Internal graph representation the compiler uses for programs. llvm uses SSA form of programs as its main low level representation. The benefits of SSA is that it enables some analysis and optimization techniques.
It is basically a simplification that makes optimizations easier by eliminating re-assignment of local variables (where possible, for loops and conditions, merging phi values might be necessary). It has been awhile since I played with this, but it made CSE (common sub-expression elimination) really easy.
