[Vm-dev] Max number of method arguments

[Nicolas Cellier]

Le mar. 8 janv. 2019 à 22:51, Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> a écrit :

> Hi all,
> particularly Clement and Eliot,
>
> One of the most annoying limit of bytecode is the number of arguments (<16
> in V3), not so much annoying for pure Smalltalk, but certainly so for FFI
> (FORTRAN 77 lacks structures so existing code base often have functions
> with many arguments).
> For scientific Smalltalk, some of those old FORTRAN libraries are still
> around nowadays (LAPACK is an example).
>
> I patched the old Squeak compiler in Smallapack to workaround this
> limitation (it was easy enough to pass a single Array, and invoke FFI with
> many args).
> In modern Pharo flavour, this is more involved with the new OpalCompiler
> (iit does not seem to be designed for extensibility as it seems necessary
> to patch many pieces/subclasses for a single feature change...).
>
> But we now have Sista V1 bytecodes which removed a lot of limitations (#
> inst vars, #literals, max jump offset ...). Alas I don't see a modified
> limit for number of arguments (source:
> https://hal.inria.fr/hal-01088801/document a bytecode set for adaptive
> optimization): there is still a limit of 4 reserved bits in compiled method
> header documented in link above.
> Though, there is an adjacent unused bit now...
> In Squeak,/Pharo, EncoderForSistaV1>>genSend:numArgs: suggests that the
> limit is 31 (sic)
>
>     (nArgs < 0 or: [nArgs > 31]) ifTrue:
>         [^self outOfRangeError: 'numArgs' index: nArgs range: 0 to: 31
> "!!"].
>
> or at least 2047 if we believe code below:
>
>     "234        11101010    i i i i i j j j    Send Literal Selector
> #iiiii (+ Extend A * 32) with jjj (+ Extend B * 8) Arguments"
>
>
> https://github.com/pharo-project/pharo/blob/50992c3e5fed790b7e660954aee983f4681da658/src/Kernel-BytecodeEncoders/EncoderForSistaV1.class.st
>
> Pharo also limit the numArgs to 15 whatever the encoding in
> CompiledMethod>>newBytes:trailerBytes:nArgs:nTemps:nStack:nLits:primitive:
>
> https://github.com/pharo-project/pharo/blob/50992c3e5fed790b7e660954aee983f4681da658/src/Kernel/CompiledMethod.class.st
>
> But Squeak does not limit nArgs at all in
>
> EncoderForSistaV1>>computeMethodHeaderForNumArgs:numTemps:numLits:primitive:
>
> So my questions:
> - is that doc up-to-date?
> - if so, couldn't we expand the limit to 31 args by using the unused bit?
>
> Note: there is another unused bit in V3 (not adjacent), and the double
> extended (send) byte code has room for 31 args in V3 too, since only the
> first 3 bits of second byte encode the type of operation...
>

Confirmed in VMMaker: max num args is still 15 in CompiledMethod header

StackInterpreter>>argumentCountOfMethodHeader: header
    <api>
    ^header >> MethodHeaderArgCountShift bitAnd: 16rF

I see that this decoding is shared whatever byte-code alternative...
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