https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68507
--- Comment #4 from Daniel Santos <daniel.santos at pobox dot com> --- According to § 3.2.1 "Registers and the Stack Frame" of the System V Application Binary Interface for AMD64 Registers %rbp, %rbx and %r12 through %r15 “belong” to the calling function and the called function is required to preserve their values. In other words, a called function must preserve these registers’ values for its caller. Remaining registers “belong” to the called function.5 If a calling function wants to preserve such a register value across a function call, it must save the value in its local stack frame. And for microsoft's "x64" calling convention (https://msdn.microsoft.com/en-us/library/9z1stfyw.aspx), the xmm registers are considered non-volatile, so it would appear that this is the correct behaviour, barring some extensive whole-program analysis that can guarantee that the xmm registers are not destroyed.
