[Vm-dev] [64 bits] Object pointers in jitted code
elpochodelagente at gmail.com
Wed Feb 28 15:33:11 UTC 2018
On Wed, Feb 28, 2018 at 12:03 PM, Eliot Miranda <eliot.miranda at gmail.com>
> Hi Javier,
> > On Feb 28, 2018, at 5:16 AM, Javier Pimás <elpochodelagente at gmail.com>
> > 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
yes, I meant a solution like this. You also need a scratch register if you
want to push a pointer but it's not a huge problem.
> > 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:
> genAndDis: Object>>#printOn: "includes 'a ' and 'an '"
> options: #(ObjectMemory Spur64BitCoMemoryManager)
Nice! I'll try it
> 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
Ciudad de Buenos Aires
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