[Ecls-list] Some improvements

[Juan Jose Garcia-Ripoll]

On Sun, Jul 5, 2009 at 3:08 AM, Gabriel Dos
Reis<gdr at integrable-solutions.net> wrote:
>> - Function calls are very expensive.
>
> Do you mean 'C' function calls,

Yes

> PIC is relatively slow compared to non-PIC; but that is the price one
> pays for using dynamically linked objects...
>> - SBCL can avoid these problems because it does not use PIC code: it
>
> Hmm, I'm not sure I understand that.

I think I did not make my point clear by using too much jargon. SBCL
knows the addresses of all functions and objects because it
manipulates the whole memory and has explicit assumptions on where it
is going to "live". Hence it does not use PIC for its own code: it
calls its own functions directly by address or by locating them on a
table whose address is also hard coded. Same goes for symbols: SBCL
knows their address and can directly find the function objects in
them.

In contrast, C compiled code, at least on OS X, has to make a function
call to find out where data and code lives. This is done only once per
function, and only if we refer to the data segment (i.e. global
variables), or when we have switch statements, but it makes function
calls, accessing data and other things slower.

It seems that some of these problems do not exist on ELF platforms,
but I will have to investigate that.

BTW, I think I have been able to hack a type inference pass into the
old compiler. The advantage of the separate path is that it keeps
track of the changes in the types of variables along different
branches of code. I am doing tests right now and the code seems to be
optimal for AREF/ASET and float operations -- the only things for
which I had time to hand code the type inference routines --. I only
hope I am not claiming victory too early :-)

Juanjo

-- 
Instituto de Física Fundamental, CSIC
c/ Serrano, 113b, Madrid 28006 (Spain)
http://juanjose.garciaripoll.googlepages.com