https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67841
--- Comment #6 from H.J. Lu <hjl.tools at gmail dot com> ---
ix86_expand_prologue in i386.c has
/* The computation of the size of the re-aligned stack frame means
that we must allocate the size of the register save area before
performing the actual alignment. Otherwise we cannot guarantee
that there's enough storage above the realignment point. */
That is where unaligned SSE load/store in prologue come from.
