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Comment by pcwalton | Hacker News Reader

This is an example of how certain optimizations (specifically, making vtables immutable) are hard in LLVM, because of how low-level it is. Language-specific high-level IRs, such as Swift SIL, can allow compilers to perform these kinds of optimizations more easily. Of course, they're a lot of work to implement.

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It's true that language specific IRs are more powerful, but that isn't the problem in this example. Interfaces aren't part of Zig at the language level, nor are object lifetimes over which to make the vtables immutable. Having a memory model where memory is not innately typed is what makes this problem difficult to optimize, the IR has very little to do with it.

> a memory model where memory is not innately typed

This is intriguing. Do you have examples of systems where memory is typed? Or some ideas?

BitC used a strictly typed memory model.

Memory is typed in C++. You must use placement new to imbue memory with a non-POD type, and that memory continues to have that type until its destructor is invoked. This is what allows the optimizer to assume that vtable pointers don't change while an object is alive.

Effects of the typedness of memory can also be seen in the strict aliasing rules. Once a piece of memory has taken on a type, it may no longer be aliased by pointers of different types, until the memory is relinquished back into typelessness by a destructor.

thanks!

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