The way the existing ppc_simd.h is written makes it hard to to switch between the old Altivec loads and stores and the new POWER7 loads and stores. This checkin rewrites the wrappers to use _ALTIVEC_, _ARCH_PWR7 and _ARCH_PWR8. The wrappers in this file now honor -maltivec, -mcpu-power7 and -mcpu=power8. It allows users to compile a source file, like chacha_simd.cpp, with a lower ISA and things just work for them.
This costs about 0.6 cpb (700 MB/s on GCC112), but it makes the faster algorithm available to more machines. In the future we may want to provide both POWER7 and POWER8
Jeffrey Walton