[Vm-dev] Max number of method arguments

[Nicolas Cellier]

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...
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20190108/1a39c1de/attachment.html>