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I'm not sure that's the best example. In that case, the knowledge that the 'array' global never has its address taken allows you to perform the optimization. Yo

I'm not sure that's the best example. In that case, the knowledge that the 'array' global never has its address taken allows you to perform the optimization. You can also rewrite it by copying 'array' to a separate local that doesn't have its address taken, and ordinary SSA construction solves the problem. I feel like a new systems language needs to come and make aliasing explicit in a way, so that it doesn't surprise programmers but still allows them to get the optimizations that they want. Tracking uniqueness like Rust is a good first step, but it doesn't work for unsafe code or anything with more subtle aliasing relationships.

zvrba 11y ago
> I'm not sure that's the best example. In that case, the knowledge that the 'array' global never has its address taken allows you to perform the optimization. Separate translation units. For this to work, you'd need to postpone all optimizations to the link stage. Even then you're not safe, because you may be producing a dynamic library and the program loading it can take the address of an object by dlsym().
Too 11y ago
You can specify aliasing explicitly with the restrict keyword. It's used in cases where you have two pointers of the same type that are otherwise allowed to alias each other. For pointers of different types it doesn't make much sense and shouldn't be required, there is just no valid reason, except trying to be a smart ass, for two pointers of different types to alias each other, for generic containers you can use void pointers.
pcwalton 11y ago
> I feel like a new systems language needs to come and make aliasing explicit in a way, so that it doesn't surprise programmers but still allows them to get the optimizations that they want. Tracking uniqueness like Rust is a good first step, but it doesn't work for unsafe code or anything with more subtle aliasing relationships. I think it could be added to Rust. :) Uniqueness can be viewed as just a starting point. It's a conservative subset of more sophisticated systems like PALE. Since it's a subset, it could be expanded to the full system eventually. I think the biggest problem is not implementing it or finding a way to slot it into the language design but rather making it easy enough to use to achieve widespread adoption. Rust already stretches the boundaries of programmers' willingness to learn new type systems/static analyses to control aliasing, and it has, as you point out, a very simple system. In fact, I think it's unproven that programmers will be willing to adopt such a system en masse at all†, since Rust is still niche. If and when Rust really takes off in widespread production, we'll have proof that programmers are willing to adopt aliasing information in the type system. † It'll be interesting to see how the ISO Core C++ lifetime checker fares here. I'm not optimistic about how much use it will get due to complexity and effectively being a different language from C++, but I'd love to be proven wrong.

zvrba 11y ago

> I'm not sure that's the best example. In that case, the knowledge that the 'array' global never has its address taken allows you to perform the optimization. Separate translation units. For this to work, you'd need to postpone all optimizations to the link stage. Even then you're not safe, because you may be producing a dynamic library and the program loading it can take the address of an object by dlsym().

Too 11y ago

You can specify aliasing explicitly with the restrict keyword. It's used in cases where you have two pointers of the same type that are otherwise allowed to alias each other. For pointers of different types it doesn't make much sense and shouldn't be required, there is just no valid reason, except trying to be a smart ass, for two pointers of different types to alias each other, for generic containers you can use void pointers.

pcwalton 11y ago

> I feel like a new systems language needs to come and make aliasing explicit in a way, so that it doesn't surprise programmers but still allows them to get the optimizations that they want. Tracking uniqueness like Rust is a good first step, but it doesn't work for unsafe code or anything with more subtle aliasing relationships. I think it could be added to Rust. :) Uniqueness can be viewed as just a starting point. It's a conservative subset of more sophisticated systems like PALE. Since it's a subset, it could be expanded to the full system eventually. I think the biggest problem is not implementing it or finding a way to slot it into the language design but rather making it easy enough to use to achieve widespread adoption. Rust already stretches the boundaries of programmers' willingness to learn new type systems/static analyses to control aliasing, and it has, as you point out, a very simple system. In fact, I think it's unproven that programmers will be willing to adopt such a system en masse at all†, since Rust is still niche. If and when Rust really takes off in widespread production, we'll have proof that programmers are willing to adopt aliasing information in the type system. † It'll be interesting to see how the ISO Core C++ lifetime checker fares here. I'm not optimistic about how much use it will get due to complexity and effectively being a different language from C++, but I'd love to be proven wrong.