[MC] Support .cfi_label

GNU assembler 2.26 introduced the .cfi_label directive. It does not
expand to any CFI instructions, but defines a label in
.eh_frame/.debug_frame, which can be used by runtime patching code to
locate the FDE. .cfi_label is not allowed for CIE's initial
instructions, and can therefore be used to force the next instruction to
be placed in a FDE instead of a CIE.

In glibc since 2018, sysdeps/riscv/start.S utilizes .cfi_label to force
DW_CFA_undefined to be placed in a FDE. arc/csky/loongarch ports have
copied this use.
```
.cfi_startproc
// DW_CFA_undefined is allowed for CIE's initial instructions.
// Without .cfi_label, gas would place DW_CFA_undefined in a CIE.
.cfi_label .Ldummy
.cfi_undefined ra
.cfi_endproc
```

No CFI instruction is associated with .cfi_label, so the `case
MCCFIInstruction::OpLabel:` code in BOLT is unreachable and onlt to make
-Wswitch happy.

Close #97222

Pull Request: https://github.com/llvm/llvm-project/pull/97922

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCDwarf.cpp b/llvm/lib/MC/MCDwarf.cpp
index 321a66e..efafd55 100644
--- a/llvm/lib/MC/MCDwarf.cpp
+++ b/llvm/lib/MC/MCDwarf.cpp
@@ -1465,6 +1465,9 @@ void FrameEmitterImpl::emitCFIInstruction(const MCCFIInstruction &Instr) {
   case MCCFIInstruction::OpEscape:
     Streamer.emitBytes(Instr.getValues());
     return;
+  case MCCFIInstruction::OpLabel:
+    Streamer.emitLabel(Instr.getCfiLabel(), Instr.getLoc());
+    return;
   }
   llvm_unreachable("Unhandled case in switch");
 }