Docs for std::rand::random should discuss performance

[huonw]

std::rand::random is quite slow, since it has to do a TLD look-up to find the task_rng, and only then generate the random number. The docs should mention that if random is called repeatedly, one should do a single look-up via the task_rng() function, and then call the Rng methods directly.

E.g.

for x in foo.mut_iter() {
    *x = rand::random()
}

// would be faster as

let mut rng = rand::task_rng();
for x in foo.mut_iter() {
    *x = rng.gen();
}

[steveklabnik]

👍

[thestinger]

This kind of thing wouldn't be an issue with a fast TLS implementation though. LLVM knows how to perform optimizations on real TLS.

[steveklabnik]

What's the story with this today? the removal of libgreen means that TLS got redone, correct?