[Vm-dev] There are something fishy with FFI plugin

[Eliot Miranda]

On Wed, Apr 25, 2012 at 4:30 AM, Esteban Lorenzano <estebanlm at gmail.com>wrote:

>
> Hi Eliot,
>
> Ok, after spending 2.5 days on this (but well... now I know a lot about
> this stuff, he), I found that the problem with our compilation is that two
> flags are needed:
>
> STACK_ALIGN_BYTES=16
> ALLOCA_LIES_SO_USE_GETSP=0 //Yep, alloca is not lying here
>
> problem is... while first flag can be set by command line, second one is
> setted on preamble for ThreadedFFIPlugin, then I need to change it...
>
> so, here my question:
>
> Why is working as is for you and not for us? (I think both of us are using
> mingw, and I installed gcc 3.4.5 to use same version as you) Do I'm missing
> something?
>

Are you sure you're actually using   gcc 3.4.5?  Otherwise the difference
could be because you're probably using CMake and I'm not.  i.e. I use the
makefiles
http://www.squeakvm.org/svn/squeak/branches/Cog/cygwinbuild/{Makefile,Makefile.plugin}


> best,
> Esteban
>
> On Apr 25, 2012, at 1:20 AM, Eliot Miranda wrote:
>
>
>
> On Tue, Apr 24, 2012 at 2:53 PM, Igor Stasenko <siguctua at gmail.com> wrote:
>
>>
>> On 24 April 2012 19:16, Torsten Bergmann <astares at gmx.de> wrote:
>> >
>> > Load latest from ConfigurationOfFFI from MetacellRepository:
>> >
>> > For me the FFIPluginTest fails with latest Pharo 1.4 VM
>> > - so at least one has a reproducable case.
>> >
>> Esteban spent whole day today trying to find the offending bug.
>> Source code match 1:1 with Eliot's code, because there was no any
>> changes in it..
>> still the results is that it refuses to work correctly on windoze.
>>
>
> You need to understand what the ThreadedFFIPlugin tries to do with alloca
> and that various compilers may implement alloca in a number of different
> ways.  You then need to look at what the compiler actually does and figure
> out how to subvert it to do what you want to do.
>
> 1.  the ThreadedFFIPlugin uses alloca to stack-allocate the space for the
> outgoing parameters of the FFI call.  Sicne on x86 all parameters are
> passed on the stack and none in registers alloca is ideal.  Notionally the
> plugin can call alloca to extend the stack downwards (stacks grow down, the
> result answered by alloca is a pointer to the start of N bytes, hence it is
> effectively the stack pointer. If this doesn't make sense to you *draw a
> picture on a whiteboard* of what you expect alloca(29) to do)
>
> 2. some C compilers are nice and implement alloca(N) as effectively
> stackPointer := stackPointer - roundUpTo(N,StackAlignment).  For these
> compilers we don't need to do anything.  Some C compilers try and be clever
> and answer stackPointer + 4, or something else.  In these cases we need to
> get the actual stack pointer.  So see the code ThreadedFFIPlugin
> class>>preambleCCode.
>
> 3.  look at what the C compiler produces for the alloca call (and/or run
> under gdb) to see what happens to the stack pointer and what ends up in the
> allocation var in ffiCall:ArgArrayOrNil:NumArgs:.
>
> 4. update ThreadedFFIPlugin class>>preambleCCode. appropriately until
> you've bent alloca and the C compiler to your will.
>
> You will not make this work without trying to understand what is going on
> and looking at what your C compiler produces.  This is low-level hackery
> that aims to avoid having to write any assembler (outside of very few C asm
> statements).
>
> HTH
> Eliot
>
>
>> > Thx
>> > T.
>> > --
>> > NEU: FreePhone 3-fach-Flat mit kostenlosem Smartphone!
>> > Jetzt informieren: http://mobile.1und1.de/?ac=OM.PW.PW003K20328T7073a
>>
>>
>>
>> --
>> Best regards,
>> Igor Stasenko.
>>
>
>
>
> --
> best,
> Eliot
>
>
>
>


-- 
best,
Eliot
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