This patch seems a bit confused. You warn that the flag is ignored in C++, but it only has an effect in C++. You have a testcase with a .c extension that is built with -x c++.
On Wed, Aug 12, 2015 at 5:23 AM, Sjoerd Meijer <sjoerd.mei...@arm.com> wrote: > [ + cfe-commits@lists.llvm.org ] > > > > Hi, > > The functionality is now available under a flag, see attached patch. Note > that the flag is ignored in C++ mode, so it will help the use case of > compiling (legacy) C code with a C++ compiler. > > Cheers, > > Sjoerd. > > > > *From:* Sjoerd Meijer [mailto:sjoerd.mei...@arm.com > <sjoerd.mei...@arm.com>] > *Sent:* 03 August 2015 11:40 > *To:* 'Richard Smith' > *Cc:* Hal Finkel; Marshall Clow; cfe-...@cs.uiuc.edu Developers; cfe > commits > *Subject:* RE: [PATCH] RE: [cfe-dev] missing return statement for > non-void functions in C++ > > > > Hi Richard, > > > > I agree with your conclusions and will start preparing a patch for option > 3) under a flag that is off by default; this enables folks to build/run C > code in C++. I actually think option 2) would be a good one too, but as it > is already available under a flag I also don’t see how useful it is > combining options 2) and 3) with another (or one more) flag that is off by > default. > > > > Cheers. > > > > *From:* meta...@gmail.com [mailto:meta...@gmail.com <meta...@gmail.com>] *On > Behalf Of *Richard Smith > *Sent:* 31 July 2015 19:46 > *To:* Sjoerd Meijer > *Cc:* Hal Finkel; Marshall Clow; cfe-...@cs.uiuc.edu Developers; cfe > commits > *Subject:* Re: [PATCH] RE: [cfe-dev] missing return statement for > non-void functions in C++ > > > > On Fri, Jul 31, 2015 at 7:35 AM, Sjoerd Meijer <sjoerd.mei...@arm.com> > wrote: > > Hi, I am not sure if we came to a conclusion. Please find attached a > patch. It simply removes the two lines that insert an unreachable statement > (which cause removal of the return statement). Please note that at -O0 the > trap instruction is still generated. Is this something we could live with? > > > > I don't think this is an improvement: > > > > This doesn't satisfy the folks who want an 'unreachable' for better code > size and optimization, and it doesn't satisfy the folks who want a > guaranteed trap for security, and it doesn't satisfy the folks who want > their broken code to limp along (because it'll still trap at -O0), and it > is at best a minor improvement for the folks who want missing returns to be > more easily debuggable (with -On, the code goes wrong in the caller, or > appears to work, rather than falling into an unrelated function, and > debugging this with -O0 was already easy). > > > > I think there are three options that are defensible here: > > 1) The status quo: this is UB and we treat it as such and optimize on that > basis, but provide a trap as a convenience at -O0 > > 2) The secure approach: this is UB but we always trap > > 3) Define the behavior to return 'undef' for C types: this allows > questionable C code that has UB in C++ to keep working when built with a > C++ compiler > > > > Note that (3) can be combined with either (1) or (2). (2) is already > available via the 'return' sanitizer. So this really reduces to: in those > cases where C says it's OK so long as the caller doesn't look at the > returned value (and where the return type doesn't have a non-trivial copy > constructor or destructor, isn't a reference, and so on), should we attempt > to preserve the C behaviour? I would be OK with putting that behind a `-f` > flag (perhaps `-fstrict-return` or similar) to support those folks who want > to build C code in C++, but I would suggest having that flag be off by > default, since that is not the usual use case for a C++ compiler. > > > > Cheers, > > Sjoerd. > > > > *From:* cfe-dev-boun...@cs.uiuc.edu [mailto:cfe-dev-boun...@cs.uiuc.edu] *On > Behalf Of *Richard Smith > *Sent:* 29 July 2015 18:07 > *To:* Hal Finkel > *Cc:* Marshall Clow; cfe-...@cs.uiuc.edu Developers > > > *Subject:* Re: [cfe-dev] missing return statement for non-void functions > in C++ > > > > On Jul 29, 2015 7:43 AM, "Hal Finkel" <hfin...@anl.gov> wrote: > > > > ----- Original Message ----- > > > From: "David Blaikie" <dblai...@gmail.com> > > > To: "James Molloy" <ja...@jamesmolloy.co.uk> > > > Cc: "Marshall Clow" <mclow.li...@gmail.com>, "cfe-dev Developers" < > cfe-...@cs.uiuc.edu> > > > Sent: Wednesday, July 29, 2015 9:15:09 AM > > > Subject: Re: [cfe-dev] missing return statement for non-void functions > in C++ > > > > > > > > > On Jul 29, 2015 7:06 AM, "James Molloy" < ja...@jamesmolloy.co.uk > > > > wrote: > > > > > > > > Hi, > > > > > > > > If we're going to emit a trap instruction (and thus create a broken > > > > binary), why don't we error instead? > > > > > > We warn, can't error, because it may be dynamically unreached, in > > > which case the program is valid and we can't reject it. > > > > I think this also explains why this is useful for optimization. > > > > 1. It is a code-size optimization > > 2. By eliminating unreachable control flow, we can remove branches and > tests that are not actual necessary > > > > int foo(int x) { > > if (x > 5) return 2*x; > > else if (x < 2) return 3 - x; > > } > > > > That having been said, there are other ways to express these things, and > the situation often represents an error. I'd be fine with requiring a > special flag (-fallow-nonreturning-functions or whatever) in order to put > the compiler is a truly confirming mode (similar to the situation with > sized delete). > > Note that we already have a flag to trap on this: -fsanitize-trap=return. > (You may also need -fsanitize=return, I don't remember.) That seems > consistent with how we treat most other forms of UB. > > > -Hal > > > > > > > > > > > > > James > > > > > > > > On Wed, 29 Jul 2015 at 15:05 David Blaikie < dblai...@gmail.com > > > > > wrote: > > > >> > > > >> > > > >> On Jul 29, 2015 2:10 AM, "mats petersson" < m...@planetcatfish.com > > > >> > wrote: > > > >> > > > > >> > > > > >> > > > > >> > On 28 July 2015 at 23:40, Marshall Clow < mclow.li...@gmail.com > > > >> > > wrote: > > > >> >> > > > >> >> > > > >> >> > > > >> >> On Tue, Jul 28, 2015 at 6:14 AM, Sjoerd Meijer < > > > >> >> sjoerd.mei...@arm.com > wrote: > > > >> >>> > > > >> >>> Hi, > > > >> >>> > > > >> >>> > > > >> >>> > > > >> >>> In C++, the undefined behaviour of a missing return statements > > > >> >>> for a non-void function results in not generating the > > > >> >>> function epilogue (unreachable statement is inserted and the > > > >> >>> return statement is optimised away). Consequently, the > > > >> >>> runtime behaviour is that control is never properly returned > > > >> >>> from this function and thus it starts executing “garbage > > > >> >>> instructions”. As this is undefined behaviour, this is > > > >> >>> perfectly fine and according to the spec, and a compile > > > >> >>> warning for this missing return statement is issued. However, > > > >> >>> in C, the behaviour is that a function epilogue is generated, > > > >> >>> i.e. basically by returning uninitialised local variable. > > > >> >>> Codes that rely on this are not beautiful pieces of code, i.e > > > >> >>> are buggy, but it might just be okay if you for example have > > > >> >>> a function that just initialises stuff (and the return value > > > >> >>> is not checked, directly or indirectly); some one might argue > > > >> >>> that not returning from that function might be a bit harsh. > > > >> >> > > > >> >> > > > >> >> I would not be one of those people. > > > >> > > > > >> > > > > >> > Nor me. > > > >> >> > > > >> >> > > > >> >>> > > > >> >>> So this email is to probe if there would be strong resistance > > > >> >>> to follow the C behaviour? I am not yet sure how, but would > > > >> >>> perhaps a compromise be possible/acceptable to make the > > > >> >>> undefined behaviour explicit and also generate the function > > > >> >>> epilogue? > > > >> >> > > > >> >> > > > >> >> "undefined behavior" is exactly that. > > > >> >> > > > >> >> You have no idea what is going to happen; there are no > > > >> >> restrictions on what the code being executed can do. > > > >> >> > > > >> >> "it just might be ok" means on a particular version of a > > > >> >> particular compiler, on a particular architecture and OS, at a > > > >> >> particular optimization level. Change any of those things, and > > > >> >> you can change the behavior. > > > >> > > > > >> > > > > >> > In fact, the "it works kind of as you expected" is the worst > > > >> > kind of UB in my mind. UB that causes a crash, stops or other > > > >> > "directly obvious that this is wrong" are MUCH easier to debug. > > > >> > > > > >> > So make this particular kind of UB explicit by crashing or > > > >> > stopping would be a good thing. Making it explicit by > > > >> > "returning kind of nicely, but not correct return value" is > > > >> > about the worst possible result. > > > >> > > > >> At -O0 clang emits a trap instruction, making it more explicit as > > > >> you suggest. At higher optimization levels it just falls > > > >> through/off. > > > >> > > > >> > > > > >> > -- > > > >> > Mats > > > >> >> > > > >> >> > > > >> >> -- Marshall > > > >> >> > > > >> >> > > > >> >> _______________________________________________ > > > >> >> cfe-dev mailing list > > > >> >> cfe-...@cs.uiuc.edu > > > >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev > > > >> >> > > > >> > > > > >> > > > > >> > _______________________________________________ > > > >> > cfe-dev mailing list > > > >> > cfe-...@cs.uiuc.edu > > > >> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev > > > >> > > > > >> > > > >> _______________________________________________ > > > >> cfe-dev mailing list > > > >> cfe-...@cs.uiuc.edu > > > >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev > > > > > > _______________________________________________ > > > cfe-dev mailing list > > > cfe-...@cs.uiuc.edu > > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev > > > > > > > -- > > Hal Finkel > > Assistant Computational Scientist > > Leadership Computing Facility > > Argonne National Laboratory > > > > _______________________________________________ > > cfe-dev mailing list > > cfe-...@cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev > > >
_______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
