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

[Gus Smith via llvm-dev]

Yes, you're completely right...I'm thinking about this 100% the wrong way.
I was not thinking at all about how the addresses I need won't be available
at compile time -- that would be silly if it were the case. Thanks for
pointing that out :)

On Tue, Mar 20, 2018 at 1:42 PM, Craig Topper <craig.topper at gmail.com>
wrote:

> That wasn't the part that confused me. What confused me was what you
> expected to be encoded into the instruction. Your math indicated that you
> multiple n by 3 and added 1 to it to get your 13 bytes. So that means you
> intend to use 4 bytes to store an address in 32 bits which implied to me
> that you intended to have a fixed address encoded. But what if the address
> in in a register as would often be the case in real code. For example, when
> a pointer is passed as operand to a function. Most X86 instructions support
> a format of (base + scale * index + displacement) where any of those pieces
> are optional, but it takes up to 6 bytes to encode them.
>
> ~Craig
>
> On Tue, Mar 20, 2018 at 10:25 AM, Gus Smith <gushenrysmith at gmail.com>
> wrote:
>
>> 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
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>
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