Thus spake Terry Lambert <[EMAIL PROTECTED]>: > David Schultz wrote: > > Thus spake Ray Kohler <[EMAIL PROTECTED]>: > > > Has anyone tried building world/kernel with high optimizations (-O2, > > > -O3) recently? What breaks? (Booby prize to whoever says "common sense" > > > ;) I last tried it quite a few months ago and the resolver died on me, > > > don't know what else. I'm not really thinking of running like that, but > > > I am curious about others' experiences. > > > > First, let me answer the question that you really meant to ask but > > forgot to, namely, ``How much of a performance difference does -O3 > > make over -O for the kernel/world?'' The answer is ``very little, > > for most purposes.'' So if you do use higher optimization levels, > > at least do a little benchmarking to make sure it was worth it. > > Actually, failure to use optimization suppresses some compilation > warnings, particularly those which normally print from using some > variables without initializing them.
I think you're thinking of dataflow analysis, which I believe gcc does with -O and higher optimization levels. So unless you're using -O0, I would expect that you'd get all the warnings you want. > There are a number of places, particularly on non-i386 platforms, > where optimization actually doesn't work. I think that's why it > was turned off for the libc compilation, and why the bug crept in. > > It's probably useful to compile world with optimization occasionally, > to make compilation-time detectable bugs like that to show up, but, as > you point out, it'd probably be a *bad* idea to actually use the > resulting code, at least until after the next GCC import, which will > supposedly fix the Alpha optimizer. Yes, the possibility of being bitten by compiler bugs is certainly higher with higher optimization levels. Alpha with -O2 seems to have been broken for years, and I have seen strange things happen on IA64 as well. But the i386 code generators have received much wider testing and debugging, so there is somewhat less danger there. To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
