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>

2 files changed, 3 insertions, 13 deletions

diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c
index c036a4a2f8e2..a1570c8bab25 100644
--- a/arch/arm/kernel/asm-offsets.c
+++ b/arch/arm/kernel/asm-offsets.c
@@ -31,15 +31,6 @@
 #if defined(__APCS_26__)
 #error Sorry, your compiler targets APCS-26 but this kernel requires APCS-32
 #endif
-/*
- * GCC 4.8.0-4.8.2: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58854
- *	      miscompiles find_get_entry(), and can result in EXT3 and EXT4
- *	      filesystem corruption (possibly other FS too).
- */
-#if defined(GCC_VERSION) && GCC_VERSION >= 40800 && GCC_VERSION < 40803
-#error Your compiler is too buggy; it is known to miscompile kernels
-#error and result in filesystem corruption and oopses.
-#endif
 
 int main(void)
 {
diff --git a/arch/mips/include/asm/unroll.h b/arch/mips/include/asm/unroll.h
index c628747d4ecd..8ed660adc84f 100644
--- a/arch/mips/include/asm/unroll.h
+++ b/arch/mips/include/asm/unroll.h
@@ -19,14 +19,13 @@
 								\
 	/*							\
 	 * We can't unroll if the number of iterations isn't	\
-	 * compile-time constant. Unfortunately GCC versions	\
-	 * up until 4.6 tend to miss obvious constants & cause	\
+	 * compile-time constant. Unfortunately clang versions	\
+	 * up until 8.0 tend to miss obvious constants & cause	\
 	 * this check to fail, even though they go on to	\
 	 * generate reasonable code for the switch statement,	\
 	 * so we skip the sanity check for those compilers.	\
 	 */							\
-	BUILD_BUG_ON((CONFIG_GCC_VERSION >= 40700 ||		\
-		      CONFIG_CLANG_VERSION >= 80000) &&		\
+	BUILD_BUG_ON((CONFIG_CLANG_VERSION >= 80000) &&		\
 		     !__builtin_constant_p(times));		\
 								\
 	switch (times) {					\