kbuild: add macro for controlling warnings to linux/compiler.h

I have occasionally run into a situation where it would make sense to
control a compiler warning from a source file rather than doing so from
a Makefile using the $(cc-disable-warning, ...) or $(cc-option, ...)
helpers.

The approach here is similar to what glibc uses, using __diag() and
related macros to encapsulate a _Pragma("GCC diagnostic ...") statement
that gets turned into the respective "#pragma GCC diagnostic ..." by
the preprocessor when the macro gets expanded.

Like glibc, I also have an argument to pass the affected compiler
version, but decided to actually evaluate that one. For now, this
supports GCC_4_6, GCC_4_7, GCC_4_8, GCC_4_9, GCC_5, GCC_6, GCC_7,
GCC_8 and GCC_9. Adding support for CLANG_5 and other interesting
versions is straightforward here. GNU compilers starting with gcc-4.2
could support it in principle, but "#pragma GCC diagnostic push"
was only added in gcc-4.6, so it seems simpler to not deal with those
at all. The same versions show a large number of warnings already,
so it seems easier to just leave it at that and not do a more
fine-grained control for them.

The use cases I found so far include:

- turning off the gcc-8 -Wattribute-alias warning inside of the
  SYSCALL_DEFINEx() macro without having to do it globally.

- Reducing the build time for a simple re-make after a change,
  once we move the warnings from ./Makefile and
  ./scripts/Makefile.extrawarn into linux/compiler.h

- More control over the warnings based on other configurations,
  using preprocessor syntax instead of Makefile syntax. This should make
  it easier for the average developer to understand and change things.

- Adding an easy way to turn the W=1 option on unconditionally
  for a subdirectory or a specific file. This has been requested
  by several developers in the past that want to have their subsystems
  W=1 clean.

- Integrating clang better into the build systems. Clang supports
  more warnings than GCC, and we probably want to classify them
  as default, W=1, W=2 etc, but there are cases in which the
  warnings should be classified differently due to excessive false
  positives from one or the other compiler.

- Adding a way to turn the default warnings into errors (e.g. using
  a new "make E=0" tag) while not also turning the W=1 warnings into
  errors.

This patch for now just adds the minimal infrastructure in order to
do the first of the list above. As the #pragma GCC diagnostic
takes precedence over command line options, the next step would be
to convert a lot of the individual Makefiles that set nonstandard
options to use __diag() instead.

[paul.burton@mips.com:
  - Rebase atop current master.
  - Add __diag_GCC, or more generally __diag_<compiler>, abstraction to
    avoid code outside of linux/compiler-gcc.h needing to duplicate
    knowledge about different GCC versions.
  - Add a comment argument to __diag_{ignore,warn,error} which isn't
    used in the expansion of the macros but serves to push people to
    document the reason for using them - per feedback from Kees Cook.
  - Translate severity to GCC-specific pragmas in linux/compiler-gcc.h
    rather than using GCC-specific in linux/compiler_types.h.
  - Drop all but GCC 8 macros, since we only need to define macros for
    versions that we need to introduce pragmas for, and as of this
    series that's just GCC 8.
  - Capitalize comments in linux/compiler-gcc.h to match the style of
    the rest of the file.
  - Line up macro definitions with tabs in linux/compiler-gcc.h.]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/linux/compiler-gcc.h

@@ -347,3 +347,28 @@
 #if GCC_VERSION >= 50100
 #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
 #endif
+
+/*
+ * Turn individual warnings and errors on and off locally, depending
+ * on version.
+ */
+#define __diag_GCC(version, severity, s) \
+	__diag_GCC_ ## version(__diag_GCC_ ## severity s)
+
+/* Severity used in pragma directives */
+#define __diag_GCC_ignore	ignored
+#define __diag_GCC_warn		warning
+#define __diag_GCC_error	error
+
+/* Compilers before gcc-4.6 do not understand "#pragma GCC diagnostic push" */
+#if GCC_VERSION >= 40600
+#define __diag_str1(s)		#s
+#define __diag_str(s)		__diag_str1(s)
+#define __diag(s)		_Pragma(__diag_str(GCC diagnostic s))
+#endif
+
+#if GCC_VERSION >= 80000
+#define __diag_GCC_8(s)		__diag(s)
+#else
+#define __diag_GCC_8(s)
+#endif

include/linux/compiler_types.h

@@ -271,4 +271,22 @@ struct ftrace_likely_data {
 # define __native_word(t) (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
 #endif
 
+#ifndef __diag
+#define __diag(string)
+#endif
+
+#ifndef __diag_GCC
+#define __diag_GCC(version, severity, string)
+#endif
+
+#define __diag_push()	__diag(push)
+#define __diag_pop()	__diag(pop)
+
+#define __diag_ignore(compiler, version, option, comment) \
+	__diag_ ## compiler(version, ignore, option)
+#define __diag_warn(compiler, version, option, comment) \
+	__diag_ ## compiler(version, warn, option)
+#define __diag_error(compiler, version, option, comment) \
+	__diag_ ## compiler(version, error, option)
+
 #endif /* __LINUX_COMPILER_TYPES_H */