[CLOSED] FOR loop unrolling

[julkas]

Code: Pascal  [Select][+][-]

1. {$optimization noloopunroll}

Thanks. But why it's undocumented?
$OPTIMIZATION : Enable Optimizations - https://www.freepascal.org/docs-html/current/prog/progsu58.html#x65-640001.2.58.

And main question - when compiler unrolls ?

In  some  cases,  we  could  unroll  loops  to  speed  up  the  code  execution.  This  is 
especially effective for short loops.

FPC AVR with -O3 unrolls for loop with 30 iterations.
GCC AVR compiler unrolls not only for loops. See (p.13) - https://ww1.microchip.com/downloads/en/AppNotes/doc8453.pdf

[Laksen]

It's a documentation bug. Please put an issue in the bugtracker

The loop unroller should probably be fine tuned for AVR. It's mostly tuned for high end processors right now. It either way only triggers on loops with known bounds

[julkas]

It's a documentation bug. Please put an issue in the bugtracker

The loop unroller should probably be fine tuned for AVR. It's mostly tuned for high end processors right now. It either way only triggers on loops with known bounds

Check - https://bugs.freepascal.org/view.php?id=36464.

[julkas]

It's a documentation bug. Please put an issue in the bugtracker

The loop unroller should probably be fine tuned for AVR. It's mostly tuned for high end processors right now. It either way only triggers on loops with known bounds

only triggers on loops with known bounds - Can you give more info?

[Laksen]

The loop unroll pass first calculates number of unrolls. It does this by dividing an estimated loop body complexity with a fixed processor specified number.
Then it looks at the lower and upper bounds. If they are constant it can find out exactly how many times to unroll and where to possibly insert an odd entry (if you are unrolling a loop 4 times but it runs 10 iterations originally)

If the lower and upper bounds are not constant then it doesn't do anything:
https://svn.freepascal.org/cgi-bin/viewvc.cgi/trunk/compiler/optloop.pas?view=markup#l234

This has constant bounds:

Code: Pascal  [Select][+][-]

1. for i:=0 to 7 do
2.  

This does not have constant bounds:

Code: Pascal  [Select][+][-]

1. for i:=0 to some_variable do
2.  

[julkas]

Thanks for help.