On Sat, Feb 08, 2020 at 11:32:40AM +0100, Uros Bizjak wrote:
> I think that the patch should also be backported to gcc-9 branch. The
> change is backward compatible, since the new code will save and
> restore zmm16+ registers at the caller site, and the old code (e.g.
> existing libraries) will then unnecessarily save and restore regs in
> the callee.
Actually, it isn't backward compatible.
If both caller and callee touch xmm16+ (sure, unlikely), then if caller
is compiled by new gcc and callee by old gcc it is the case you described,
caller doesn't try to preserve values across the call and callee uselessly
saves them anyway (unless it fails to assemble due to SEH).
But if caller is compiled by old gcc and callee by new gcc, caller assumes
it can preserve values across the call in those registers and callee doesn't
save them and clobbers them.
Jakub
