Your point is that if we can convince the compiler to optimize out the initialization of a register, we can get printf() to "leak" the previous value of that register. The example of %esi is specific to this variable being the first to occur in format string and the fact that you are running on a non-Windows 64-bit system, where the format string is passed in %rdi and the first format variable in %rsi (and %ax is zeroed to specify that no vector arguments are involved).

It happens that current Clang makes this and only this omission, although GCC and ICC leave crash because they try to execute 'idiv' by zero. Confusingly, this error is reported as a "floating point exception" even though no floating point numbers are involved. If one wanted to see the uninitialized value for these other compilers, you could probably just use a bare printf("%llx") without any argument.