> I have done quite a few experiments with this to > narrow down the problem. The performance numbers are > slower compared to *No Feedback optimization with just > -O3* Here are some of them. All the experiments were > done on a new build-area in order to eliminate effects > of old feedback files. > > 1. I built the app using -O3 and -fprofile-generate to > generate the feedback data. I then ran the workload > and then recompiled the app using -O3 and > -fprofile-use [app was 20% slower] > > 2. I built the app using -O3 and -fprofile-generate to > generate the feedback data. I then ran the workload > and then recompiled the app using -O3 and > -fprofile-use -fno-vpt -fno-unroll-loops > -fno-peel-loops -fno-tracer (Which is turn off all the > flags used by -fprofile-use) [App was still 20% > slower] > > 3. I have tried selectively turning of some of the > other flags in the above list as well, but the > performance regression persists. > > 4. I tried with the older flags namely -fprofile-arcs > and -fbranch-probabilities still no help.
So it looks like the slowdown is caused by one of the profile based optimizations that are enabled by default (basic block reordering or register allocation). If you are getting such a noticable slodown, it probably means that your app has pretty small inner loop. Can you just look into assembly generated for it with and without profiling and try to spot what is gong wrong? Thanks, Honza > > Can someone help me out on how to proceed with this. > > Thanks > -girish > > > --- Jan Hubicka <[EMAIL PROTECTED]> wrote: > > > > I started with a clean slate in my build > > environment > > > and did not have any residual files hanging > > around. > > > Are the steps I have indicated in my earlier email > > > correct. Is there a way I can break down the > > problem > > > into a smaller sub-set of flags and eliminate the > > flag > > > causing the performance problem. What I mean is > > since > > > -fprofile-generate and -fprofile-use enable a > > bunch of > > > flags, would it make sense to avoid profiling and > > try > > > out some of the individual flags on a trial and > > error > > > basis. If so what would be the flags to start the > > It would be probably better to just turn off the > > individual > > optimizations with -fprofile-use (for optimizations > > that are implied by > > this flag there should be no need to re-profile each > > time). > > If you can find particular optimization that gets > > out of control, it > > would be lot easier to fix it... > > > > Honza > > > trials with. > > > > > > -girish > > > > > > --- Jan Hubicka <[EMAIL PROTECTED]> wrote: > > > > > > > > On Wed, Jul 20, 2005 at 10:45:01AM -0700, > > girish > > > > vaitheeswaran wrote: > > > > > > > --- Steven Bosscher <[EMAIL PROTECTED]> > > wrote: > > > > > > > > > > > > > > > On Wednesday 20 July 2005 18:53, girish > > > > vaitheeswaran wrote: > > > > > > > > > I am seeing a 20% slowdown with > > feedback > > > > optimization. > > > > > > > > > Does anyone have any thoughts on this. > > > > > > > > > > > > > > > > My first thought is that you should > > probably > > > > first > > > > > > > > tell what compiler > > > > > > > > you are using. > > > > > > > > > > > > I am using gcc 3.4.3 > > > > > > -girish > > > > > > > > > > Which platform? I've seen slower code for > > > > profile-directed optimizations > > > > > on powerpc64-linux with GCC 4.0 and mainline. > > > > It's a bug, but I haven't > > > > > looked into it enough to provide a small test > > case > > > > for a problem report. > > > > > > > > Actually I would be very interested in seeing > > > > testcases such as those. > > > > (and the Girish' slowdown too if possible). In > > > > general some slowdowns > > > > in side corners are probably unavoidable but > > both > > > > 3.4.3 and 4.0 seems to > > > > have pretty consistent improvements with > > profiling > > > > at least for SPEC and > > > > i386 I am testing pretty regularly. > > > > Such slodowns usually indicate problems like > > > > incorrectly updated profile > > > > or incorrectly readed in profile because of > > > > missmatch in CFGs in between > > > > profile and feedback run that are rather > > dificult to > > > > notice and hunt > > > > down... > > > > > > > > Honza > > > > > > > > > > Janis > > > > > >
