[Nasm-bugs] [Bug 3392781] New: Local labels get emitted in object and break dead striping on macOS.

[noreply-nasm at dev.nasm.us]

https://bugzilla.nasm.us/show_bug.cgi?id=3392781

            Bug ID: 3392781
           Summary: Local labels get emitted in object and break dead
                    striping on macOS.
           Product: NASM
           Version: 2.15.xx
          Hardware: All
                OS: Mac OS
            Status: OPEN
          Severity: normal
          Priority: Medium
         Component: Assembler
          Assignee: nobody at nasm.us
          Reporter: afish at apple.com
                CC: chang.seok.bae at intel.com, gorcunov at gmail.com,
                    hpa at zytor.com, nasm-bugs at nasm.us
     Obtained from: Binary from nasm.us

I hit this issue again so I tracked down the owner of the macOS linker to get
more data...

Looks like I was wrong about what was happening in my older BZ. Turns out in
mach-O (and we think ELF too) the assembler does not emit local labels (1: etc)
into the object. Also for mach-O symbols starting with L (.L for ELF) don't get
emitted into the object by the assembler. In addition to this the machOS linker
skips labels starting with L when dead striping. 

macOS example:
$ cat CopyMem.S
.globl _InternalMemCopyMem
_InternalMemCopyMem:
    movq    %rdi, %rax                  # rax <- Destination as return value
    jae     0
    cmpq    %rdi, %r9
    jmp     L0
0:
L0:
    ret

.subsystem_via_symbols
$ clang -c -o CopyMem.S.o CopyMem.S 
$ nm -m  CopyMem.S.o 
0000000000000000 (__TEXT,__text) external _InternalMemCopyMem


Nasm does not strip local labels it name mangles them, and it also does not do
the assembly stripping for the L etc.

$ cat CopyMem.nasm 
global _InternalMemCopyMem
_InternalMemCopyMem:
    mov     rax, rdi
    jae     .0
    cmp     r9, rdi
    jmp     L0
.0:
L0:
    ret

%pragma macho subsections_via_symbols
$ nasm  -f macho64 -o CopyMem.o CopyMem.nasm
$ nm -m  CopyMem.o 
0000000000000011 (__TEXT,__text) non-external L0
0000000000000000 (__TEXT,__text) external _InternalMemCopyMem
0000000000000011 (__TEXT,__text) non-external _InternalMemCopyMem.0

In my case the linker will deal with L0, but the _InternalMemCopyMem.0 causes
the `%pragma macho subsections_via_symbols` to strip out the end of the
function and boom.

I realize nasm has some features that may depend on the local name mangling so
I'd be OK if a flag was required to be more compatible with mach-O and ELF
assemblers in regards to assembler-stripped symbols. Just leaving them out of
the object file would make the code more compatible.

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