Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference

Jakub Jelinek jakub@redhat.com
Wed Feb 18 19:29:00 GMT 2015

On Wed, Feb 18, 2015 at 11:21:56AM -0800, Jeff Prothero wrote:
> Starting with gcc 4.9, -O2 implicitly invokes
>     -fisolate-erroneous-paths-dereference:
> which
>     https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> documents as
>     Detect paths that trigger erroneous or undefined behavior due to
>     dereferencing a null pointer. Isolate those paths from the main control
>     flow and turn the statement with erroneous or undefined behavior into a
>     trap. This flag is enabled by default at -O2 and higher.
> This results in a sizable number of previously working embedded programs mysteriously
> crashing when recompiled under gcc 4.9.  The problem is that embedded
> programs will often have ram starting at address zero (think hardware-defined
> interrupt vectors, say) which gets initialized by code which the
> -fisolate-erroneous-paths-deference logic can recognize as reading and/or
> writing address zero.

If you have some pages mapped at address 0, you really should compile your
code with -fno-delete-null-pointer-checks, otherwise you can run into tons
of other issues.
Also, there is -fsanitize=undefined that allows discovery of such invalid
calls at runtime, though admittedly it isn't supported for all targets.


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