Using reserved keywords as label names break LLVM based back end

[dvanhorn]

The old nasm printer used $ to preface label names so that reserved words could serve as label names without breaking things, e.g.

        default rel
        section .text
        global $entry
$entry:
        mov rax, 42
$rax:
        sub rax, 1
        cmp rax, 0
        jne $rax
        ret

But the move to clang/LLVM dropped this feature which means you get:

        .intel_syntax noprefix
        .text
        .global entry
entry:
        mov rax, 42
rax:
        sub rax, 1
        cmp rax, 0
        jne rax
        ret

and jne rax is not a valid instruction. There is a failing test case in the repo that catches this regression.

(Note that this test won't fail on Macs because the _ prefix that gets inserted saves it from being a malformed instruction.)

The clang assembler let's you use double quotes for label names, so the example could be jne "rax".

[dvanhorn]

Just confirmed that changing the printer to use this:

(define label-symbol->string
  (match (system-type 'os)
    ['macosx
     (λ (s)
       (string-append "\"" "_" (symbol->string s) "\""))]
    [_
     (λ (s)
       (string-append "\"" (symbol->string s) "\""))]))

fixes the bug.

This makes the printed assembly code less nice to look at, but I think we should move the materials away from showing the concrete x86 syntax at all, in which case, this seems fine.