[boost] [context] SSE/AVX registers preservation

Hi, I've been looking at Boost.Context and wondering whether it preserves SSE/AVX state on context switches. From the code it doesn't look like the case - only jump_x86_64_ms_pe_masm.asm contains the code that saves some (not all) xmm registers and ymm registers are never saved. Is this intentional or the library simply doesn't support SSE/AVX? And why not use xsave/xrstor to consistently support all future extensions?