Alexander Kornienko
2012-Jul-02 16:59 UTC
[LLVMdev] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases
Hi llvmdev, llvm-commits, There was a discussion on this topic a while ago, and now I've decided to make a formal proposal and post it here. I propose to add the LLVM_FALLTHROUGH macro for specifying intended fall-through locations between switch cases. *INTRODUCTION* The switch construct of C/C++ languages allows fall-throughs between switch labels when control flow is not directed elsewhere by a break, return or continue statement or other ways. There are certain coding idioms which utilize this behavior, e.g. Duff's device<http://en.wikipedia.org/wiki/Duff's_device>, but much more frequently the use of switch statements doesn't involve a fall-through. (A commonly used case - when several switch labels mark one statement - is not considered a fall-through here.) As there's no special way to mark a fall-through, it's relatively easy to make a mistake by just missing a break/return statement before the next switch label. This kind of mistake is sometimes difficult to spot by manual code examination, and the compiler can't provide any help either. It's common to mark fall-through locations with a comment, but parsing comments would incur a significant performance penalty and is probably an unhealthy approach overall. So the use of comment-only annotations is limited to manual inspections. Some time ago I've added the 'Unintended fall-through in switch statement' diagnostics to clang ( http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120430/057180.html ). This is a proposal to introduce clang's unintended fall-through diagnostic to llvm/clang code. *DESCRIPTION OF 'UNINTENDED FALL-THROUGH IN SWITCH STATEMENTS' DIAGNOSTIC IN CLANG* Related functionality in clang introduces several diagnostic messages and a syntax for intended fall-through annotation, based on C++11 attributes ([dcl.attr], http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3376.pdf, 7.6). The [[clang::fallthrough]] attribute should be applied to a null statement placed in a point of execution where fall-through to the next switch label occurs, e.g.: switch (n) { case 0: ... // no break here case 1: // warning: unannotated fall-through between switch labels if (x) break; else if (y) { ... *[[clang::fallthrough]];* // annotated fall-through to case 2 } else return 13; case 2: // no warning here ... *[[clang::fallthrough]];* // annotated fall-through to case 3 case 3: ... } So, the [[clang::fallthrough]]; annotation can be used in mostly in the same way as break; or return; statements (but it doesn't change control-flow, it just annotates a fall-through). The following rules are checked for this annotation: * the clang::fallthrough attribute can only be attached to a null-statement; * the [[clang::fallthrough]]; annotation should be placed inside a switch body; * it should be placed on an execution path between any statement inside a switch body and a case/default label (this means there *is* a fall-through to the corresponding case/default label); * no statements should exist on an execution path between the [[clang::fallthrough]]; annotation and the next case/default label. *PROPOSAL FOR LLVM/CLANG CODE* I ran the unintended fall-through diagnostics against llvm/clang code and found a number of fall-through locations, most of them are probably intended. But some look like bugs (see the attached * fallthrough-bugs-llvm.diff* file). To start using the proposed diagnostics code has to be prepared. First, we need to mark all intended fall-through locations with the clang::fallthrough attribute. It makes sense to wrap [[clang::fallthrough]]to a macro, so the code would continue to work in c++98 mode, but in c++11 mode it would also allow to run this diagnostic (btw, is llvm compilable in c++11 mode?). Sample implementation of the macro (see * fallthrough-macro.diff*): *#ifdef __clang__ **#if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough") * *#define LLVM_FALLTHROUGH [[clang::fallthrough]] **#endif **#endif ****#ifndef LLVM_FALLTHROUGH **#define LLVM_FALLTHROUGH do { } while (0) **#endif* After this we can start using *-Wimplicit-fallthrough*. Please express your opinions on this proposal. I'm also ready to provide diffs for marking all fall-through locations with the LLVM_FALLTHROUGH macro, if this proposal gets approval. -- Best regards, Alexander Kornienko -------------- next part -------------- An HTML attachment was scrubbed... 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Alexander Kornienko
2012-Jul-26 18:17 UTC
[LLVMdev] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases
Is anyone interested in introducing fall-through annotations and the related diagnostic in LLVM/Clang code? On Mon, Jul 2, 2012 at 6:59 PM, Alexander Kornienko <alexfh at google.com>wrote:> Hi llvmdev, llvm-commits, > > There was a discussion on this topic a while ago, and now I've decided to > make a formal proposal and post it here. > > I propose to add the LLVM_FALLTHROUGH macro for specifying intended > fall-through locations between switch cases. > > *INTRODUCTION* > > The switch construct of C/C++ languages allows fall-throughs between > switch labels when control flow is not directed elsewhere by a break, > return or continue statement or other ways. There are certain coding > idioms which utilize this behavior, e.g. Duff's device<http://en.wikipedia.org/wiki/Duff's_device>, > but much more frequently the use of switch statements doesn't involve a > fall-through. (A commonly used case - when several switch labels mark one > statement - is not considered a fall-through here.) As there's no special > way to mark a fall-through, it's relatively easy to make a mistake by just > missing a break/return statement before the next switch label. This kind of > mistake is sometimes difficult to spot by manual code examination, and the > compiler can't provide any help either. It's common to mark fall-through > locations with a comment, but parsing comments would incur a significant > performance penalty and is probably an unhealthy approach overall. So the > use of comment-only annotations is limited to manual inspections. > > Some time ago I've added the 'Unintended fall-through in switch statement' > diagnostics to clang ( > http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120430/057180.html > ). > > This is a proposal to introduce clang's unintended fall-through diagnostic > to llvm/clang code. > > *DESCRIPTION OF 'UNINTENDED FALL-THROUGH IN SWITCH STATEMENTS' DIAGNOSTIC > IN CLANG* > > Related functionality in clang introduces several diagnostic messages and > a syntax for intended fall-through annotation, based on C++11 attributes > ([dcl.attr], > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3376.pdf, 7.6). > > The [[clang::fallthrough]] attribute should be applied to a null > statement placed in a point of execution where fall-through to the next > switch label occurs, e.g.: > > switch (n) { > case 0: > ... // no break here > case 1: // warning: unannotated fall-through between switch labels > if (x) > break; > else if (y) { > ... > *[[clang::fallthrough]];* // annotated fall-through to case 2 > } > else > return 13; > case 2: // no warning here > ... > *[[clang::fallthrough]];* // annotated fall-through to case 3 > case 3: > ... > } > > So, the [[clang::fallthrough]]; annotation can be used in mostly in the > same way as break; or return; statements (but it doesn't change > control-flow, it just annotates a fall-through). The following rules are > checked for this annotation: > * the clang::fallthrough attribute can only be attached to a > null-statement; > * the [[clang::fallthrough]]; annotation should be placed inside a > switch body; > * it should be placed on an execution path between any statement inside > a switch body and a case/default label (this means there *is* a > fall-through to the corresponding case/default label); > * no statements should exist on an execution path between the > [[clang::fallthrough]]; annotation and the next case/default label. > > *PROPOSAL FOR LLVM/CLANG CODE* > > I ran the unintended fall-through diagnostics against llvm/clang code and > found a number of fall-through locations, most of them are probably > intended. But some look like bugs (see the attached * > fallthrough-bugs-llvm.diff* file). > > To start using the proposed diagnostics code has to be prepared. > > First, we need to mark all intended fall-through locations with the > clang::fallthrough attribute. It makes sense to wrap > [[clang::fallthrough]] to a macro, so the code would continue to work in > c++98 mode, but in c++11 mode it would also allow to run this diagnostic > (btw, is llvm compilable in c++11 mode?). Sample implementation of the > macro (see *fallthrough-macro.diff*): > *#ifdef __clang__ > **#if __has_feature(cxx_attributes) && > __has_warning("-Wimplicit-fallthrough") > * > *#define LLVM_FALLTHROUGH [[clang::fallthrough]] > **#endif > **#endif > ****#ifndef LLVM_FALLTHROUGH > **#define LLVM_FALLTHROUGH do { } while (0) > **#endif* > > After this we can start using *-Wimplicit-fallthrough*. > > Please express your opinions on this proposal. > > I'm also ready to provide diffs for marking all fall-through locations > with the LLVM_FALLTHROUGH macro, if this proposal gets approval. > > -- > Best regards, > Alexander Kornienko >-- Alexander Kornienko | Software Engineer | alexfh at google.com | +49 151 221 77 957 Google Germany GmbH | Dienerstr. 12 | 80331 München -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120726/05ca15b5/attachment.html>
Alexander Kornienko
2012-Jul-26 22:21 UTC
[LLVMdev] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases
On Thu, Jul 26, 2012 at 8:53 PM, Cameron McInally <cameron.mcinally at nyu.edu>wrote:> Hey Alex, > > Sorry if this is a silly question... are you asking if anyone "wants the > functionality proposed" or "wants to write the code for the functionality > proposed"? >*-Wimplicit-fallthrough* diagnostic is already implemented, and the patch in this thread is a portability macro for *[[clang::fallthrough]]*attribute (which is also implemented, of course). I only need an approval from community to add this macro and replace comment-only fall-through annotations with the *LLVM_FALLTHROUGH;* macro.> -Cameron > > On Thu, Jul 26, 2012 at 2:17 PM, Alexander Kornienko <alexfh at google.com>wrote: > >> Is anyone interested in introducing fall-through annotations and the >> related diagnostic in LLVM/Clang code? >> >> >> On Mon, Jul 2, 2012 at 6:59 PM, Alexander Kornienko <alexfh at google.com>wrote: >> >>> Hi llvmdev, llvm-commits, >>> >>> There was a discussion on this topic a while ago, and now I've decided >>> to make a formal proposal and post it here. >>> >>> I propose to add the LLVM_FALLTHROUGH macro for specifying intended >>> fall-through locations between switch cases. >>> >>> *INTRODUCTION* >>> >>> The switch construct of C/C++ languages allows fall-throughs between >>> switch labels when control flow is not directed elsewhere by a break, >>> return or continue statement or other ways. There are certain coding >>> idioms which utilize this behavior, e.g. Duff's device<http://en.wikipedia.org/wiki/Duff's_device>, >>> but much more frequently the use of switch statements doesn't involve a >>> fall-through. (A commonly used case - when several switch labels mark one >>> statement - is not considered a fall-through here.) As there's no special >>> way to mark a fall-through, it's relatively easy to make a mistake by just >>> missing a break/return statement before the next switch label. This kind of >>> mistake is sometimes difficult to spot by manual code examination, and the >>> compiler can't provide any help either. It's common to mark fall-through >>> locations with a comment, but parsing comments would incur a significant >>> performance penalty and is probably an unhealthy approach overall. So the >>> use of comment-only annotations is limited to manual inspections. >>> >>> Some time ago I've added the 'Unintended fall-through in switch >>> statement' diagnostics to clang ( >>> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120430/057180.html >>> ). >>> >>> This is a proposal to introduce clang's unintended fall-through >>> diagnostic to llvm/clang code. >>> >>> *DESCRIPTION OF 'UNINTENDED FALL-THROUGH IN SWITCH STATEMENTS' >>> DIAGNOSTIC IN CLANG* >>> >>> Related functionality in clang introduces several diagnostic messages >>> and a syntax for intended fall-through annotation, based on C++11 >>> attributes ([dcl.attr], >>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3376.pdf, 7.6). >>> >>> The [[clang::fallthrough]] attribute should be applied to a null >>> statement placed in a point of execution where fall-through to the next >>> switch label occurs, e.g.: >>> >>> switch (n) { >>> case 0: >>> ... // no break here >>> case 1: // warning: unannotated fall-through between switch labels >>> if (x) >>> break; >>> else if (y) { >>> ... >>> *[[clang::fallthrough]];* // annotated fall-through to case 2 >>> } >>> else >>> return 13; >>> case 2: // no warning here >>> ... >>> *[[clang::fallthrough]];* // annotated fall-through to case 3 >>> case 3: >>> ... >>> } >>> >>> So, the [[clang::fallthrough]]; annotation can be used in mostly in the >>> same way as break; or return; statements (but it doesn't change >>> control-flow, it just annotates a fall-through). The following rules are >>> checked for this annotation: >>> * the clang::fallthrough attribute can only be attached to a >>> null-statement; >>> * the [[clang::fallthrough]]; annotation should be placed inside a >>> switch body; >>> * it should be placed on an execution path between any statement >>> inside a switch body and a case/default label (this means there *is* a >>> fall-through to the corresponding case/default label); >>> * no statements should exist on an execution path between the >>> [[clang::fallthrough]]; annotation and the next case/default label. >>> >>> *PROPOSAL FOR LLVM/CLANG CODE* >>> >>> I ran the unintended fall-through diagnostics against llvm/clang code >>> and found a number of fall-through locations, most of them are probably >>> intended. But some look like bugs (see the attached * >>> fallthrough-bugs-llvm.diff* file). >>> >>> To start using the proposed diagnostics code has to be prepared. >>> >>> First, we need to mark all intended fall-through locations with the >>> clang::fallthrough attribute. It makes sense to wrap >>> [[clang::fallthrough]] to a macro, so the code would continue to work >>> in c++98 mode, but in c++11 mode it would also allow to run this diagnostic >>> (btw, is llvm compilable in c++11 mode?). Sample implementation of the >>> macro (see *fallthrough-macro.diff*): >>> *#ifdef __clang__ >>> **#if __has_feature(cxx_attributes) && >>> __has_warning("-Wimplicit-fallthrough") >>> * >>> *#define LLVM_FALLTHROUGH [[clang::fallthrough]] >>> **#endif >>> **#endif >>> ****#ifndef LLVM_FALLTHROUGH >>> **#define LLVM_FALLTHROUGH do { } while (0) >>> **#endif* >>> >>> After this we can start using *-Wimplicit-fallthrough*. >>> >>> Please express your opinions on this proposal. >>> >>> I'm also ready to provide diffs for marking all fall-through locations >>> with the LLVM_FALLTHROUGH macro, if this proposal gets approval. >>> >>> -- >>> Best regards, >>> Alexander Kornienko >>> >>-- Best regards, Alexander Kornienko -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120727/a1e49c93/attachment.html>
Chris Lattner
2012-Jul-26 23:07 UTC
[LLVMdev] [llvm-commits] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases
<dropping llvm-commits> On Jul 2, 2012, at 9:59 AM, Alexander Kornienko wrote:> Hi llvmdev, llvm-commits, > > There was a discussion on this topic a while ago, and now I've decided to make a formal proposal and post it here.I missed the earlier discussion, so I'm sorry for chiming in late.> I propose to add the LLVM_FALLTHROUGH macro for specifying intended fall-through locations between switch cases.I don't really see that the tradeoff here is worthwhile. It is possible that we have some fallthrough bugs, but the cost of sprinkling this macro everywhere doesn't seem like the right tradeoff. -Chris
Jim Grosbach
2012-Jul-26 23:24 UTC
[LLVMdev] [llvm-commits] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases
On Jul 26, 2012, at 4:07 PM, Chris Lattner <clattner at apple.com> wrote:> <dropping llvm-commits> > > On Jul 2, 2012, at 9:59 AM, Alexander Kornienko wrote: > >> Hi llvmdev, llvm-commits, >> >> There was a discussion on this topic a while ago, and now I've decided to make a formal proposal and post it here. > > I missed the earlier discussion, so I'm sorry for chiming in late. > >> I propose to add the LLVM_FALLTHROUGH macro for specifying intended fall-through locations between switch cases. > > I don't really see that the tradeoff here is worthwhile. It is possible that we have some fallthrough bugs, but the cost of sprinkling this macro everywhere doesn't seem like the right tradeoff. >While I tend to agree with you, it's also true that for many (most?) of the locations where we have an intentional fall through, there's already a comment to that effect as a matter of style. This would simply formalize that currently voluntary bit of style and use the macro rather than a comment. Depending on how many bugs this would enable find (perhaps some experiments are in order?), I could be convinced the tradeoff is worthwhile. -Jim
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