Awesome, Chris.  Would you believe we were just talking internally about this topic yesterday?  We were considering the amount of waste if there were big gaps of "off" points in the middle.  But I think your solution is quite a bit more elegant than what we were discussing (a list of "on" points).  I like how it devolves into (begin,end] for the common case of all points on.

It's a pretty big overhaul, touches every shadeop and a lot of templates, and the call signatures have to be changed (to what, exactly? passing a std::vector<int>& ? or a combo of int *begend and int segments?).  Ugh, and we may wish to change/add OIIO texture routines that take runflags, too.  But your experiment is quite convincing.

What does everybody else think?  Chris/Cliff/Alex?  (I'm happy to do the coding, but I want consensus because it touches so much.)

       -- lg

On Jan 21, 2010, at 8:21 AM, Chris Foster wrote:

> Hi all,
>
> I've been looking through the OSL source a little, and I'm interested to see
> that you're using runflags for the SIMD state.  I know that's a really
> conventional solution, but there's an alternative representation which I reckon
> is significantly faster, and I wondered if you considered it.
>
> Imagine a SIMD runstate as follows
>
> index: [ 0  1  2  3  4  5  6  7  8  9 ]
>
> flags: [ 0  0  1  1  1  1  0  0  1  0 ]
>
>
> An alternative to the flags is to represent this state as a list of active
> intervals.  As a set of active start/stop pairs, the state looks like
>
> active = [ 2 6  8 9 ]
>
> ie,
>
> state: [ 0  0  1  1  1  1  0  0  1  0 ]
>               ^           v     ^  v
>               2           6     8  9
>
> The advantage of doing this is that the inner SIMD loops become tighter since
> there's one less test.  Instead of
>
> for (int i = 0; i < len; ++i)
> {
>    if (state[i])
>        do_something(i);
> }
>
> we'd have
>
> for (int j = 0; j < nActiveTimes2; j+=2)
> {
>    for (int i = active[j]; i < active[j+1]; ++i)
>        do_something(i);
> }
>
> and the inner loop is now completely coherent.
>
> I can see you have the beginnings of this idea in the current OSL code, since
> you pass beginpoint and endpoint along with the runflags everywhere.  However,
> why not take it to its logical conclusion?
>
> Given that you already have beginpoint and endpoint, the particularly big win
> here is when most of the flags are turned on.  If do_something() is a simple
> arithmetic operation (eg, float addition) the difference between the two
> formulations can be a factor of two in speed.
>
>
> I'm attaching some basic test code which I whipped up.  Timings on my core2 duo
> with gcc -O3 look like:
>
>
> All flags on (completely coherent):
>
> run flags:        1.310s
> active intervals: 0.690s
>
>
> Random flags on (completely incoherent):
>
> run flags:        5.440s
> active intervals: 3.310s
>
>
> Alternate flags on (maximum number of active intervals -> worst case):
>
> run flags:        1.500s
> active intervals: 2.150s
>
>
> Thoughts?
>
> ~Chris.

> <ATT00001..txt><runstate.cpp>

--
Larry Gritz
l...@...





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