[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v4 08/11] xen/compiler: import 'fallthrough' keyword from linux
On 13.01.2021 00:30, Stefano Stabellini wrote: > On Tue, 12 Jan 2021, Jan Beulich wrote: >> On 08.01.2021 15:46, Rahul Singh wrote: >>> -Wimplicit-fallthrough warns when a switch case falls through. Warning >>> can be suppress by either adding a /* fallthrough */ comment, or by >>> using a null statement: __attribute__ ((fallthrough)) >> >> Why is the comment variant (which we use in many places already, >> albeit with varying wording) not the route of choice? > > See previous discussion: > > https://marc.info/?l=xen-devel&m=160707274517270 > https://marc.info/?l=xen-devel&m=160733742810605 > https://marc.info/?l=xen-devel&m=160733852011023 > > We thought it would be best to introduce "fallthrough" and only resort > to comments as a plan B. The usage of the keyword should allow GCC to do > better checks. Hmm, this earlier discussion was on an Arm-specific thread, and I have to admit I can't see arguments there pro and/or con either of the two alternatives. >>> Define the pseudo keyword 'fallthrough' for the ability to convert the >>> various case block /* fallthrough */ style comments to null statement >>> "__attribute__((__fallthrough__))" >>> >>> In C mode, GCC supports the __fallthrough__ attribute since 7.1, >>> the same time the warning and the comment parsing were introduced. >>> >>> fallthrough devolves to an empty "do {} while (0)" if the compiler >>> version (any version less than gcc 7) does not support the attribute. >> >> What about Coverity? It would be nice if we wouldn't need to add >> two separate constructs everywhere to make both compiler and static >> code checker happy. > > I don't think I fully understand your reply here: Coverity doesn't come > into the picture. Given that GCC provides a special keyword to implement > fallthrough, it makes sense to use it when available. When it is not > available (e.g. clang or older GCC) we need to have an alternative to > suppress the compiler warnings. Hence the need for this check: > > #if (!defined(__clang__) && (__GNUC__ >= 7)) I'm not sure how this interacts with Coverity. My point bringing up that one is that whatever gets done here should _also_ result in Coverity recognizing the fall-through as intentional, or else we'll end up with many unwanted reports of new issues once the pseudo- keyword gets made use of. The comment model is what we currently use to "silence" Coverity; I'd like it to be clear up front that any new alternative to be used is also going to "satisfy" it. Jan
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