What is ssa?

SSA, or Single Static Assignment, is a property of an intermediate representation (IR) used in compilers. In SSA form, each variable is assigned a value exactly once in the program's code. If a variable needs to have multiple values at different points in the program, new versions of that variable are created.

Here are some key aspects of SSA:

• Unique Assignment: The defining characteristic. This makes many compiler optimizations simpler and more efficient.

• Φ (Phi) Functions: These are special functions inserted at control flow merge points. A Φ function takes multiple variable versions as input and selects the appropriate version based on which control flow path was taken to reach that point. For example, x3 = φ(x1, x2) means x3 takes the value of x1 if the path leading to the φ function came from where x1 was defined, and x2 otherwise.

• Advantages: SSA form simplifies various compiler analyses and optimizations, including dead code elimination, constant propagation, and strength reduction. The unique assignment property allows for more straightforward data flow analysis.

• Construction: Algorithms exist to convert any imperative program into SSA form. These typically involve identifying dominance frontiers and inserting φ functions accordingly.

• Use in Compilers: Many modern compilers use SSA as their primary IR for optimization. Examples include GCC, LLVM, and many Java Virtual Machines.