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

[Clément Bera]

On Fri, Mar 2, 2018 at 3:44 PM, Guido Chari <charig at gmail.com> wrote:

>
> Hi Eliot,
>
> 2018-02-28 16:03 GMT+01:00 Eliot Miranda <eliot.miranda at gmail.com>:
>
>>
>> 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.
>>
>
> There are plenty of scripts. I ran a few that failed while loading code.
> Could you point us to the one you are using?
>

My normal workflow from loading is to run StackVMSimulator from VM
simulation workspace, then Cog Simulator from VM simulation workspace, then
in-image compilation from In-image compilation workspace. I frequently pick
up a new image and annoy Eliot if it does not work, so normally things
should be working. Sometimes the last one does not work if the first ones
are not run first. In any case in-image compilation requires a VM with a
processor simulator external plugins (else it fails on prim New CPU). To
build such a VM go in the build folder and follow HowToBuild file
instructions, especially the section on Bochs.

StackVMSimulator script looks like

| cos |
cos := StackInterpreterSimulator newWithOptions: #( ...

CogSimulator script looks like
| cos |
cos := CogVMSimulator newWithOptions: #(...

In-image compilation are the scripts using #genAndDis:options:


>
>>
>> > 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
>>
>
>
>


-- 
Clément Béra
Pharo consortium engineer
https://clementbera.wordpress.com/
Bâtiment B 40, avenue Halley 59650 Villeneuve d'Ascq
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