Re: [PATCH 2/2] ci: let pedantic job compile with -Og

From: Jeff King <peff@peff.net>
Date: Jun 6, 2024, 06:52 UTC
Message-ID: <20240606065236.GA646308@coredump.intra.peff.net>
In-Reply-To: <351dec4a4d5a5619e7627e11a8e674e32125125e.1717655210.git.ps@pks.im>

On Thu, Jun 06, 2024 at 08:30:34AM +0200, Patrick Steinhardt wrote:

Show 16 quoted lines

> We have recently noticed that our CI does not always notice variables
> that may be used uninitialized. While it is expected that compiler
> warnings aren't perfect, this one was a but puzzling because it was
> 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, only adapt the "pedantic" job to compile with `-Og`.

Hmm. This is the first time I've ever seen lower optimization levels produce more warnings. It is almost always the opposite case in my experience. So it's not clear to me that moving to "-Og" will generally find more warning spots, and that this isn't a bit of a fluke.

As you note, we'll still compile with -O2 in other jobs. But isn't the point of the pedantic job to enable a bunch of extra warnings that aren't available elsewhere? We wouldn't have any coverage of those.

So for the pedantic warnings, we're left with a guess as to whether -Og or -O2 will yield more results. And in my experience it is probably -O2.

If we want to get coverage of -Og, I'd suggest doing it in a job that is otherwise overlapping with another (maybe linux-TEST-vars, which I think is otherwise a duplicate build?).

Show 17 quoted lines

> diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
> index 98dda42045..e78e19e4a6 100755
> --- a/ci/run-build-and-tests.sh
> +++ b/ci/run-build-and-tests.sh
> @@ -44,6 +44,15 @@ pedantic)
>  	# Don't run the tests; we only care about whether Git can be
>  	# built.
>  	export DEVOPTS=pedantic
> +	# Warnings generated by compilers are unfortunately specific to the
> +	# optimization level. With `-O0`, many warnings won't be shown at all,
> +	# whereas the optimizations performed by our default optimization level
> +	# `-O2` will mask others. We thus use `-Og` here just so that we have
> +	# at least one job with a different optimization level so that we can
> +	# overall surface more warnings.
> +	cat >config.mak <<-EOF
> +	export CFLAGS=-Og
> +	EOF

Writing config.mak is unusual, though I guess just setting CFLAGS in the environment isn't enough, because we set it unconditionally in the Makefile. Doing "make CFLAGS=-Og" would work, but we'd need help from the code that actually runs "make".

I do suspect the "export" is unnecessary; it should just be used by the Makefile recipes themselves.

Your command above also loses the "-g" and "-Wall" from the default CFLAGS. Maybe OK, since DEVELOPER=1 implies -Wall anyway, and "-g" isn't important. But one thing I've done for a long time in my config.mak is:

  O ?= 2
  CFLAGS = -g -Wall -O$(O)

Then you can "make O=0" or "make O=g" if you want. And I think that setting O=g in the environment (exported) would work, as well.

-Peff