audit use of __GNUC__

[nickdesaulniers]

ag GNUC shows a handful of comparisons against ancient versions of gcc below the minimum (4.6). There's also some checks against newer versions of GCC that should be audited to see what the implications are for clang.

[nickdesaulniers]

@Villemoes pointed out some cleanups:

• https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=88ca66d8540ca26119b1428cddb96b37925bdf01
• https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=2e905c7abdcd5ff09b9df33d25eb7148c85bed2a

@sedat pointed out they're already upstream:

• https://git.kernel.org/linus/88ca66d8540ca26119b1428cddb96b37925bdf01
• https://git.kernel.org/linus/2e905c7abdcd5ff09b9df33d25eb7148c85bed2a

[nickdesaulniers]

Also, KBUILD_CFLAGS := (reassignment).

[nickdesaulniers]

also GCC_VERSION, for example:
drivers/gpu/drm/i915/i915_utils.h has:

 83 #if defined(GCC_VERSION) && GCC_VERSION >= 70000                                
 84 #define add_overflows_t(T, A, B) \                                              
 85   __builtin_add_overflow_p((A), (B), (T)0)                                      
 86 #else                                                                           
 87 #define add_overflows_t(T, A, B) ({ \                                           
 88   typeof(A) a = (A); \                                                          
 89   typeof(B) b = (B); \                                                          
 90   (T)(a + b) < a; \                                                             
 91 })                                                                              
 92 #endif 

[nickdesaulniers]

There's also a ton of __GNUC_MINOR__ checks against unsupported GCC versions.

[nickdesaulniers]

Ok, I sent a few small cleanups related to the GCC 4.9 minimum version I found via grepping for __GNUC_MINOR__:

1. https://lore.kernel.org/lkml/20201111024140.1483879-1-ndesaulniers@google.com/T/#u
2. https://lore.kernel.org/lkml/20201111025120.1694158-1-ndesaulniers@google.com/T/#u
3. https://lore.kernel.org/lkml/20201111030557.2015680-1-ndesaulniers@google.com/T/#u
4. https://lore.kernel.org/lkml/20201111032105.2346303-1-ndesaulniers@google.com/T/#u

Nothing super important. I don't think it's worth checking for __GNUC__ since clang defines it; it's not possible to isolate code from Clang with __GNUC__ preprocessor guards alone. Closing.