Messages in this thread | | | Subject | Re: [PATCH 000/141] Fix fall-through warnings for Clang | From | "Gustavo A. R. Silva" <> | Date | Fri, 20 Nov 2020 13:04:55 -0600 |
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Hi,
On 11/20/20 12:53, Jakub Kicinski wrote: > On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote: >> This series aims to fix almost all remaining fall-through warnings in >> order to enable -Wimplicit-fallthrough for Clang. >> >> In preparation to enable -Wimplicit-fallthrough for Clang, explicitly >> add multiple break/goto/return/fallthrough statements instead of just >> letting the code fall through to the next case. >> >> Notice that in order to enable -Wimplicit-fallthrough for Clang, this >> change[1] is meant to be reverted at some point. So, this patch helps >> to move in that direction. >> >> Something important to mention is that there is currently a discrepancy >> between GCC and Clang when dealing with switch fall-through to empty case >> statements or to cases that only contain a break/continue/return >> statement[2][3][4]. > > Are we sure we want to make this change? Was it discussed before? > > Are there any bugs Clangs puritanical definition of fallthrough helped > find? > > IMVHO compiler warnings are supposed to warn about issues that could > be bugs. Falling through to default: break; can hardly be a bug?!
The justification for this is explained in this same changelog text:
Now that the -Wimplicit-fallthrough option has been globally enabled[5], any compiler should really warn on missing either a fallthrough annotation or any of the other case-terminating statements (break/continue/return/ goto) when falling through to the next case statement. Making exceptions to this introduces variation in case handling which may continue to lead to bugs, misunderstandings, and a general lack of robustness. The point of enabling options like -Wimplicit-fallthrough is to prevent human error and aid developers in spotting bugs before their code is even built/ submitted/committed, therefore eliminating classes of bugs. So, in order to really accomplish this, we should, and can, move in the direction of addressing any error-prone scenarios and get rid of the unintentional fallthrough bug-class in the kernel, entirely, even if there is some minor redundancy. Better to have explicit case-ending statements than continue to have exceptions where one must guess as to the right result. The compiler will eliminate any actual redundancy.
Note that there is already a patch in mainline that addresses almost 40,000 of these issues[6].
[1] commit e2079e93f562c ("kbuild: Do not enable -Wimplicit-fallthrough for clang for now") [2] ClangBuiltLinux#636 [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432 [4] https://godbolt.org/z/xgkvIh [5] commit a035d552a93b ("Makefile: Globally enable fall-through warning") [6] commit 4169e889e588 ("include: jhash/signal: Fix fall-through warnings for Clang")
Thanks -- Gustavo
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