As noted, GCC does these checks using information gathered with the optimization passes. A bug in GCC 4.5 and earlier was that it did the warning before all optimization passes had been done and therefore could give warnings for code paths that weren't relevant (= dead, would never happen). This was (at least mostly) fixed in GCC 4.6 and there is/are bugs about it in the GCC bugzilla.
Even if kernel people wouldn't fix GCC bugs instead of kludging kernel code, at least they could file bugs against GCC, or if there's already a bug on the issue, add a pointer to it and comment in code about which GCC version fixes that bug.
(On quick glance of these kind of bugs in GCC bug tracker, most of them seem to be pretty old and might be already fixed with GCC 4.6, I think they need a bit of a re-testing & cleanup.)
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