llvm dev - Jun 2016 - x86: How to Force 2-byte `jmp` instruction in lowering

I have a bit of a riddle:

In http://reviews.llvm.org/D19904 I'm trying to spell the following
assembly:

  .palign 2, 0x90
  jmp +0x9
  nopw 512(%rax,%rax,1)
  // rest of the code

I try the following snippet to accomplish this:

  OutStreamer->EmitLabel(CurSled);
  OutStreamer->EmitCodeAlignment(4);
  auto Target = OutContext.createLinkerPrivateTempSymbol();

  // Use a two-byte `jmp`. This version of JMP takes an 8-bit relative
offset as
  // an operand (computed as an offset from the jmp instruction).
  OutStreamer->EmitInstruction(
      MCInstBuilder(X86::JMP_1)
          .addExpr(MCSymbolRefExpr::create(Target, OutContext)),
      getSubtargetInfo());
  EmitNops(*OutStreamer, 9, Subtarget->is64Bit(), getSubtargetInfo());
  OutStreamer->EmitLabel(Target);

Which turns into:

.Lxray_sled_0:
  .palign 2, 0x90
  jmp .Ltmp0
  nopw 512(%rax,%rax,1)
.Ltmp0:
  // rest of the code

Is there a way of forcing the lowered JMP instruction to turn into a
two-byte jump that does a short relative jump (one that fits within 8
bits)? When I run the binary and disassemble the function I'm seeing it
turn into a 5-byte jump (jmpq <32-bit offset>) instead of a 2-byte jump
(jmp <8-bit offset>).

Thanks in advance!
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This appears to work:

auto Target = OutContext.createLinkerPrivateTempSymbol();

with

auto Target = OutContext.createTempSymbol();

-Nirav


On Wed, Jun 22, 2016 at 3:10 AM, Dean Michael Berris via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I have a bit of a riddle:
>
> In http://reviews.llvm.org/D19904 I'm trying to spell the following
> assembly:
>
>   .palign 2, 0x90
>   jmp +0x9
>   nopw 512(%rax,%rax,1)
>   // rest of the code
>
> I try the following snippet to accomplish this:
>
>   OutStreamer->EmitLabel(CurSled);
>   OutStreamer->EmitCodeAlignment(4);
>   auto Target = OutContext.createLinkerPrivateTempSymbol();
>
>   // Use a two-byte `jmp`. This version of JMP takes an 8-bit relative
> offset as
>   // an operand (computed as an offset from the jmp instruction).
>   OutStreamer->EmitInstruction(
>       MCInstBuilder(X86::JMP_1)
>           .addExpr(MCSymbolRefExpr::create(Target, OutContext)),
>       getSubtargetInfo());
>   EmitNops(*OutStreamer, 9, Subtarget->is64Bit(), getSubtargetInfo());
>   OutStreamer->EmitLabel(Target);
>
> Which turns into:
>
> .Lxray_sled_0:
>   .palign 2, 0x90
>   jmp .Ltmp0
>   nopw 512(%rax,%rax,1)
> .Ltmp0:
>   // rest of the code
>
> Is there a way of forcing the lowered JMP instruction to turn into a
> two-byte jump that does a short relative jump (one that fits within 8
> bits)? When I run the binary and disassemble the function I'm seeing it
> turn into a 5-byte jump (jmpq <32-bit offset>) instead of a 2-byte
jump
> (jmp <8-bit offset>).
>
> Thanks in advance!
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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On Wed, Jun 22, 2016 at 6:05 AM Nirav Davé <niravd at google.com> wrote:
> This appears to work:
>
> auto Target = OutContext.createLinkerPrivateTempSymbol();
>
> with
>
> auto Target = OutContext.createTempSymbol();
>
> -Nirav
>
>
Thanks Nirav -- I tried this but I'm still getting a "jmpq
<address>" with
this incantation when I load and disassemble from gdb. I'm seeing a
5-instruction jump, followed by the nops.

If I disassemble with llvm-objdump though I see the following:

_Z3foov:
  400c10:       e9 09 00 00 00  jmp     9 <_Z3foov+0xE>
  400c15:       66 0f 1f 84 00 00 02 00 00      nopw    512(%rax,%rax)

I'm not sure whether the extra 0's after '0xe9 0x09' are
alignment padding
(though I was expecing 0x90 to show up if this was an alignment issue).

Is there anything else I can try here?

Thanks in advance!
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