[pypy-dev] CLI code generation (was: Svn account?)

[Antonio Cuni]

holger krekel wrote:
> Hi Antonio, 
> 
> On Sun, Mar 19, 2006 at 20:53 +0100, Antonio Cuni wrote:
>> as I said I've begun writing the .NET CLI backend; it is still very 
>> experimental but it can already compile correctly some code snippets 
>> such as the algorithm for computing fibonacci's numbers.
> 
> cool!  I would be interested to hear a bit more about your concrete
> current approach. 

I respond here so that other can read, if they are interested.

The first decision I took is whether to generate IL code (to be 
assembled with ilasm) or C# code: I choose the first mainly because C# 
lacks the goto statement and it would be difficult to implement flow 
control.

Given this, my current approach if fairly naive and certainly not 
efficient: at the moment the compiler does a 1-to-1 translation between 
low level operation expressed in SSA; for example the simple function:

def bar(a,b):
     return a+b

is compiled into the following IL code:

.method static public int32 bar(int32 a_1, int32 b_1) il managed
{
     .locals (int32 v6, int32 v12)

block0:
     ldarg.s 'a_1'
     ldarg.s 'b_1'
     add
     stloc.s 'v12'
     ldloc.s 'v12'
     stloc.s 'v6'
     br.s block1

block1:
     ldloc.s 'v6'
     ret
}

As you can see there are many unnecessary operations: the result of 
'add' is stored to v12, loaded from v12, stored in v6 and then loaded 
from v6! The same is true for the branch instruction, which is unneeded.
I think it should be fairly simple to translate from the SSA form to a 
more "stack-friendly" form useful for stack-based machines; the question 
is: where to put such code?
Since it could be useful even for other backends, it would be nice to 
put it in some place where it can be shared from several backends: one 
option could be to write it as a backend optimization, but in this case 
we have to introduce new low level operations for stack manipulation, 
such as 'push', 'pop' or 'dup'.

ciao Anto