It will vary a lot from compiler to compiler, as different compilers implement it with different levels of aggression. GCC is fairly aggressive about it: enabling strict aliasing will cause it to think that pointers that are "obviously" equivalent to a human (as in, foo *a; bar *b = (bar *) a;) cannot alias, which allows for some very aggressive transformations, but can obviously break non-carefully written code. Apple's GCC disables strict aliasing by default for this reason.
LLVM, by contrast, does not even have strict aliasing, and, while it is planned, the developers have said that they plan to implement it as a fall-back case when nothing else can judge equivalence. In the above example, it would still judge a and b equivalent. It would only use type-based aliasing if it could not determine their relationship in any other way.
In my experience, the performance impact of strict aliasing mostly has to do with loop invariant code motion, where type information can be used to prove that in-loop loads can't alias the array being iterated over, allowing them to be pulled out of the loop. YMMV.
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