[fpc-devel] Optimizations

Martin Frb lazarus at mfriebe.de
Fri Jan 24 00:04:27 CET 2014

On 23/01/2014 22:26, August Oktobar wrote:
> Hello, I have seen your mails about peephole optimization, so I wonder 
> if you could look at this reports
> http://bugs.freepascal.org/view.php?id=23595
> or perhaps optimize slow array access using operator [] (it is faster 
> to use pointer arithmetics)
> thanks!

1) I am just getting started on this, so I can only answer with limited 
     @experts, please correct me below, where needed
2) The peephole opt is only something I do "on the side" as it currently is.

 From a quick glance, this does not look like something for the peephole 

The peephole opt currently looks at statements that are close together 
(follow each other immediately).
I am not sure to which extend (if at all) it would be acceptable to 
break that limit (it is doable, question is if desired).

In this specific case:
1) between the "fstpl" and the "mov (half_the_data), %edi" are other 
   detecting the connection would either:
   - need scanning several statements ahead. This would be slow, because 
it had to be done after each storing "something to memory" (so very often)
   - keeping state of all the involved registers and memory (do-able / 
interesting at least from a theoretical view / but not sure if desired)
2) only half the mem is accessed, and then the other half. That means to 
detect the connection between the mem read and the register, it is 
needed to analyse 4 statements. Very unlikely to see this in the 
peephole opt.

The livelihood of "a[i]" needs to be checked where the code is generated.

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