[LLVMdev] PROPOSAL: LLVM_FALLTHROUGH macro for intended fall-throughs between switch cases

[Alexander Kornienko]

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
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