[llvm-dev] MachineFunction Instructions Pass using Segment Registers

[K Jelesnianski via llvm-dev]

Dear All,

Currently I am trying to inject custom x86-64 assembly into a
functions entry basic block. More specifically, I am trying to build
assembly in a machine function pass from scratch.

While the dumped machine function instruction info displays that %gs
will be used, when I perform objdump -d on my executable I am see that
%gs is replaced by %ebp? Why is this happening?

I know it probably has something to do with me not specifying operands
properly, but I cannot find enough documentation on this besides
looking through code comments such as X86BaseInfo.cpp. I feel there
isn't enough for me to be able to connect the dots.

Below I have sample code: %gs holds a base address to a memory
location where I am trying to store information. I am trying to update
the %gs register pointer location before saving more values, etc.

LLVM C++ codeMachine Function pass code:
MachineInstrBuilder sss = BuildMI(MBB, MBB.begin(), DL,
TII->get(X86::SUB32ri),X86::GS)
                    .addReg(X86::GS)
                    .addImm(0x8);

machine function pass dump:
 %gs = SUB32ri %gs, 8, implicit-def %eflags

Objdump -d assembly from executable
  400510:   81 ed 04 00 00 00       sub    $0x8,%ebp


TLDR: I am trying to create custom assembly via BuildMI() and manipulate segment
registers via a MachineFunctionPass.

I have looked at LLVMs safestack implementation, but they are taking a
fairly complicated hybrid approach between an IR Function pass with
Backend support. I would like to stay as a single machinefunction
pass.

Believe me I would do this at the IR level if I didnt need to
specifically use the segment registers.

Thanks for the help in advance!

Sincerely,

Christopher Jelesnianski
Graduate Research Assistant
Virginia Tech