[llvm-dev] adding prefixes to certain instructions x86 -- where to start?

[mats petersson via llvm-dev]

What is it you are ACTUALLY trying to do?

In other words, why would you want a different address size...
Understanding that would probably help provide a better answer (I have
absolutely no idea how to solve the actual question, but I suspect
understanding what the overall goal is will help a whole lot)

--
Mats

On 11 September 2015 at 15:38, Scott A. Carr via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I’ve been using LLVM IR passes for my research for about a year now, but
> for my next step I think I might have to dig into a backend.  I'm hoping
> someone could give me a pointer on how to get started.
>
> The thing I would like to do is add and override address-size override
> prefix [1] to a given x86-64 instruction.  I’m hoping I can do something
> like:
>
> 1) Mark some IR instructions with metadata in my pass
> 2) Hack the backend to look for my metadata, and if found add the prefix
> when the machine instruction is emitted
>
> Does this seem feasible?  Does the LLVM x86 backend currently have the
> capability of adding instruction prefixes and could someone please point
> out where I should look in the code for it?
>
> Thanks,
> Scott A. Carr
> PhD Student
> Purdue University CS
>
> [1]
> http://wiki.osdev.org/X86-64_Instruction_Encoding#Operand-size_and_address-size_override_prefix
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