Add Phi-Nodes to Your SSA IR

Phi-Nodes Merge Values at Joins

In SSA, each variable has one definition, but control flow merges multiple paths. Phi-nodes select the right value based on which predecessor block executed. LLVM's phi instruction and GCC's PHI_EXPR are everywhere in optimized IR.

Placement and Semantics

Insert phi at join blocks where different paths assign the same variable. x3 = phi(x1 from B1, x2 from B2) means x3 takes x1 if control came from B1, else x2 from B2.

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• One phi operand per CFG predecessor
• Operands pair (value, label) in LLVM text form
• Place at dominance frontier of definitions
• Later passes copy-propagate through phi chains

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will add phi-nodes to your SSA IR at control-flow merge points. This exercise requires inserting phi instructions and wiring predecessor values so SSA form correctly represents variables across branches.