Why don't you use -O2 -finline-functions then?
I did a quick experiment, compiling one C file with -O2 -finline-functions and another with -O3, using the -S flag so I could see the assembler output.
The only difference I saw was this:
.comm free_list,8,8
.comm free_list,8,16
It is basically CSE "around loops". You can generalize it to subsume loop store motion and strength reduction, but most compilers (including GCC) don't bother.
For example, it transforms
for (int i = 0; i < 50; i++)
a[i+2] = a[i] + a[i+1]
p0 = a[0]
p1 = a[1]
for (int i = 0; i < 50; i++) {
a[i+2]=p2=p0+p1;
p0=p1;
p1=p2;
}
It's been a while since i looked at GCC's implementation, but it did pretty well in the past (whether you can do commoning depends on your ability to identify and group sequences, etc)
-ftree-vectorize does the obvious thing (turn on vectorization). How effective it is depends on a lot of factors.