While there is a reference to -ftree-dse being turned on with -O optimization in both the manpage and the online docs (see e.g. http://gcc.gnu.org/onlinedocs/gcc-4.2.0/gcc/Optimize-Options.html#Optimize-Options), that is the only reference to this option. The specific behavior of this option does not appear in either the manpage or the online docs. Confirmed to exist in 4.2.0 and 4.2.2 docsets. Maybe more?
*** This bug has been marked as a duplicate of 32965 ***