David Edelsohn wrote: > On Fri, Nov 20, 2009 at 10:05 AM, Ian Bolton <bol...@icerasemi.com> > wrote: > > From some simple experiments (see below), it appears as though GCC > aims > > to > > create a lop-sided tree when there are constants involved (func1 > below), > > but a balanced tree when there aren't (func2 below). > > > > Our assumption is that GCC likes having constants all near to each > other > > to > > aid with tree-based optimisations, but I'm fairly sure that, when it > > comes > > to scheduling, it would be better to have a balanced tree, so sched > has > > more > > choices about what to schedule next? > > I think this would depend on the target architecture and instruction > set: CISC vs RISC, many registers vs few registers, etc. I do not > believe that GCC intentionally is trying to optimize for either, but I > do not think there is a single, right answer. > Regardless of the architecture, I can't see how an unbalanced tree would ever be a good thing. With a balanced tree, you can still choose to process it in either direction (broad versus deep) - whichever is better for your architecture - but, as far as I can see (bearing in mind that I'm very new to GCC development!), a tall lop-sided tree gives few scheduling options due to all the extra dependencies. I guess I must be missing something?
Regards, Ian
