On Mon, 2007-04-23 at 10:56 -0700, Mark Mitchell wrote: > Kaveh R. GHAZI wrote: > > On Mon, 23 Apr 2007, Mark Mitchell wrote: > > > >> I'm certainly not trying to suggest that we run SPEC on every > >> architecture, and then make -O2 be the set of optimization options that > >> happens to do best there, however bizarre. > > > > Why not? Is your objection because SPEC doesn't reflect real-world apps > > or because the option set might be "bizarre"? > > Some of both, I guess. Certainly, SPEC isn't a good benchmark for some > CPUs or some users. Also, I'd be very surprised to find that at -O2 my > inline functions weren't inlined, but I'd not be entirely amazed to find > out that on some processor that was for some reason negative on SPEC. > I'd be concerned that a SPEC run on Pentium IV doesn't necessarily guide > things well for Athlon, so, in practice, we'd still need some kind of > fallback default for CPUs where we didn't do the Acovea thing. > > I'd also suspect if we move various chips to differing options, we'll > see more bugs due to rarely-tested combinations. Yes, that will help us > fix latent bugs, but it may not improve the user experience. We'll see > packages that used to compile on lots of platforms start to break more > often on one platform or another due either to latent bugs in GCC, or > latent bugs in the application. > > So, I think there's a middle ground between "exactly the same passes on > all targets" and "use Acovea for every CPU to pick what -O2 means". > Using Acovea to reveal some of the suprising, but beneficial results, > seems like a fine idea, though. If every target had a different set of optimizations on at -O2, then I'd claim that we've gone horribly wrong somewhere -- both from a design and implementation standpoint.
If we're going to go down this path, I'd much rather see us tune towards target characteristics than towards targets themselves. Enabling/disabling a generic optimizer on a per-target basis should be the option of last resort. Acovea can be helpful in the process of identifying issues, but I strongly believe that using it in the way some have suggested is a cop-out for really delving into performance issues. Jeff
