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

Stephen Checkoway via llvm-dev llvm-dev at lists.llvm.org
Sat Feb 27 08:08:28 PST 2016

> 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):

$ 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
	jmp	_label                  # encoding: [0xeb,A]
                                       #   fixup A - offset: 1, value: _label-1, kind: FK_PCRel_1
$ clang -c b.s
$ otool -tvj 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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