[FFmpeg-devel] [PATCH]Use -parallel when compiling with icc

[Måns Rullgård]

Carl Eugen Hoyos <cehoyos at ag.or.at> writes:

> Reimar D?ffinger <Reimar.Doeffinger <at> stud.uni-karlsruhe.de> writes:
>
>> > > > +  # Use -parallel option, as recommended by Intel, but keep quiet
>> > > > about it
>> > > > +  check_cflags -parallel
>> > > > +  check_cflags -par-report=0
>> > > > +  check_ldflags -parallel
>> > > >  fi
>> > > 
>> > > What effect does that flag have?
>> > 
>> > (From man icc)
>> > It enables the auto-parallelizer to generate multi-threaded code for loops
>> > that can be safely executed in parallel.
>> 
>> You should _very_ carefully benchmark that. It is not unlikely to slow
>> things down, at least when used globally.
>
> Have you any suggestions on what to test?

Benchmark as many samples, and as varied, as you can.

> I was not able to see any slowdown, and both the manual and the
> optimization guide (that do mention the possible slowdown of -O3) do
> not warn of any negative effects.

Whenever the compiler is allowed to get "creative", there is always a
risk of negative effects.

-- 
M?ns Rullg?rd
mans at mansr.com