[tylerhou]

It is not critical for register assignment -- in fact, SSA makes register assignment more difficult (see the swap problem; the lost copy problem).

Lifetime analysis is important for register assignment, and SSA can make lifetime analysis easier, but plenty of non-SSA compilers (lower-tier JIT compilers often do not use SSA because SSA is heavyweight) are able to register allocate just fine without it.

[tonfa]

One nice thing is that it makes register assignment polynomial (the coloring of SSA variables graph is polynomial since it's not an arbitrary graph).