Re: GCC and out-of-range constant array indexes?

[Manuel López-Ibáñez <lopezibanez at gmail dot com>]

> Would it be possible to compute enough of the control flow graph
> to process warnings like this one, without running the
> actual optimizations, unless those optimizations are requested?
> Would the cost be too high?

It is possible to do it quite fast. Clang implements all warnings,
including Wuninitialized, in the FE using fast analysis and they claim
very low false positives.
However, there are various reasons why it has not been attempted in GCC:

* GCC is too slow already at -O0, slowing it down further would not be
acceptable. So you need a really high-performing implementation.

* The FEs are quite complex, and both C and C++ construct gimple in a
different way. It would be easier to do the analysis once the FE has
finished building generic/gimple. However,

* The FEs fold/transform expressions as they go, so you don't have a
1-to-1 relationship between the intermediate representation generated
by the FEs and the code.

Yet, anything is possible in principle. First, it would need someone
to do the work. As far as I know, there is no one planning or willing
to work on this. And second, it would need to be accepted by the
maintainers. I suggest you clarify the latter before implementing your
idea, or you will be seriously disappointed.

An alternative would be to move the heavier analysis to an external
tool that can be invoked by the compiler and share the same
infrastructure. As http://clang-analyzer.llvm.org/ does. However, GCC
is many years far away to enable implementing such technology. So
perhaps implementing some analysis on the FE would be a more promising
approach (despite the caveats mentioned above). But you have to find
out if the overhead would be acceptable for the respective
maintainers.

Cheers,

Manuel.