+    return StringRef("v4");
   default:
     return std::nullopt;
   }