[LLVMdev] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases (original) (raw)

Alexander Kornienko alexfh at google.com
Mon Jul 2 09:59:58 PDT 2012

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:

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... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120702/e3b7093a/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: fallthrough-bugs-llvm.diff Type: application/octet-stream Size: 2402 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120702/e3b7093a/attachment.obj> -------------- next part -------------- A non-text attachment was scrubbed... Name: fallthrough-macro.diff Type: application/octet-stream Size: 1798 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120702/e3b7093a/attachment-0001.obj>

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