Re: Gcc 3.1 performance regressions with respect to 2.95.3

[Daniel Berlin <dan at dberlin dot org>]

On Mon, 18 Mar 2002, Peter Schmid wrote:

> 
> 
> > It is more or less special case. Overall 3.1 groks better the code
> > with lots of abstraction, but in this case 2.95 got particulary lucky.
> > It runs into similar slowdown only on slightly modified Stephanov as
> > long as I can remember.
> >
> 
> That is only partly true. When I benchmark gcc 3.1 at the -O versus
> -O2 optimization level with the help of the bench++ test suite, the
> performance is better in 45 cases at -O. Interestingly, the checks
> measuring loop overhead (L*) run all faster at the -O level. L00004 runs
> two times faster. And even more interesting is that o000007[a,b].cpp which
> checks the strength reduction capabilities of the compiler, o000007a.cpp
> which checks dead code elimination, o000010b.cpp which checks redundant
> code and o000011a.cpp checking the unreachable code optimizating facility
> run slower at the -O2 optimization level although this level provides
> specific optimizations for these problems. Furthermore, function calls are
> slower at the -O2 level:
>   p000005.cpp  Static Class Method Call: 1-int Arg: Catches Exceptions
>   p000006.cpp  Static Class Method Call: 1-int *Arg: Catches Exceptions
>   p000008.cpp  Procedure Call: No Parameters: Called thru pointer, Catches Exceptions
>   p000012.cpp  Procedure Call: 10-(3-int) Args: Catches Exceptions
>   p000023.cpp  Same as p000022: called in loop to see if lookup is optimized
> 
> And in addition to the decrease in the performance of the stepanov tests
> there is a substantial decrease in the performance for processing
> complex numbers (S000004a).
> 
> It looks that there are some flaws in the -O2 optimizer passes. Is
> there a chance that this is fixed for the upcoming gcc 3.1 release?

There are so many possible reasons for these problems it's not 
funny.
Especially on x86, where register pressure matters a lot.
If you had smaller test cases, it would be helpful.
In the meanwhile, i'm going to run these on powerpc and see if i come up 
with the same relative times.