4 ms·

I think this is mostly a philosphical question, rather than optimizer quality. Once an optimization becomes part of the interface and it is guaranteed, is it r

I think this is mostly a philosphical question, rather than optimizer quality. Once an optimization becomes part of the interface and it is guaranteed, is it really an optimization? Or did it just became part of the language/library/database/whatever? One example is return value optimization in C++. In C++17 the "optimization" became mandatory in some contexts. What really happened though is that the rules of temporary materialization changed, and in those contexts it just never happens prematurely by the language rules. This ceased to be an optimization and became a mechanism in the language. What I'm getting at is that unreliability is a defining quality of optimizations. Sure, there are certain optimizations that become load-bearing, in which case it would be better if they became part of the language's semantics and guarantees, therefore they ceased to be optimizations.

gizmo686 2y ago
It is still a useful distinction. Programming languages are complex enough to understand. It is useful to have a base of 'this is the simplest description of how programs will work' that is as simple as possible. Then, a separate set of 'and here is a separate description of its performance characteristics; nothing in this portion should be understood to change the defined behavior of any program". Even if that second description is stable and part of the guarantees you make, keeping it seperate is still incredibly useful from a user perspective. From an implementation perspective, there is also a useful distinction. Optimizations take a valid representation, and turn it into a different valid representation of the same type that shares all defined behavior. This is a fairly different operation than compilation, which converts between representations. In particular, for the compilation step, you typically have only one compilation function for a given pair of representations; and if you have multiple, you select one ahead of time. For optimizations, each representation has a set of optimization functions, and you need to decided what order to apply them and how many times to do so. Compilation functions, for their part, need to deal with every difference between the two representations, whereas optimization functions get to ignore everything except the part they care about.
account42 2y ago
C++ return value optimization has always been a language feature rather than a true optimization because it is/was allowed to change the observable behavior of the program.

gizmo686 2y ago

It is still a useful distinction. Programming languages are complex enough to understand. It is useful to have a base of 'this is the simplest description of how programs will work' that is as simple as possible. Then, a separate set of 'and here is a separate description of its performance characteristics; nothing in this portion should be understood to change the defined behavior of any program". Even if that second description is stable and part of the guarantees you make, keeping it seperate is still incredibly useful from a user perspective. From an implementation perspective, there is also a useful distinction. Optimizations take a valid representation, and turn it into a different valid representation of the same type that shares all defined behavior. This is a fairly different operation than compilation, which converts between representations. In particular, for the compilation step, you typically have only one compilation function for a given pair of representations; and if you have multiple, you select one ahead of time. For optimizations, each representation has a set of optimization functions, and you need to decided what order to apply them and how many times to do so. Compilation functions, for their part, need to deal with every difference between the two representations, whereas optimization functions get to ignore everything except the part they care about.

account42 2y ago

C++ return value optimization has always been a language feature rather than a true optimization because it is/was allowed to change the observable behavior of the program.