LLVM's `MCParser` does not understand `.cfi_label` directives

[alexrp]

❯ cat test.s
.globl foo
.align 2
.type foo,@function
.cfi_startproc
.cfi_label .Ldummy
.cfi_endproc
❯ riscv64-linux-gnu-gcc test.s -c
❯ /opt/llvm/bin/clang -target riscv64 test.s -c
test.s:5:1: error: unknown directive
.cfi_label .Ldummy
^
❯ /opt/llvm/bin/clang --version
clang version 18.1.6 (https://github.com/llvm/llvm-project 1118c2e05e67a36ed8ca250524525cdb66a55256)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/llvm/bin

This is currently a blocker for compiling glibc with Clang for ARC, C-SKY, LoongArch, and RISC-V, as these all make use of .cfi_label in their start.S (example).

[MaskRay]

Created #97922 to support .cfi_label

In glibc, https://sourceware.org/PR23125 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

Technically, .cfi_label .Ldummy isn't really necessary. It moves the instruction from CIE to FDE and potentially decrease the number of CIEs, usually without making the FDE larger (since the FDE has to be padded).

ARC, C-SKY, LoongArch, and RISC-V, as these all make use of .cfi_label in their start.S ([example]

I believe ARC, C-SKY, LoongArch just copy RISC-V without understanding the rationale :)

/opt/llvm/bin/clang -target riscv64 test.s -c

Use --target=riscv64. -target has been deprecated for a long time since around Clang 3.4