[llvm-dev] Encoding an X86 format with long operands

[Gus Smith via llvm-dev]

Whoops - sorry for the confusion. n would be set in stone beforehand. I
basically meant to indicate that we'd either be looking at a 32 bit or 64
bit system, ie 4 byte or 8 byte addresses.

On Tue, Mar 20, 2018, 1:07 PM Craig Topper <craig.topper at gmail.com> wrote:

> Hi Gus,
>
> When you say "n byte destination" you mean you want to encode an n byte
> address as a constant within the instruction? That would mean you couldn't
> encode an address that comes from a register.
>
> ~Craig
>
> On Tue, Mar 20, 2018 at 9:48 AM, Gus Smith via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all.
>>
>> tl;dr: I would like to add a long x86 instruction which doesn't conform
>> to any existing format that I know; I'm not sure where to start.
>>
>> I am attempting to add an instruction into X86, to be simulated in gem5.
>> I've already added a simple, opcode-only instruction which I can
>> successfully decode and run in gem5, so I am roughly familiar with .td
>> files and how backends are built out of them.
>>
>> My goal now is to make a more complex instruction -- specifically, I need
>> to add large operands. The format would look something like this:
>>
>>    - 1 byte opcode (0x06, which I hijacked from PUSHES, which isn't
>>    implemented in gem5)
>>    - n byte destination (memory location)
>>    - n byte source (memory location)
>>    - n byte source (memory location or  immediate)
>>
>> If n=4, then the total opcode length is 13 bytes, which is under the 15
>> byte x86 limit.
>>
>> As far as I know, this doesn't conform to any existing x86 format.
>> Because that's the case, I'm not sure how to go about encoding an
>> instruction like this; presumably, I can't use the existing I<..> class,
>> which is what I'd used previously.
>>
>> Can anyone point me in the general direction of what I will need to do to
>> encode this rather arbitrary instruction format? Should I look into
>> implementing a new Instruction class? Is there an easier way?
>>
>> Thanks,
>> Gus Smith, PSU
>>
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>>
>>
>
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