Linear types support is coming along nicely, there is a fork of GHC with a prototype implementation.

https://github.com/tweag/ghc/tree/linear-types

However, to allow the compiler to optimise pure operations to impure ones, I don't think linear types are sufficient. You need something like uniqueness types.

I think C++ compiles down to a pure functional language that is then optimized into an efficient beast. So I'm not sure that pure code is hard to optimize.

SSA is quite different from a pure FP; for example it doesn’t do anything about effects, it only gets rid of local imperative assignments that don’t really interfere with purity anyways.

Most imperative languages have an intermediate language in which local variables are immutable. However, purity is about more than immutability. For example, in C++

    x = doSomething(x) + 1

Can be written to not overwrite x

    int x2 = doSomething(x) + 1

This is equivalent in some ways to Haskell

    let x' = doSomething x + 1

However, I know that in Haskell, evaluating 'doSomething x' will not turn off the computer, display anything to the user, or launch missiles. I have no idea what evaluating 'doSomething(x)' does in C++. It may add things to caches, exit the program, etc.