On Wed, Feb 28, 2018 at 12:03 PM, Eliot Miranda eliot.miranda@gmail.com wrote:
Hi Javier,
On Feb 28, 2018, at 5:16 AM, Javier Pimás elpochodelagente@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
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:
StackToRegisterMappingCogit 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