Allow atomic memory count update with C11 builtins
From mailing list post https://groups.google.com/forum/#!topic/redis-db/QLjiQe4D7LA In zmalloc.c the following primitives are currently used to synchronize access to single global variable: __sync_add_and_fetch __sync_sub_and_fetch In some architectures such as powerpc these primitives are overhead intensive. More efficient C11 __atomic builtins are available with newer GCC versions, see http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/_005f_005fatomic-Builtins.html#_005f_005fatomic-Builtins By substituting the following __atomic… builtins: __atomic_add_fetch __atomic_sub_fetch the performance improvement on certain architectures such as powerpc can be significant, around 10% to 15%, over the implementation using __sync builtins while there is only slight uptick on Intel architectures because it was already enforcing Intel Strongly ordered memory semantics. The selection of __atomic built-ins can be predicated on the definition of ATOMIC_RELAXED which Is available on in gcc 4.8.2 and later versions.
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