gcc, strict-aliasing, and casting through a union

Question

Do you have any horror stories to tell? The GCC Manual recently added a warning regarding -fstrict-aliasing and casting a pointer through a union:

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Answer 1

Aliasing occurs when the compiler has two different pointers to the same piece of memory. By typecasting a pointer, you're generating a new temporary pointer. If the optimizer reorders the assembly instructions for example, accessing the two pointers might give two totally different results - it might reorder a read before a write to the same address. This is why it is undefined behavior.

You are unlikely to see the problem in very simple test code, but it will appear when there's a lot going on.

I think the warning is to make clear that unions are not a special case, even though you might expect them to be.

See this Wikipedia article for more information about aliasing: http://en.wikipedia.org/wiki/Aliasing_(computing)#Conflicts_with_optimization