Re: Small LAP peephole optimization

[Ken Raeburn]

On May 10, 2007, at 10:21, Dmitry Antipov wrote:
> What do you think about such 'unsafe' optimizations in general ? As  
> I know,
> some CL systems (such as from Franz) allows byte compiler to be  
> very aggressive
> at the cost of safety.

Generally, in the absence of either a directive from the code author  
saying such assumptions are valid, or some language spec (or other  
documentation readily available to the code authors) that says that  
such assumptions may be made in compilation, I don't think such  
optimizations to valid code are a good idea.

> As I understand, branching to +0 is impossible if there is no TAG  
> between
> previous byteop an (byte-constant 0), so we might safely optimize the
> sequences like
>
> <numeric-on-top-op> (byte-constant 0) (byte-plus . 0) -> <numeric- 
> on-top-op>

I'm not familiar with the representation of byte code in the  
compiler, but if labels are explicit in the sequence so that you can  
see that there isn't one there, then yes, that looks correct.

> Less obvious cases are also interesting, but I'm not sure that  
> saving 2 ops
> might push someone to implement substantially more complex logic.

Right, probably not worthwhile, at least right now.

Ken