Re: [PATCH] config.mak.dev: enable -Wunreachable-code

On Mon, Mar 10, 2025 at 11:50:20AM -0700, Junio C Hamano wrote:

Yes, exactly. I started to prepare a patch for that, but then I realized I'd probably be adding support in config.mak.dev. So we could also just handle it automatically there, skipping the flag on macOS.

That would use the flag in more situations (blocking the known-bad case, rather than enabling it in a known-good one). It might hit more false positives, but I'd rather experiment in that direction and see if anybody setting DEVELOPER=1 complains. After all, in either case it is still a big question of whether this is the only false positive we'll see, or if this is opening up a can of worms. So I consider it all kind-of exploratory.

So that patch could look like this (on top of what you've queued already in jk/use-wunreachable-code-for-devs).

-- >8 --
Subject: [PATCH] config.mak.dev: disable -Wunreachable-code on macOS

We've seen false positives here related to calling sigfillset(); even though POSIX specifies that it may return an error, it transparently (to the compiler) always returns success on macOS. As a result, the compiler flags the error path in something like:

  if (sigfillset(&set))
	die(...);

as unreachable (which it is on this platform, but not in the general case). We could work around it, but let's just disable the warning on this platform. There are plenty of CI jobs that will still trigger it (e.g., all of the linux+clang jobs).

Signed-off-by: Jeff King <peff@peff.net>
---
It's possible FreeBSD might share the same problem, but their manpage
does not seem to have the same "it always returns 0" language. But we
might need to expand this list if people report more problems.

 config.mak.dev | 5 +++++
 1 file changed, 5 insertions(+)