Last weekend I managed to try GCC 4 on my Gentoo box. My inspiron 6000 comes with Pentium M, but with gcc version of <3.4, I can only use pentium3 optimisation flag instead of pentium-m, whic is not very nice. My Pentium M 1.86 outperforms Pentium 4 2.8 (ht) by 40% in a test program compiled with pentium3 (my laptop at that time) and the Pentium 4 desktop compiled the same program without cpu optimisation. Later when we tried it with -march pentium3 and -march pentium4 respectively, my Inspiron 6000 + gentoo was just 3%-5% slower than the Pentium 4 2.8 running SuSe 
That gives the idea that this is not just for fun but might as well give performance benefit at the end. Driven by that, I wish to see if my machine can outperform the much higher clocked Pentium 4 when compiled with gcc 4 and get the pentium-m flag to work.
Methodology:
Currently my gentoo is installed with GCC 3.3.6. With gentoo upgrading the compiler and glibc was easy. First of all, unmask the to be installed gcc and glibc version.
Sure, we’ll need to emerge:
#emerge binutils glibc gcc
Next up, choose the gcc version that we would like to use.
To get the listing of available versions:
#gcc-config -l [1] i686-pc-linux-gnu-3.3.6 * [2] i686-pc-linux-gnu-3.3.6-hardened [3] i686-pc-linux-gnu-3.3.6-hardenednopie [4] i686-pc-linux-gnu-3.3.6-hardenednopiessp [5] i686-pc-linux-gnu-3.3.6-hardenednossp [6] i686-pc-linux-gnu-3.4.4 [7] i686-pc-linux-gnu-3.4.4-hardened [8] i686-pc-linux-gnu-3.4.4-hardenednopie [9] i686-pc-linux-gnu-3.4.4-hardenednopiessp [10] i686-pc-linux-gnu-3.4.4-hardenednossp [11] i686-pc-linux-gnu-4.0.2
The one marked with an asterisk is the one currently used. Now, choose the desired version:
#gcc-config i686-pc-linux-gnu-4.0.2
and then rebuild the whole system..
#emerge -e system && emerge -e world
Don’t forget to also change the cpu type in kernel configuration to pentium-m, and we’re done. Ah yes, do re-compile the kernel..
Results and Discussion:
Many programs failed to compile during the emerge -e world. gcc 4 is more strict when it comes to adhering to the letter of the C/C++ specifications, and that might be the reasons for some kde package and ati-drivers, among many others, failed to compile. As some of the failed to compile packages are essential packages, I chose not to go on with gcc 4.
I need to switch to earlier version of gcc and hope everything to work as normal again. As pentium-m flag works with gcc 3.4 and above, I fall back to gcc 3.4 instead of the original 3.3.6.
#gcc-config i686-pc-linux-gnu-3.4.4 #emerge -e system && emerge -e world
Things compiled flawlessly. I couldn’t find the original program that we used to benchmark my laptop and the other desktop previously, and so I just test if the flag and gcc really boost some speed with glxgears. It managed to get the gears to rotate at 1900+ fps on average, as previously it was only rotating at the average of 1800+ fps. 100 fps (frame per second) increase is not so bad after all.