Strict pointer aliasing: is access through a 'volatile' pointer/reference a solution?

Question

On the heels of a specific problem, a self-answer and comments to it, I\'d like to understand if it is a proper solution, workaround/hack or just plain wrong.

Specif

Answer 1

Volatile can't help you avoid undefined behaviour here. So, if it works for you with GCC it's luck.

Let's assume T is a POD. Then, the proper way to do this is

T x = …;
int i;
memcpy(&i,&x,sizeof i);
if (i==0)
  …

There! No strict aliasing problem and no memory alignment problem. GCC even handles memcpy as an intrinsic function (no function call is inserted in this case).

Answer 2

Volatile can't help you avoid undefined behaviour here.

Well, anything regarding volatile is somewhat unclear in the standard. I mostly agreed with your answer, but now I would like to slightly disagree.

In order to understand what volatile means, the standard is not clear for most people, notably some compiler writers. It is better to think: when using volatile (and only when), C/C++ is pretty much high level assembly.

When writing to a volatile lvalue, the compiler will issue a STORE, or multiple STORE if one is not enough (volatile does not imply atomic).

When writing to a volatile lvalue, the compiler will issue a LOAD, or multiple LOAD if one is not enough.

Of course, where there is no explicit LOAD or STORE, the compiler will just issue instructions which imply a LOAD or STORE.

sellibitze gave the best solution: use memcpy for bit reinterpretations.

But if all accesses to a memory region are done with volatile lvalues, it is perfectly clear that the strict aliasing rules do not apply. This is the answer to your question.