Raise gcc version requirement to 4.9

I realize that we fairly recently raised it to 4.8, but the fact is, 4.9
is a much better minimum version to target.

We have a number of workarounds for actual bugs in pre-4.9 gcc versions
(including things like internal compiler errors on ARM), but we also
have some syntactic workarounds for lacking features.

In particular, raising the minimum to 4.9 means that we can now just
assume _Generic() exists, which is likely the much better replacement
for a lot of very convoluted built-time magic with conditionals on
sizeof and/or __builtin_choose_expr() with same_type() etc.

Using _Generic also means that you will need to have a very recent
version of 'sparse', but thats easy to build yourself, and much less of
a hassle than some old gcc version can be.

The latest (in a long string) of reasons for minimum compiler version
upgrades was commit 5435f73d5c4a ("efi/x86: Fix build with gcc 4").

Ard points out that RHEL 7 uses gcc-4.8, but the people who stay back on
old RHEL versions persumably also don't build their own kernels anyway.
And maybe they should cross-built or just have a little side affair with
a newer compiler?

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 3 insertions, 29 deletions

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 4671fbf28842..7f475d59a097 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -18,8 +18,7 @@
  * position @h. For example
  * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
  */
-#if !defined(__ASSEMBLY__) && \
-	(!defined(CONFIG_CC_IS_GCC) || CONFIG_GCC_VERSION >= 49000)
+#if !defined(__ASSEMBLY__)
 #include <linux/build_bug.h>
 #define GENMASK_INPUT_CHECK(h, l) \
 	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 1c74464c80c6..0b1dc61f3955 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -11,7 +11,7 @@
 		     + __GNUC_PATCHLEVEL__)
 
 /* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 */
-#if GCC_VERSION < 40800
+#if GCC_VERSION < 40900
 # error Sorry, your compiler is too old - please upgrade it.
 #endif
 
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c3bf7710f69a..01dd58c74d80 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -252,32 +252,8 @@ struct ftrace_likely_data {
  * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
  *			       non-scalar types unchanged.
  */
-#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)
 /*
- * We build this out of a couple of helper macros in a vain attempt to
- * help you keep your lunch down while reading it.
- */
-#define __pick_scalar_type(x, type, otherwise)					\
-	__builtin_choose_expr(__same_type(x, type), (type)0, otherwise)
-
-/*
- * 'char' is not type-compatible with either 'signed char' or 'unsigned char',
- * so we include the naked type here as well as the signed/unsigned variants.
- */
-#define __pick_integer_type(x, type, otherwise)					\
-	__pick_scalar_type(x, type,						\
-		__pick_scalar_type(x, unsigned type,				\
-			__pick_scalar_type(x, signed type, otherwise)))
-
-#define __unqual_scalar_typeof(x) typeof(					\
-	__pick_integer_type(x, char,						\
-		__pick_integer_type(x, short,					\
-			__pick_integer_type(x, int,				\
-				__pick_integer_type(x, long,			\
-					__pick_integer_type(x, long long, x))))))
-#else
-/*
- * If supported, prefer C11 _Generic for better compile-times. As above, 'char'
+ * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
  * is not type-compatible with 'signed char', and we define a separate case.
  */
 #define __scalar_type_to_expr_cases(type)				\
@@ -293,7 +269,6 @@ struct ftrace_likely_data {
 			 __scalar_type_to_expr_cases(long),		\
 			 __scalar_type_to_expr_cases(long long),	\
 			 default: (x)))
-#endif
 
 /* Is this type a native word size -- useful for atomic operations */
 #define __native_word(t) \