Re: [Lightning] On the benefits of inlining

[Paolo Bonzini]

> While it works, it turns out to be quite inefficient.  It's even
> slightly less efficient than the function-call-style equivalent.
>  
the overhead is high and patching is not lightweight, either.  i have 
not understood exactly when you do it.

but be careful, you do need to call jit_prolog to set up lightning 
before generating the first instruction.  GNU Smalltalk has some 
examples of compiling prolog-less subroutines that are used by the 
jit-ed code.

Instead of the goto you can put an asm ("jmp *%0" : : "r" (&&abc) : 
"eax", "ecx", "edx"); so you can avoid the push/pop and GCC does not 
think that no instruction will execute after the jump (which it does, if 
you have a goto).  It is unportable, however. :-(

> Have you ever tried such things?  Clearly, this would only make sense
> for very small pieces of code like this one (there's actually only one
> useful instruction so making it a function induces too much overhead).
>  
This would *not* make sense for very small pieces of code like this one, 
or did I miss something?

> I believe it would be useful to have back-ends export a mapping between
> the Lightning register names and the corresponding GCC register names
> for that architecture.  In the above example, it would allow to pass the
> input operand directly into the appropriate register (via an `asm'
> statement) and fetch the result likewise.
>
> What do you think?
>  
Yes, that's right.  I have not thought about it, though.

Paolo