Removing uninitialized_var() [LWN.net]

Removing uninitialized_var()

Posted Jan 4, 2013 12:05 UTC (Fri) by heijo (guest, #88363)
In reply to: Removing uninitialized_var() by mjthayer
Parent article: Removing uninitialized_var()

It's impossible (http://en.wikipedia.org/wiki/Rice_theorem).

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Removing uninitialized_var()

Posted Jan 4, 2013 23:24 UTC (Fri) by nix (subscriber, #2304) [Link] (1 responses)

It's impossible to get it right *in the general case*. You can get it right in an arbitrarily large percentage of special cases (and indeed, GCC often does, when it says 'is used uninitialized' rather than 'may be used uninitialized').

Removing uninitialized_var()

Posted Jan 13, 2013 0:39 UTC (Sun) by oak (guest, #2786) [Link]

GCC not detecting all the cases where variable might be uninitialized (and it requiring optimizations enabled to find what it can find) is one thing, but it's not what annoys people. Wrong warnings are what annoy them.

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.)