Re: Massive performance regression from switching to gcc 4.5

[Taras Glek <tglek at mozilla dot com>]

On 06/24/2010 12:06 PM, Andrew Pinski wrote:
> On Jun 24, 2010, at 11:50 AM, Taras Glek <tglek@mozilla.com> wrote:
>
> > Hi,
> > Just wanted to give a heads up on what might be the biggest 
> > compiler-upgrade-related performance difference we've seen at Mozilla.
> >
> > We switched gcc4.3 for gcc4.5 and our automated benchmarking 
> > infrastructure reported  4-19% slowdown on most of our performance 
> > metrics on 32 and 64bit Linux.
> >
> > A lone 8% speedup was measured on the Sunspider javascript benchmark 
> > on 64bit linux.
> >
> > Here are some of the slowdowns reported:
> > http://groups.google.com/group/mozilla.dev.tree-management/browse_thread/thread/77951ccb76b5e630# 
> >
> > http://groups.google.com/group/mozilla.dev.tree-management/browse_thread/thread/624246d7d900ed41# 
> >
> >
> >
> > Most of the code is compiled with   -fPIC  -fno-rtti -fno-exceptions -Os
>
> Stop right there. You are compiling at -Os, that is tuned for size and 
> not speed. So the question is did the size go down? Not the speed 
> decreased. Try at -O2 and report back. I doubt we are going to do a 
> tradeoff for speed at -Os at all.
> Thanks,
> Andrew Pinski
Good point.

Looks like the actual problem is that at -Os there is less inclining 
happening in 4.5 vs 4.3, which results in a bigger binary and slower code.

I tried 4.5 -O2 and it's actually faster than 4.3 -Os.

I am happy that -O2 performance is actually pretty good, but -Os 
regression is going to hurt on mobile.

Taras