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I'm surprised no one mentioned a potentially difficult problem using x86 segments and C: pointer aliasing issues.
It's entirely possible for two pointers to point to precisely the same memory location and still be unequal.
This can make certain compiler optimizations next to impossible even if the programmer knows that two pointers can't possibly point to the same memory location. This goes for OoO processing at the processor level as well.
Dependencies are created that limit the degree to which operations can be reordered or elided.
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