Re: [9fans] Plan 9 on Blue Gene

["ron minnich" <rminnich@gmail.com>]

On Wed, Jul 30, 2008 at 11:03 AM, don bailey <don.bailey@gmail.com> wrote:
>> Can you elaborate here? What tricks can the IBM compilers use
>> that the Plan 9 ones can't? Are we talking optimization?

yes. Quite impressive optimization. Which results in very high
measured performance. At least when I've looked at it. The XLF/XLC
compilers represent a few decades of very good work from IBM.

> No, really, that's not troll bait. I'm actually interested in
> understanding the project's basis for discriminating against
> specific compiler capability. Obviously Plan 9's compiler
> isn't optimal.. but what really are the requirements people
> want?
>

it's not taken as such.

The simplest requirement is "XLC version x.y.z". Why? Because these
programs are beasts. Once they are working people are not inclined to
change the compiler and runtime, as that is a frequent source of
breakage. Sometimes you really do want the compiler bug.

That said, if you look at the code XLF/XLC emit, it's pretty
impressive. I suggest you grab the released papers on them. There's an
ugly effect that can occur on big parallel machines. Very small
performance deltas on small numbers of nodes can result in  huge
deltas on the whole machine. So the kinds of things you don't care
about on, e.g., your desktop, become utterly crucial on lots of nodes.

ron