[Pharo-dev] [Vm-dev] Methods with more than 15 args sketch

Tudor Girba tudor at tudorgirba.com
Sun Jun 19 05:35:58 EDT 2016


Hi,

Does anyone want to pick this up on the Pharo image side?

I took a brief look at this and while playing I noticed that the errors when we send more than 15 arguments seem a bit strange. Take a look at this snippet:
http://ws.stfx.eu/958BXP5GR136
(you can paste it in Spotter to get the playground)

In summary:
- If we compile a method with a signature containing more than 15 arguments we get: 
	"SyntaxErrorNotification: Too many arguments’”.
This is good.

- If we compile a method with a message send containing between 16-31 arguments we get:
	"'InMidstOfFileinNotification’”. 
Ok-ish.

- If we compile a method with a message send containing more than 31 arguments we get: 
	"'Error: genSend:numArgs: numArgs index 32 is out of range 0 to 31’"
This is wrong.


Did I misunderstand something?

Cheers,
Doru



> On Jun 16, 2016, at 8:10 AM, Clément Bera <bera.clement at gmail.com> wrote:
> 
> Hi everyone.
> 
> It seems that some people are struggling with the fact that methods in Pharo (and other Smalltalk on top of Cog) cannot have more than 15 arguments. Typically the problem arises while generating Smalltalk code or porting code from other Smalltalks.
> 
> I have been thinking for a while about this problem. In this mail I write a small sketch to solve it, it's something that could go to production very quickly and that requires almost only image-side changes. Eliot has already reviewed and approved the sketch, but I''d like to know if someone is strongly against that change.
> 
> The generic idea is to change the send calling convention at the bytecode level, using a temp vector to pass overflowing arguments. 
> 
> The normal calling convention in the Smalltalk bytecode is as follows:
> 
> push receiver
> push arg 1 
> push arg 2 
> ....
> push arg N
> send selector
> 
> N is limited to 15 arguments due compiled method header & bytecode encoding. 
> 
> In the new design, if the send has less than 15 arguments, the calling convention remains the same. Over 15 arguments, the overflowing arguments are passed in a temp vector in a similar way to the closure temp vectors.
> 
> The convention will look like that:
> 
> push receiver
> push arg 1 
> push arg 2 
> ....
> push arg N
> popIntoArray N-15 values
> send selector
> 
> The compilation of a method with more than 15 arguments is then changed:
> - As for temp vectors, the 15th arg array can't be become or read-only, hence the temp vector instructions can be used to access the overflowing arguments through the 15th arg.
> - the compiled method header is still marked with 15 arguments
> 
> In addition one needs to change CompiledMethod to:
> - allow larger frames (instead of 56 we could allow 128 or 256)
> - have numArgs answering the right number of methods with many arguments. I think the number of arguments could be in the additional method state in this case as it is very uncommon.
> 
> And to change the fall-back of a couple primitives, such as:
> - Object>>perform: withArgs:
> - Object>>perform: withArgs:inSupedClass:
> - BlockClosure>>valueWithArgs:
> 
> This design would allow methods to up to 141 arguments.
> 
> What do you guys think ? 

--
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