[llvm-dev] Fwd: X86 assembler cannot jump NEAR?

[Jun Koi via llvm-dev]

On Feb 28, 2016 12:08 AM, "Stephen Checkoway" <s at pahtak.org> wrote:
>
>
>
> > On Feb 27, 2016, at 4:36 AM, Jun Koi via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> > The problem is that llvm-mc always compiles "jmp" this as short jump,
no matter where the target is. Hence my question. I dont know if there is
any way to change this behavior. Looks like a bug to me so far.
>
> It isn't. It's just created some assembly which, when assembled, may end
up as a near jump or a short jump.
>
> > Craig said there is a trick with object file, but I dont know how to do
that. Also, why compiling to object file changes the result?
>
> Here's an object file (although I don't know why it's producing ELF
rather than Mach-o):

Oh this is cool! Which code in MC is responsible for replacing EB with E9
when dumping to object file like this???

Thanks.

>
> $ echo "jmp _label"|llvm-mc-mp-3.7 -assemble -triple=i386  -filetype=obj
> a.o
> $ x86_64-pc-elf-objdump -d a.o
>
> a.o:     file format elf32-i386
>
>
> Disassembly of section .text:
>
> 00000000 <.text>:
>   0:    e9 fc ff ff ff          jmp    0x1
>
> And here's assembly which is then run through the compiler.
>
> $ echo "jmp _label"|llvm-mc-mp-3.7 -assemble -triple=i386  -show-encoding
|tee b.s
>         .text
>         jmp     _label                  # encoding: [0xeb,A]
>                                        #   fixup A - offset: 1, value:
_label-1, kind: FK_PCRel_1
> $ clang -c b.s
> $ otool -tvj b.o
> b.o:
> (__TEXT,__text) section
> 0000000000000000        e900000000              jmp     0x5
>
> As you can see, both produced near jumps. (I assume the difference
between the offsets has to do with a difference in relocations between ELF
and Mach-o.)
>
> --
> Stephen Checkoway
>
>
>
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