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This is a thing; it's called shadow call stack. Both ARM and now Intel have extensions for it.

But the shadow stack concept seems much dumber to me. Why write the address to the regular stack and the shadow stack and then compare? Why not only use the shadow stack and not put return addresses on the main stack at all.

ABI compatibility.

When you're compiling from scratch, especially when you're not working on a shared library, ABI compatability doesn't matter as much. Doesn't explain why there's no -fshadow-stack-only option to pass in.

> Doesn't explain why there's no -fshadow-stack-only option to pass in.

I thought you were asking about the design of the hardware: it's designed that way because compatibility means that the vast majority of people want something backwards compatible.

Very little is fully "compiled from scratch" when you consider for example that libc is in the set of things that must be recompiled.

Because that would break compatibility. The way shadow stacks are implemented means they can be enabled in existing software without code changes.

If one were to design a modern ISA from scratch it would make sense though.

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