[Vm-dev] [64 bits] Object pointers in jitted code

[Eliot Miranda]

Hi Javier,


> On Feb 28, 2018, at 5:16 AM, Javier Pimás <elpochodelagente at gmail.com> wrote:
> 
> Hi! This time I'm investigating how cog jit handles pointers to objects in native code. In x86-32 its easier because you have immediates of the size of a pointer, but in x64 the immediates are restricted to 32bits (and I think less in arm).

That's not quite right.  On x86_64 instructions can load 64-bit constants into registers.  What is restricted is loading/storing through a 64-bit immediate address.  That can only be done to/from %rax.  So when loading an arbitrary register from memory the JIT often generates sequences like:

    xchgq %r15,%rax
    moveq 123456789AB0,%rax
    xchgq %r15,%rax

> So I wonder how people works around that, if using a movabs instruction every time you need a pointer or if doing something else. I found a mail in the list dated from 2011 (titled "questions about cog internals") where you (Eliot) said that pointers were inlined in jit code, but I don't know if that's still the case.

Yes.  The easy way to see this is to use in-image compilation.  e.g. in a VMMaker.oscog image (scripts to build them being in the image directory) run the following with a Transcript open:

StackToRegisterMappingCogit
    genAndDis: Object>>#printOn: "includes 'a ' and 'an '"
    options: #(ObjectMemory Spur64BitCoMemoryManager)

and the generated machine code method will be output to the transcript.

> Looking at the slang code I found CogOutOfLineLiteralsX64Compiler, but it seems it is not used (yet?).

Yes, we should implement this and see how it compares.  It's not particularly compelling in x86_64 because we can load 64-bit immediates inline but performance might differ significantly.

> Cheers!
> Pocho
> 
> -- 
> Javier Pimás
> Ciudad de Buenos Aires