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by jcelerier 1871 days ago
> A correctly-specified ABI should pass large structures by immutable reference

is just not possible. CPUs don't know about `const`. So you have to work with the assumption that functions that you call can do anything to their arguments. Thus copies cannot be avoided.
3 comments
mhh__ 1871 days ago
The CPU also doesn't know what an ABI is
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CuriousCosmic 1871 days ago
An ABI also has a concept of defined and undefined behaviour. You can design an ABI that is fully protected against abuse but often the performance penalty for that will be huge.

Instead what you'll do is specify the constrained inputs and expected output behaviour. From there you can out anything that violates those constraints as non-conformant. As long as you maintain those constraints between versions, there's no ABI breakage.

Also you can absolutely have constant references in an ABI. There may be ways of ignoring the const depending on how you design the ABI but they will be obvious abuse.

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wizzwizz4 1871 days ago
CPUs actually do know about const; it's called a read-only page.

Besides, that's irrelevant. There's nothing stopping my function from following every pointer on the stack and smashing up its contents; are you going to defend against that, too? If not, how is this any different?

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