No, p+1 is OK because you are allowed to make a pointer one past the end in C (so that common pointer loops work as expected) but you cannot dereference them.

    uintptr_t ip = (uintptr_t)(p+1);

Is perfectly defined. What is undefined is converting ip back to a pointer and writing to it, which the original program never does.

Hmm, I think I understand now.

So now I think it's the first optimization the one that is wrong. If you replace a variable with another, don't you need to keep information of the original variable?

  I mean, if a==b and you do *a=1 you can replace it with *b=1, but then you need to keep the information that 'a' was written to, so other optimizations (the third one) don't think it wasn't. Or am I missing something else here?

Edit: sorry for the code block, otherwise the asterisks are removed.

> If you replace a variable with another, don't you need to keep information of the original variable?

Note that ip and iq are integers. If you can't replace integers freely, it makes it really hard to optimize arithmetic.

There's clearly potential for pointer aliasing in the program, isn't that UB?