[CLOSED] FOR loop unrolling

[julkas]

When compiler unroll loops?
Is it general behavior or ... ?

[Thaddy]

 As with any compiler that supports it: what is the complexity of the loop?
A loop of 5-6 iterations may be unrolled but 10000 iterations is silly.
On small loops - with a determinate finish - FPC does an excellent job.

[marcov]

Unroll till 1500 muops ?

[julkas]

And how disable / enable loop unrolling in code?
I compile with -O3 option.
Can be unrolled while and repeat loops or only for?

[Thaddy]

And how disable / enable loop unrolling in code?
I compile with -O3 option.
Can be unrolled while and repeat loops or only for?

-OoLOOPUNROLL-

[Handoko]

According to the documentations:
https://wiki.freepascal.org/Optimization
https://www.freepascal.org/docs-html/current/prog/progsu58.html

if (-O3 or -O4) and (FPC version >= 3.04) then loop unrolling on

or you can manually turn it on by

{$OPTIMIZATION LOOPUNROLL}

[lucamar]

Can be unrolled while and repeat loops or only for?

AFAIK only FOR loops are candidates to be unrolled.

[julkas]

Can be unrolled while and repeat loops or only for?

AFAIK only FOR loops are candidates to be unrolled.

Yes . AVR compiler unrolls simple for loops (number iterations - 30) but not while loops.

[julkas]

According to the documentations:
https://wiki.freepascal.org/Optimization
https://www.freepascal.org/docs-html/current/prog/progsu58.html

if (-O3 or -O4) and (FPC version >= 3.04) then loop unrolling on

or you can manually turn it on by

{$OPTIMIZATION LOOPUNROLL}

Ok. Assume I have LOOPUNROLL enabled, how disable unroll.
I want manually control UNROLL in my code.

[Laksen]

Code: Pascal  [Select][+][-]

1. {$optimization noloopunroll}

[lucamar]

Code: Pascal  [Select][+][-]

1. {$optimization noloopunroll}

Does that even exists? AFAICT the only recognized $optimization arguments (beyond ON/OFF) are these, from compiler/globtypes.pas:

Code: Pascal  [Select][+][-]

1. const
2.        OptimizerSwitchStr : array[toptimizerswitch] of string[17] = ('',
3.          'LEVEL1','LEVEL2','LEVEL3',
4.          'REGVAR','UNCERTAIN','SIZE','STACKFRAME',
5.          'PEEPHOLE','ASMCSE','LOOPUNROLL','TAILREC','CSE',
6.          'DFA','STRENGTH','SCHEDULE','AUTOINLINE','USEEBP','USERBP',
7.          'ORDERFIELDS','FASTMATH','DEADVALUES','REMOVEEMPTYPROCS',
8.          'CONSTPROP',
9.          'DEADSTORE','FORCENOSTACKFRAME'
10.        );

[Laksen]

Looks like that isn't documented, but adding 'NO' in front of an optimization will turn it off. The same way as it's done on the commandline(this is documented)

[lucamar]

Looks like that isn't documented, but adding 'NO' in front of an optimization will turn it off. The same way as it's done on the commandline(this is documented)

Thanks. Found it (at last, and only for the command tail) in Appendix A of the User Guide:

-O<x>  Optimizations:
      [...]
      -Oo[NO]<x> Enable or disable optimizations; see fpc -i or fpc -io for possible values

So yes, adding NO in front of a $optimization argument should work.

Thanks again for the tip, Laksen

[valdir.marcos]

Unroll till 1500 muops?

What muops stands for?

[Hartmut]

I found this topic but had no idea, what "Loop unrolling" could be. The provided links of the FPC documentation

https://wiki.freepascal.org/Optimization
https://www.freepascal.org/docs-html/current/prog/progsu58.html

only tell the syntax, how to enable/disable this function, but not what it means.

After some research I found https://en.wikipedia.org/wiki/Loop_unrolling which explains this method in detail. Maybe this helps other readers.