Just out of curiosity I tried to build gnome-shell with "-O3" compiler option. jhbuild quit with this error:

make[3]: Entering directory `/home/christian/gnome-shell/source/gnome-shell/src'
  CC     libst_1_0_la-st-box-layout.lo
cc1: warnings being treated as errors
st/st-box-layout.c: In Funktion »st_box_layout_get_property«:
st/st-box-layout.c:211: Fehler: »adjustment« may be used uninitialized in this function
[...]

When initialized the variable with 0 it silenced this particular warning, but other unitialized variables warnings popped up. I have no idea of C at all, so I don't know how to solve it correctly (initialize all with null?). I guess the problem is that with "-O3" GCC tries to inline these functions, which changes the program flow.

Might be no big deal if you think that noone will ever build GS with "-O3" (but the gentoo guys will surely complain ...).