[skillina]

Source for what? The volatile keyword is explicitly telling the compiler "don't optimize read/write to this memory location". That's the whole point. Its use for manipulating hardware registers is covered in any intro embedded systems course. I don't know the history of C compilers but it would seem reasonable to assume that compilers started out plainly translating the C to machine code. Optimization would have happened later as the compilers became more mature.

https://www.gnu.org/software/c-intro-and-ref/manual/html_nod...

[saagarjha]

Source for "compilers basically always did volatile since everything was always spilled".