Sort of the point is, in pure languages only the symbolic operations are important. And the side effects (memory operations) are unimportant and beyond the control of the programmer. So you can do all sorts of transformation on the code without effecting the end result.
C is definitely impure. There is a large set usage cases where the memory layouts are important. Where the orders of operation and memory accesses are important.
What I worry about is when the optimizer is allowed to make assumptions about undefined behavior, that may be actually important on certain targets. Consider referencing a null pointer.
Winows7, Linux, in user land if you do that and you'll get a seg fault. And if the default handler runs your program dies.
The ARM cortex processor I've been slopping code for, reading a null pointer returns the initial stack pointer. Writing generates a bus fault. Since I actually trap that, and error message gets written into non-initialize memory and then the processor gets forcibly reset.
On an AVR reading a null pointer gives you the programs entry point. Writing to that location typically does nothing. Though you can write to it if running out of a special 'boot sector' and you've done some magic incantations.
So that's the problem I have with the idea that 'undefined means the optimizer can make hash of your program' instead of trusting the back end of the compiler will do something target appropriate.
