[Bug c/59933] for loop goes wild with assert() enabled

["ian at g0tcd dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59933

--- Comment #9 from Ian Hamilton <ian at g0tcd dot com> ---
Yes, that's all proper and correct. The invalid C code induces undefined
behaviour. I don't think anyone is disputing that.

However, to be pragmatic for a moment, the experience of thousands of
developers out there, working with legacy code, and trying to update their
toolset to include gcc 4.8 is that code which compiled without warnings and
worked with the old gcc compiler now still compiles without warnings, but fails
at runtime with the 4.8 series compiler.

Sometimes, the runtime failures are occasional and difficult to track down if
(for example) it lies on an error handling path. This makes it even harder for
these developers to figure out what's going on.

If the compiler could provide a warning when it encounters this sort of invalid
code, that would be a good thing, as it would highlight the old latent bugs and
give developers the opportunity to fix them.

However, it doesn't, so the developers working on legacy code really have no
alternative to either using the -fno-aggressive-loop-optimizations switch to
stabilse their legacy code (even assuming they understand what's happening), or
sticking with the old version of the compiler.

So I think the request to the gcc developers is to find a way of providing a
compiler warning when the loop optimiser encounters problem code, to give
developers a fighting chance of debugging their legacy code.