[PATCH 1/2] Compiler Attributes: add support for __fallthrough (gcc >= 7.1)

[Miguel Ojeda]

>From the GCC manual:

  fallthrough

    The fallthrough attribute with a null statement serves as a
    fallthrough statement. It hints to the compiler that a statement
    that falls through to another case label, or user-defined label
    in a switch statement is intentional and thus the -Wimplicit-fallthrough
    warning must not trigger. The fallthrough attribute may appear
    at most once in each attribute list, and may not be mixed with
    other attributes. It can only be used in a switch statement
    (the compiler will issue an error otherwise), after a preceding
    statement and before a logically succeeding case label,
    or user-defined label.

  https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html

Currently, most of the kernel uses fallthrough comments to silence
the -Wimplicit-fallthrough warnings (enabled by -Wextra, in W=1).
However, C++17 standarized an "statement attribute" (the first
of its kind) to deal with this: [[fallthrough]] is meant to be
a new control keyword in the form of an extension.

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

While comment parsing is a good idea to deal with old codebases
that used such a comment as documentation for humans, the best
solution is to use the attribute:

  * It is a "real" part of the AST (=> better for tooling).

  * It does not follow arbitrary rules for parsing (e.g. regexps
    for the comment parsing).

  * It may even become standarized in C as well: there are ongoing
    proposals to import some C++ standard attributes into
    the C standard, e.g. for fallthrough:

      http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf

On top of that, it is also a better solution for the kernel, because:

  * We can actually use a #define for it like for the rest of
    attributes/extensions, which is not possible with a comment,
    so that its naming/usage is consistent across the entire kernel.

  * Whenever the migration from the comments to the attribute
    is complete, we may increase the level of the GCC warning up to 5,
    i.e. comments will not longer be considered for warning
    surpression:  only the attribute must be used. This would enforce
    consistency by leveraging the compiler directly (instead of
    enforcing it with other tools).

  * Further into the future, we can consider moving the warning
    up to W=0 or even making it an error.

It is worth noting that clang >= 3.2 supports the warning and
the attribute, but only in C++ mode (and it is not enabled by
-Wall/-Wextra/-Wpedantic like in gcc). Hopefully, they will also
support it for C as well.

Further, icc >= 18 does not seem to know anything about the warning;
except that it accepts (i.e. ignores) [[fallthrough]] in C++17 mode
(to be conformant, probably).

Link: 20181017062255.oiu44y4zuuwilan3@mwanda/">https://lore.kernel.org/lkml/20181017062255.oiu44y4zuuwilan3@mwanda/
Suggested-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
---
 include/linux/compiler_attributes.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 6b28c1b7310c..9e2153f85462 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -32,6 +32,7 @@
 # define __GCC4_has_attribute___assume_aligned__      (__GNUC_MINOR__ >= 9)
 # define __GCC4_has_attribute___designated_init__     0
 # define __GCC4_has_attribute___externally_visible__  1
+# define __GCC4_has_attribute___fallthrough__         0
 # define __GCC4_has_attribute___noclone__             1
 # define __GCC4_has_attribute___optimize__            1
 # define __GCC4_has_attribute___nonstring__           0
@@ -133,6 +134,23 @@
 # define __visible
 #endif
 
+/*
+ * Currently, most of the kernel uses fallthrough comments to silence
+ * the -Wimplicit-fallthrough warnings (enabled by -Wextra, in W=1).
+ * For new instances, please use this attribute instead.
+ *
+ * Optional: only supported since gcc >= 7.1
+ * Optional: not supported by clang
+ * Optional: not supported by icc
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-fallthrough-statement-attribute
+ */
+#if __has_attribute(__fallthrough__)
+# define __fallthrough                  __attribute__((__fallthrough__))
+#else
+# define __fallthrough
+#endif
+
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-format-function-attribute
  * clang: https://clang.llvm.org/docs/AttributeReference.html#format
-- 
2.17.1