SSA form

Basic idea

In LLVM IR every register is assigned exactly once. To represent values that depend on which control-flow predecessor was taken, LLVM uses phi instructions at the start of basic blocks.

Key formulas

• $\phi$-function: %x = phi i32 [%a, %bb1], [%b, %bb2]

• Dominance frontier $DF(B)$: where $\phi$-nodes are inserted.

See SSA LLVM does not allow multiple assignments to registers

LLVM offers the phi function to choose between values.