It depends on the compiler. The great thing about SSA conversion is that you only have to do it once, whereas a classical def/use-chaining analysis may have to

It depends on the compiler. The great thing about SSA conversion is that you only have to do it once, whereas a classical def/use-chaining analysis may have to be redone many times in an old-style optimizer. SSA is a conversion of the program into a representation of its data flow. I've used it in multiple compilers and found it to be a big win for easing other analyses (e.g., induction variable recognition become trivial) and reducing bugs due to the update problem.