[PATCH 1/2] [RFC] kbuild: add macro for controlling warnings to linux/compiler.h

[Arnd Bergmann]

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.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
I'm marking this RFC for now, as I'd like to get consensus about
whether we want to go down this particular route first, and maybe
if someone can come up with better naming for the macros.
---
 include/linux/compiler-gcc.h   | 64 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/compiler_types.h | 10 +++++++
 2 files changed, 74 insertions(+)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 2272ded07496..5d595cfdb2c4 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -329,3 +329,67 @@
  * code
  */
 #define uninitialized_var(x) x = x
+
+/*
+ * turn individual warnings and errors on and off locally, depending
+ * on version.
+ */
+#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))
+
+/* compilers before gcc-4.6 do not understand "#pragma GCC diagnostic push" */
+#define __diag_GCC_4_6(s) __diag(s)
+#else
+#define __diag(s)
+#define __diag_GCC_4_6(s)
+#endif
+
+#if GCC_VERSION >= 40700
+#define __diag_GCC_4_7(s) __diag(s)
+#else
+#define __diag_GCC_4_7(s)
+#endif
+
+#if GCC_VERSION >= 40800
+#define __diag_GCC_4_8(s) __diag(s)
+#else
+#define __diag_GCC_4_8(s)
+#endif
+
+#if GCC_VERSION >= 40900
+#define __diag_GCC_4_9(s) __diag(s)
+#else
+#define __diag_GCC_4_9(s)
+#endif
+
+#if GCC_VERSION >= 50000
+#define __diag_GCC_5(s) __diag(s)
+#else
+#define __diag_GCC_5(s)
+#endif
+
+#if GCC_VERSION >= 60000
+#define __diag_GCC_6(s) __diag(s)
+#else
+#define __diag_GCC_6(s)
+#endif
+
+#if GCC_VERSION >= 70000
+#define __diag_GCC_7(s) __diag(s)
+#else
+#define __diag_GCC_7(s)
+#endif
+
+#if GCC_VERSION >= 80000
+#define __diag_GCC_8(s) __diag(s)
+#else
+#define __diag_GCC_8(s)
+#endif
+
+#if GCC_VERSION >= 90000
+#define __diag_GCC_9(s) __diag(s)
+#else
+#define __diag_GCC_9(s)
+#endif
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6b79a9bba9a7..7e7664d57adb 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -271,4 +271,14 @@ 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
+
+#define __diag_push()			__diag(push)
+#define __diag_ignore(version, option)	__diag_ ## version (ignored option)
+#define __diag_warn(version, option)	__diag_ ## version (warning option)
+#define __diag_error(version, option)	__diag_ ## version (error   option)
+#define __diag_pop()			__diag(pop)
+
 #endif /* __LINUX_COMPILER_TYPES_H */
-- 
2.9.0