[Vm-dev] 64 bits

[Igor Stasenko]

On 4 May 2012 18:27, Eliot Miranda <eliot.miranda at gmail.com> wrote:
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> On Fri, May 4, 2012 at 7:44 AM, Bert Freudenberg <bert at freudenbergs.de> wrote:
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>> On 04.05.2012, at 16:33, David T. Lewis wrote:
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>> > On Fri, May 04, 2012 at 02:36:28PM +0200, Bert Freudenberg wrote:
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>> >> The OP's question was about a Cog VM compiled for x86_64 - is that easily available?
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>> >> - Bert -
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>> > No, that would be a larger project. Eliot could speak to this better
>> > than I can, but in general terms I expect that this would involve first
>> > building a 64-bit StackVM (i.e. the current StackVM compiled in 64-bit
>> > mode), and then developing the code generation for Cog. Both of these
>> > would involve work above and beyond what I described above for the
>> > interpreter VM. My assumption would be that these are large enough
>> > chunks of work that we should not expect them to happen on a strictly
>> > volunteer basis.
>> >
>> > Dave
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>> I thought that x86-64 was a strict extension of the 32 bit x86 model. So Cog's generated 32 bit code should work just fine even under x86-64.
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> That's not quite true.  x86-64 is actually a different ISA that adds 8 registers and a pc-relative addressing mode drops some instructions (inc dec, others?).  IIRC one has to go through a mode switch to But the processor can execute both ISAs. On x86-64, x86 is a "sub-mode" and is entered via interrupts or "gate descriptors" (?). See e.g. http://wiki.osdev.org/X86-64#Entering_Long_Mode_directly.  So executing 32-bit code inside a 64-bit process isn't trivial.  Instead most x86-64 platforms do provide support for running x86 binaries in the x86 submode.
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> I wish it was easy, but I fear not.  At the very least we would want a 32-bit to 64-bit transition on calling the FFI.    Alternatively we could look at extending the Cog glue so that generated code is x86 but the glue calls x86-64 code in the rest of the VM.  I don't have cycles for this, and personally would rather see work on a real x86-64 code generator with a full 64-bit image.
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Yes, 64bit mode is similar but not the same as 32bit. And of course
you cannot run binary compiled
for 32bits and expect that it will work correctly.

[a little OT] FWIW recently Camillo (with a bit of my help) added
support for 64bit assembler in AsmJit project.
So, actually we can produce 64-bit compatible code using smalltalk :)
But of course it is too far from having JIT compiler in VM.


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>> Cog would have to be cleaned up for pointer operations of course, just as you did for the interpreter.
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>> - Bert -
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> --
> best,
> Eliot
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-- 
Best regards,
Igor Stasenko.