[Vm-dev] 64bit FFI?

Eliot Miranda eliot.miranda at gmail.com
Thu Feb 12 17:22:55 UTC 2009

On Thu, Feb 12, 2009 at 9:15 AM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

> On Thu, Feb 12, 2009 at 1:32 AM, Andreas Raab <andreas.raab at gmx.de> wrote:
>> Folks -
>> Has anyone tried porting the FFI to 64 bit? I'm loosely considering using
>> a 64bit VM for our servers but I do need the FFI (mostly for ODBC
>> integration) and I've never looked at any of the ABIs. Alternatively, is it
>> reasonable to try to use 64bit libs from within a 32bit Squeak VM?
> Depends on what one wants to pass through the FFI.  On x86-64/EMT64
> structure passing conventions are efficient and complex, distributing
> structure fields across integer and floating-point registers.  It is
> possible to interpret such calls given a type signature for the structure to
> be passed (I did it for VisualWorks) but it is not easy and it is slow.  I
> would counsel writing a proper ABI compiler in Smalltalk that generates e.g.
> RTL that is either interpreted or compiled by the VM to make the actual
> call.
> In theory the use of 64-bit libs, provided one has the relevant FFI support
> should work fine.  Smalltalk has the ability to deal with 64-bit and 128-bit
> values easily.  But one does have to write all the support.

I should expand on this, and could be completely wrong.  One would have to
use a 64-bit address space, which implies compiling the 32-bit VM using a
64-bit compiler.  When DEC and ParcPlace co-implemented 32-bit VisualWorks
(my first job on joining ParcPlace was to productise this port) we used the
DEC C compiler's ability to define "short" (32-bit) pointers and put the
executable in the lower 32-bits of the address space (google for e.g. -xtaso
or taso).  Hence the VM was a 64-bit executable which had a 32-bit Smalltalk
living in the bottom 32-bits.  If x86-64 compilers have the same short
pointer support then the compilation of a 32-bit Squeak VM should be
relatively easy.  Otherwise one will have to define the longAt: longAt:put:
et al machinery to extend 32-bit addresses into full 64-bit pointers and
genuflect to the linker and/or memory allocator to get it to place the heap
low in the address space.

> To get acquainted with the de facto standard x86-64 ABI read the
> System V Application Binary Interface AMD64 Architecture Processor
> Supplement <http://www.x86-64.org/documentation/abi-0.98.pdf>
> pages 13 through 22 (section Function Calling Sequence) paying particular
> attention to the 3 1/2 page algorithm for marshalling :)
> and start going to bed earlier ;)
>> Thanks for any info!
>> Cheers,
>>  - Andreas
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