[kevinastock]

Aliasing information is one place the programmer can help the compiler. The 'restrict' keyword in C/C++ is a great example of this. Also information provided by 'const' can be very helpful for the compiler. While both of these aren't strictly hints because they limit what you can do and still get correct results, they do tell the compiler facts that would be otherwise difficult or impossible to prove.

[jeffreyrogers]

'const' is a very good example and one I hadn't thought about. To some extent functional languages pass every function argument with const, since the arguments are call by value, rather than call by reference. At first glance this might seem to be disastrous in terms of performance if, for example, you pass in a list with 1,000,00 entries or something like that, but any reasonable implementation of a functional language provides ways of doing this without the excessive space and time costs that a naive implementation would have.

[stormbrew]

I'm not sure any compiler actually implements any real optimizations based on const. I suppose possibly on static const variables of basic integer types, but that'd be about all it can do. The existence of the volatile escape hatch, to say nothing of const_cast<>, makes it a pretty useless hint for the compiler.