[llvm-dev] How to get emitted size of MachineInstr in MachineFunction pass?

[K Jelesnianski via llvm-dev]

Dear Paul,

I guess I used the wrong term when I said "alignment". I exactly am looking
to ensure that all the instructions I am transforming (specifically jumps:
JMP64r, JMP64m, etc) to become 5 bytes long, with JMP the length can vary
from 2 bytes long to 5 from what I am seeing when I objdump the executable
I am trying to instrument. I want to get the size of the MachineInstr I
have created so that I know how many extra NOOPs I should insert afterwards
to maintain a 5 byte total size.

On Thu, Oct 25, 2018 at 2:58 PM <paul.robinson at sony.com> wrote:

> There should be a way for you to specify a desired alignment, either on
> the machine instruction itself, or as a directive inserted prior to the
> instruction.  You should not need to compute the exact number of bytes
> yourself, the MC assembler phase knows how to insert appropriate padding.
>
> --paulr
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *K
> Jelesnianski via llvm-dev
> *Sent:* Thursday, October 25, 2018 10:19 AM
> *To:* llvm-dev
> *Subject:* [llvm-dev] How to get emitted size of MachineInstr in
> MachineFunction pass?
>
>
>
> Dear dev-list,
>
>
>
> I currently have a X86 machineFunctionPass and realized there are more
> corner cases than I realized. I need to keep my custom jump transformations
> aligned and therefore would like to fill the rest of the space with NOOPs.
>
>
>
> I understand that hexcode code ASM is emitted/encoded by the ASMPrinter.
> In the past I have gotten the size via 2 compilations (1 to collect asm
> size data, 2nd to perform fixup alignment where needed), but am looking for
> a cleaner self contained solution.
>
>
>
> In the past I used this code, creating a function in X86AsmPrinter class
>
> unsigned int instSize;
> SmallString<256> Code;
> SmallVector<MCFixup, 4> Fixups;
> raw_svector_ostream VecOS(Code);
> CodeEmitter->encodeInstruction(Inst, VecOS, Fixups, STI);
> instSize = Code.size();
>
> So far, I have already tried to get the size of the opcode at least via:
>
> size= MachineInstr->getDesc().getSize();
>
> but it returns zero, indicating that the encoding size cannot be
> determined yet according to the LLVM doxygen, but why? We are already know
> our target ISA is X86 and if I do a dump of the MachineInstr, the opcodes
> and operands/registers seem already finalized....
>
>
>
> So the questions are:
>
> 1) Is it possible to temporarily encode a given MachineInstr to a
> temporary buffer in a machineFunctionPass?? If so, how?
>
> 2) Is it possible to create an instance of CodeEmitter within a
> machineFunctionPass to make it possible to run the above code? If so, how?
>
>
>
> Thanks in advance.
>
> Sincerely,
>
>
>
> K Jelesnianski
>
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