[llvm-dev] MachineFunction Instructions Pass using Segment Registers

[Craig Topper via llvm-dev]

The SUB32ri can't instruction can't operate on segment registers. It
operates on EAX/EBX/EDX/ECX/EBP, etc. When it gets encoded only 3 or 4 bits
of the register value make it into the binary encoding. Objdump just
extracts those 3 or 4 bits back out and prints one of the
EAX/EBX/EDX/ECX/EBP registers that those bits correspond to.

~Craig


On Sat, Jun 23, 2018 at 5:28 PM K Jelesnianski via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
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> llvm-dev at lists.llvm.org
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