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

[Clément Bera]

On Wed, Feb 28, 2018 at 2:16 PM, 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). 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. Looking at the slang code
> I found CogOutOfLineLiteralsX64Compiler, but it seems it is not used
> (yet?).
>

Just generate & disassemble any method with literals from the Squeak image
(in-image compilation workspace scripts) with different ISA and look at the
disassembled code, it takes 1 second and you'll see by yourself how pointer
size constants (literals) are dealt with.

The ARM JIT dumps pointer size constants somewhere unreachable by machine
code execution but near by (in-between jumps for example) and access it
with PC-relative instructions, that's one good way to work around it.

Best,

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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