ffc team mailing list archive

[Kristian Ølgaard <k.b.oelgaard@xxxxxxxxx>]

On 12 September 2011 21:36, Marie E. Rognes <meg@xxxxxxxxx> wrote:
> On 09/12/11 20:00, Marie E. Rognes wrote:
>>
>> On 09/12/11 19:54, Garth N. Wells wrote:
>>>
>>> On 12 September 2011 18:49, Marie E. Rognes<meg@xxxxxxxxx>  wrote:
>>>>
>>>> On 09/12/11 19:40, Garth N. Wells wrote:
>>>>>
>>>>> Which compiler options did you use when evaluating the speed up?
>>>>>
>>>> Tested Extrapolation.h with vanilla dolfin (which is dominated by
>>>> evaluate_basis calls). No additional compiler options set.
>>>>
>>>> What are the default compiler options?
>>>>
>>> '-g' for plain JIT, which is dead slow.  You should test with at least:
>>>
>>>     parameters["form_compiler"]["cpp_optimize"] = True
>>>
>>> in the Python code. This will use '-O2'.

Isn't this limited in a way? Would it be a problem to let users do:

parameters["form_compiler"]["cpp_optimize"] = '-O2 -funroll-loops'
parameters["form_compiler"]["cpp_optimize"] = '-O3'

and then perhaps let

parameters["form_compiler"]["cpp_optimize"] = True

default to '-O2' as we do now?
Just a thought.

>>
>> Ok, thanks -- I'll take a closer look.
>>
>
> Take a look at the attached results in old_evaluate_basis.txt (results with
> "old" FFC),
> and new_evaluate_basis.txt (results with "new" FFC) from running the
> attached
> test_evaluate_basis.py.
>
> Acceptable?

Looks good, and the generated code is much nicer now. :)
It could have been fun to see the impact of the '-O2 -funroll-loops'
option on the old code, but then you'll have to switch to C++. Anyway,
I'm quite sure that the old code will never perform as well as the new
code even with this option.

As you have probably found out, the generated code was simply a mirror
of what is going on in FIAT (translated to C++).
Perhaps there are more places where we can simplify the generated code?

Another thing in relation to improving the evaluate_basis* functions
that I have thought about is if it's really necessary to support
derivatives of arbitrary order. If we only generate code for the first
derivative by default (and support arbitrary derivatives by a command
line argument) the code will be a lot simpler (easier on C++ compiler)
and much faster irrespective of which gcc optimisation is being used.

Kristian

> --
> Marie
>
>
>
>
>
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