This is the mail archive of the
mailing list for the GCC project.
Re: Resurrecting -Wunreachable
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: GCC <gcc at gcc dot gnu dot org>
- Date: Thu, 8 May 2014 11:18:53 +0200
- Subject: Re: Resurrecting -Wunreachable
- Authentication-results: sourceware.org; auth=none
- References: <5368ED25 dot 7090901 at redhat dot com> <CAFiYyc0+1TwHdez4hiop=PVyebTArswFKfebDPwAn_xpoad=qQ at mail dot gmail dot com> <536A1F83 dot 5050703 at redhat dot com> <CAFiYyc3U8SjqBFcEf9WKTyi3r1rmLdEagL=q=DJk65fHMS9iKg at mail dot gmail dot com> <536A2213 dot 9020104 at redhat dot com> <CAFiYyc3CUivZGc+fmZqMuYLPbNjA0t0HS5mRbEap3J8GuTWobg at mail dot gmail dot com> <536A26FE dot 7080204 at redhat dot com> <CAFiYyc0T3RkPRX3b8ssHnw_UkhA1k5dTOdAedhkPO1wizzAdWA at mail dot gmail dot com> <536B3E7B dot 6020608 at redhat dot com>
On Thu, May 8, 2014 at 10:21 AM, Florian Weimer <firstname.lastname@example.org> wrote:
> On 05/07/2014 02:43 PM, Richard Biener wrote:
>>> The more challenging issue with early GIMPLE is that loops have already
>>> lowered to gotos, so adopting the syntax-based Java reachability rules is
>>> impossible. Oh dear.
>> Perfect is the enemy of the good (no false positives and no false
>> negatives). I don't think you can get all you want starting at GIMPLE.
>> And the "earlier" you start the more you need to implement a whole
> We already had an unreachable code warning at a later stage in the compiler.
> Its reporting was so confusing that it head to be removed. I don't think
> this approach works.
>> And you have of course to precisely define what you consider
>> "unreachable" (considering a global const bool debug = true/false; and
>> if (debug) guarded code - compared to using the preprocessor).
> I plan to follow the Java rules, with necessary adjustments due to language
> They are based on syntax (except for the infinite loop case), so they are
> much more predictable from a developer perspective.
> There are other warnings which benefit greatly from information collected
> during optimization, but unreachable code doesn't.
Then I suggest to implement this warning in the frontends - as surely
you'll have differently modified "Java rules" for different frontends.
At the time a frontend calls cgraph_finalize_function () its AST is
supposed to be in GENERIC (+ FE extensions to GENERIC) form
(so for Fortran it's already lowered too much I guess). So that could
be a (kind-of) common point to implement this for C/C++ at least.
> Florian Weimer / Red Hat Product Security Team