Re: [PATCH v2 2/2] ci: compile "linux-gcc-default" job with -Og

> rather obvious that the variable can be uninitialized.
> 
> Many compiler warnings unfortunately depend on the optimization level
> used by the compiler. While `-O0` for example will disable a lot of
> warnings altogether because optimization passes go away, `-O2`, which is
> our default optimization level used in CI, may optimize specific code
> away or even double down on undefined behaviour. Interestingly, this
> specific instance that triggered the investigation does get noted by GCC
> when using `-Og`.
> 
> While we could adapt all jobs to compile with `-Og` now, that would
> potentially mask other warnings that only get diagnosed with `-O2`.
> Instead, adapt the "linux-gcc-default" job to compile with `-Og`. This
> job is chosen because it uses the "ubuntu:latest" image and should thus
> have a comparatively recent compiler toolchain, and because we have
> other jobs that use "ubuntu:latest" so that we do not loose coverage for

> warnings diagnosed only on `-O2` level.
> 
> To make it easier to set up the optimization level in our CI, add
> support in our Makefile to specify the level via an environment
> variable.
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
> 
> I was a little torn whether we really want to name the variable `O`
> here because it feels so easy to set it by accident. We could rename
> this to `OPTIMIZATION` or `OPTIMIZATION_LEVEL`, but that's quite a
> mouthful.
> 
> Alternatively, if we don't want to have this variable in the first
> place, then I'm also happy to adapt the script itself to pass the
> optimization level via an argument.