28.6.3. GCC inline assembly early-clobbers
An example of using the & early-clobber modifier: link:userland/arch/aarch64/earlyclobber.c
More details at: https://stackoverflow.com/questions/15819794/when-to-use-earlyclobber-constraint-in-extended-gcc-inline-assembly/54853663#54853663
The assertion may fail without it. It actually does fail in GCC 8.2.0.