In the bug report, @DavidKreitzer wrote:
clang should be giving an error for this test, because we have no good way to efficiently save & restore the non-GPR state.
The interrupt handler is required to save & restore all the register state that it uses. And according to the ABI, the call to subroutine1() may clobber arbitrary XMM, YMM, or ZMM state. The only way to reliably save & restore that state is to use xsave/xrstor, which would be very inefficient and is probably not what we want.
This patch implements the check described. There's a similar check for arm architecture and they give a warning. Folks tend to ignore warnings so I'll propose this as an error in the first go-round. @ABataev wrote a similar error check so I'll add him as reviewer