>But for a properly informed programmer for a given piece of code, not so much.
There are a couple reasons that even for "informed" programmers this is still important
1) For most dynamic languages, even simple operations can take a highly variable amount of time to execute. How many instructions does an array access take in Javascript? The answer depends on everything from the state of the JIT to the types involved, both of which are usually impossible to know before hand. In C we can answer this pretty easily.
2) The modern trend is towards writing more and more generic code. Even for statically compiled languages like C++ and Haskell, the actual underlying operations are purposely* abstracted away from you. Unless you know every possible instance that your code could be used it is impossible to know how long any operation will take.
And all this is assuming that the programmer knows everything about their compiler, assembler, standard library, imported libraries, ect, which isn't true for all but the most expert programmers.
*Admittedly, the actual length of time it takes is dependent on the state of the processor which can be very difficult to predict, but we will have a lot more information than we would have had otherwise.
There are a couple reasons that even for "informed" programmers this is still important
1) For most dynamic languages, even simple operations can take a highly variable amount of time to execute. How many instructions does an array access take in Javascript? The answer depends on everything from the state of the JIT to the types involved, both of which are usually impossible to know before hand. In C we can answer this pretty easily.
2) The modern trend is towards writing more and more generic code. Even for statically compiled languages like C++ and Haskell, the actual underlying operations are purposely* abstracted away from you. Unless you know every possible instance that your code could be used it is impossible to know how long any operation will take.
And all this is assuming that the programmer knows everything about their compiler, assembler, standard library, imported libraries, ect, which isn't true for all but the most expert programmers.
*Admittedly, the actual length of time it takes is dependent on the state of the processor which can be very difficult to predict, but we will have a lot more information than we would have had otherwise.