Question

I’ve read the essay Surviving the Release Version.

Under the “Aliasing bugs” clause it says:

You can get tighter code if you tell
the compiler that it can assume no
aliasing….

I’ve also read Aliasing (computing).

What exactly is a variable alias? I understand it means using a pointer to a variable is an alias, but, how/why does it affect badly, or in other words – why telling the compiler that it can assume no aliasing would get me a “tighter code”

Answer 1

Disallowing aliasing means if you have a pointer char* b, you can assume that b is the only pointer in the program that points to that particular memory location, which means the only time that memory location is going to change is when the programmer uses b to change it. The generated assembly thus doesn’t need to reload the memory pointed to by b into a register as long as the compiler knows nothing has used b to modify it. If aliasing is allowed it’s possible there’s another pointer char* c = b; that was used elsewhere to mess with that memory