Register spilling mechanism

[Igor Stasenko]

2008/4/24  <bryce at kampjes.demon.co.uk>:
>
> Igor Stasenko writes:
>   > 2008/4/22  <bryce at kampjes.demon.co.uk>:
>   > >
>   > > Igor Stasenko writes:
>   > >   > More broader explanation why i need this..
>   > >   > I want to allow direct stack manipulation for native methods to some extent.
>   > >   >
>   > >   > In code, one can write:
>   > >   >
>   > >   > (object1 expression1) push.
>   > >   > (object2 expression2) push.
>   > >   > address call.
>   > >   >
>   > >   > Now i need to make sure that Exupery will produce correct stack frame
>   > >   > for a call, regardless what code inlined in expression1/expression2
>   > >   > it should not interfere with top stack layout, which should be:
>   > >   >
>   > >   > <result of expression1>
>   > >   > <result of expression2>
>   > >   > <return address>
>   > >   >
>   > >   > when entering routine at (address call).
>   > >
>   > >  Are you talking about the C stack or the Smalltalk stack?
>   > >
>   > >  If you're talking about the Smalltalk stack then in Exupery it's only
>   > >  dealt with in the front half of Exupery. It's handled in the
>   > >  ByteCodeReader and IntermediateSimplifier. Exupery uses exactly the
>   > >  same stack locations as the interpreter so doesn't need to check the
>   > >  stack height. It'll break for exactly the same methods that will cause
>   > >  the interpreter to crash. Exupery does model the stack in both classes
>   > >  so it would be fairly easy to monitor stack height if you were
>   > >  interested.
>   > >
>   >
>   > No, forget about Smalltalk stack, i plan to use Exupery at lower
>   > levels to produce machine code fed by my own compiler.
>   >
>   > >  The C stack is maintained by Exupery, and is currently a fixed size
>   > >  but will be dynamically sized sometime in the future. Exupery will
>   > >  fail to compile if it uses more C stack than exists. The amount of C
>   > >  stack used isn't known until after register allocation because that's
>   > >  where registers are spilled to.
>   > >
>   >
>   > I don't know what you mean under 'C stack'.
>   > Isn't Exupery compiles directly to machine code? Then why you speaking
>   > about C stack at all? ;)
>
>  The machine stack which C also uses. The stack follow's C's
>  conventions for calls and returns. It's the stack that the C
>  in the interpreter uses.
>
>
>   > As i understand (correct me if i'm wrong) by fixed size stack you
>   > mean, that after done register allocation, Exupery determines the
>   > exact size of stack, and then it inserts an instruction in method's
>   > preamble to allocate it, like:
>   >
>   > push %ebp
>   > mod %ebp, %esp
>   > sub %esp, stacksize
>   >
>   > and at return point it does reverse:
>   >
>   > add %esp, stacksize
>   > pop %ebp
>   > ret
>   >
>   > or, does just 'ret' , in case if caller are responsible from cleaning
>   > the stack and can restore %ebp.
>
>  It generates sequences like you suggest but they're generated
>  as low level intermediate then instruction selected before
>  register allocation. So the size is set before the number of
>  spills is known. I've been using a fixed size stack frame as
>  at temporary measure, it's been good enough for now, there isn't
>  a great deal of pressure on that memory especially as Exupery
>  can now spill directly into the context frame.
>
>  If you're planning on generating C style calls in code be careful.
>  If a GC happens as part of the call any object may be moved. Exupery
>  doesn't really save the C (machine) stack because it doesn't have any
>  state at any time it would risk making calls, it moves all the state
>  back into real objects so the GC can see it.
>

Don't worry about it. I'm planning to overthrow the rule of C and make
every bit in VM be implemented as native methods
which can be compiled at run time. :)
So, i care little about what calling convention C or other libraries having.
All foreign calls will be handled by FFI class(es) (compiled as
anything else as well), while inside i'll have methods compiled to
machine code and i'm free to choose any calling convention for them,
as well as choose own stack layout which will be convenient for GC and
object memory.
That's why i asked how i can control these aspects with Exupery.

>
>   > Can be there any contracts between Exupery and caller, where i can
>   > state that code should compile under certain rules, like:
>   > - specify a set of registers, which should be preserved after given
>   > code done working
>   > - specify, what registers code expects to be preserved when doing calls
>   > - specify, what registers should be reverted just before ret/non-local
>   > jump instructions
>
>  It would be possible to control what you want to, but there's no
>  published interface. The intermediate languages will change as
>  Exupery evolves.
>

Thank you for explanation. I will look forward for any updates
concerning these features.
Maybe, as temporary solution, i can patch register allocator to get
some feedback on how many stack space i need, and then
re-run it again.
Or i can try to measure maximum number of live temps in code by own.

>
>
>  Bryce
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-- 
Best regards,
Igor Stasenko AKA sig.