http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51971
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-24 13:01:03 UTC --- GCC explicitely allows you to use const/pure to enable CSE even if it would not consider the function const/pure itself (thus, if you are happy to loose a second assert (), or a debug printf). About returning normally it wants to exclude side-effects that happen via the return, such as if the function calls longjmp/fork (return twice). It also wants to exclude functions that do not return because they loop infinitely. For example int foo (int b) { if (b) while (1); return b; } is not const, as GCC would, if you declare it so, happily remove foo (1); as dead code (GCC will do so for all pure/const functions if the result is not needed).