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- From: Florian Weimer <fweimer at redhat dot com>
- To: GCC <gcc at gcc dot gnu dot org>
- Date: Tue, 06 May 2014 16:09:41 +0200
- Subject: Resurrecting -Wunreachable
- Authentication-results: sourceware.org; auth=none
I would like to resurrect -Wunreachable, using an algorithm which is
roughly based on the Java rules for reachable statements and normal
completion, augmented to deal with labels and gotos, no-return
functions, statement expressions, and whatever else is relevant to C and
C++. This analysis would be based mostly on syntax (except that
constant folding is applied to detect trivially infinite loops), so it
wouldn't suffer from the dependence on target and optimization levels
the previous option had to deal with.
I think computing this information is a prerequisite for a high-quality
switch warning, to detect cases that do not fall through because they do
not complete normally, but this is a separate matter.
The question I have if it would be feasible right now to implement this
as an early GIMPLE pass, or if I should bury this project until we have
mostly eliminated early folding (Jeff Law thinks it's too difficult…).
As far as I can tell, the target dependence we have in fold right now
(see <http://gcc.gnu.org/ml/gcc-help/2014-05/msg00014.html>) would not
interfere too much with Java-style reachability analysis: The CFG is
target-dependent, but under the Java rules, both branches of an if
statement a reachable even if the condition is a compile-time constant.
(It makes sense to keep this behavior because just as in Java,
constant conditions are used for conditional compilation in C and C++,
it's not always the preprocessor that is used for this.) So I hope that
the known target dependency issues would not impact the generated warnings.
Florian Weimer / Red Hat Product Security Team