Re: Benchmark of gcc 4.0

[Vladimir Makarov <vmakarov at redhat dot com>]

Biagio Lucini wrote:

> I run for my personal pleasure (since I am a number cruncher) the
> Scimark2 tests on my P4 Linux machine. I tested GCC 4.0 (today's CVS) vs. GCC 
> 3.4.1 vs. Intel's ICC 8.1
>
> For GCC, I used in both cases the flags
> -march=pentium4 -mfpmath=sse -O3 -fomit-frame-pointer -ffast-math
>
> Should be of some interest, for ICC I used
> -ipo -tpp7 -xW -align -Zp16 -O3
>
> The results were surprisingly bad, and this is why I am writing this message:
>
>
> 			    GCC 4.0	GCC 3.4.1 	ICC	
> Composite Score:   270.51	345.28		430.47
> FFT          Mflops:   192.10	203.77		206.66
> SOR         Mflops:    257.61	252.88		258.30
> MC           Mflops:      58.61	  67.96 		312.13	
> matmult    Mflops:    376.64 	557.75		564.97
> LU            Mflops:    467.58 	644.03		810.29
>
> I leave aside any personal comments, except that being involved in Monte Carlo 
> calculations, I would love if GCC were not outperformed by a factor of ~ 4.5 
> in MC by ICC. 
>
> I also would like to ask whether you see anything wrong with those benchmarks 
> and/or you have suggestions to improve them.
>
>  
Thanks for reporting this.  Although it would be more usefull if you 
made some analysis what is wrong with gcc.  For example, icc reports 
loop vectorization.  Or may be it is  a memory heirarchy optimization, 
or usage of better standard function like random function (I am not 
familiar well with MC).  Usually vectorization is a reason of such big 
difference.  People in gcc community work on vectorization.  Although I 
don't know when it will be used for x86.  We have no such resources as 
Intel has (several hundred engineers mainly working on optimizations 
only for 3 their architectures).

As for gcc4 vs gcc3.4,  degradataion on x86 architecture is most 
probably because of higher register pressure created with more 
aggressive SSA optimizations in gcc4.  The current  register allocator 
does not deal well with such problem.  So code generated by gcc4 can be 
worse for architectures with few registers.  For architectures with many 
registers (like ia64), gcc4 generates a better code than gcc3.4.  Again 
gcc community works on the register allocator problem too.

Vlad