For gcc 4.1 & later, expands to __attribute__((flatten)) which forces the compiler to inline everything it can into the function. This is useful in combination with noinline when you want to control the depth of inlining, or create a single function where inline expansions will occur. (see http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#index-g_t_0040code_007bflatten_007d-function-attribute-2512) Normally, it's best to leave this type of thing up to the compiler. However, the generic rbtree code uses inline functions just to be able to inject compile-time constant data that specifies how the caller wants the function to behave (via struct rb_relationship). This data can be thought of as the template parameters of a C++ templatized function. Since some of these functions, once expanded, become quite large, gcc sometimes decides not to perform some important inlining, in one case, even generating a few bytes more code by not doing so. (Note: I have not eliminated the possibility that this was an optimization bug, but the flatten attribute fixes it in either case.) Combining __flatten and noinline insures that important optimizations occur in these cases and that the inline expansion occurs in exactly one place, thus not leading to unnecissary bloat. However, it also can eliminate some opportunities for optimization should gcc otherwise decide the function its self is a good candidate for inlining. Signed-off-by: Daniel Santos <daniel.santos@xxxxxxxxx> --- include/linux/compiler-gcc4.h | 7 ++++++- include/linux/compiler.h | 4 ++++ 2 files changed, 10 insertions(+), 1 deletions(-) diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h index ad610f2..5a0897e 100644 --- a/include/linux/compiler-gcc4.h +++ b/include/linux/compiler-gcc4.h @@ -15,7 +15,12 @@ #if GCC_VERSION >= 40102 # define __compiletime_object_size(obj) __builtin_object_size(obj, 0) -#endif + +/* flatten introduced in 4.1, but broken in 4.6.0 (gcc bug #48731)*/ +# if GCC_VERSION != 40600 +# define __flatten __attribute__((flatten)) +# endif +#endif /* GCC_VERSION >= 40102 */ #if GCC_VERSION >= 40300 /* Mark functions as cold. gcc will assume any path leading to a call diff --git a/include/linux/compiler.h b/include/linux/compiler.h index fd455aa..268aeb6 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -244,6 +244,10 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); #define __always_inline inline #endif +#ifndef __flatten +#define __flatten +#endif + #endif /* __KERNEL__ */ /* -- 1.7.3.4 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
