[cfe-dev] [PATCH] Automatic detection of compatibility macros for non-portable diagnostics

[Matthieu Monrocq]

On Tue, Jul 24, 2012 at 7:11 PM, Alexander Kornienko <alexfh at google.com>wrote:

> This patch adds an automatic detection of a compatibility macros used in
> specific projects to hide constructions based on non-portable features
> (e.g. custom C++11 attributes). It helps to adapt diagnostic messages and
> fix-it hints to use these compatibility macros instead of the actual
> construct.
>
> This feature is implemented in AnalysisBasedWarnings.cpp, as there's
> currently only one diagnostic which gets profit from this - diagnostic of
> unannotated fall-through between switch labels. But the code of the
> CompatibilityMacroFinder class was intentionally made reasonably generic,
> and doesn't contain any specific bindings to this diagnostic. The class is
> a lightweight handler of PPCallbacks::MacroDefined calls. An instance of it
> is registered via Preprocessor::addPPCallbacks for each token
> sequence (specified in plain text) to find in macros (currently only
> one). It keeps all macros with the specified expansion token sequence and
> then can determine which one can be used instead of the actual construct in
> a specific code location.
>
> A motivating example for this feature:
> There's the -Wimplicit-fallthrough warning, which detects
> [[clang::fallthrough]]; construct as an annotation of an intended
> fall-through. In projects which should be compiled both in C++11 mode and
> in C++03 mode, this construct can not be used as is, so it should be
> wrapped in a macro, e.g.:
>
> #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
>
>
> Prior to this feature diagnostic messages would say:
> test.cpp:156:5: *warning: unannotated fall-through between switch labels*
>     case 223:
>     ^
> test.cpp:156:5: *note: insert '[[clang::fallthrough]];' to silence this
> warning*
>     case 223:
>     ^
>     *[[clang::fallthrough]];*
> test.cpp:156:5: *note: insert 'break;' to avoid fall-through*
>     case 223:
>     ^
>     break;
>
> Applying the first of these fix-it hints will lead to broken builds in
> C++03 mode, which is usually not desired.
>
> But with the automatic detection feature the diagnostic will suggest the
> correct compatibility macro available in the corresponding source location:
> ...
> test.cpp:156:5: *note: insert 'LLVM_FALLTHROUGH;' to silence this warning*
>     case 223:
>     ^
>     *LLVM_FALLTHROUGH; *
> ...
>
> Please, review this patch. Thank you!
>
> --
> Best regards,
> Alexander Kornienko
>
>
Wow! I would love to see this generalized.

I have had to use a couple of such macros in the past (notably for
__attribute__(noreturn)) and it's always annoying to have to grep through
the headers to find where that compatibility macro had been registered.

-- Matthieu
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