Constant Folding

Fold Constants at Compile Time

Constant folding evaluates expressions with compile-time-known operands. Even -O0 Clang folds 2 + 3 to 5. JavaScript minifiers like Terser apply it on every production bundle. It enables further propagation and dead code removal.

What to Fold

Arithmetic, comparisons, and bitwise ops on literals. Boolean logic. Some sizeof and address-difference expressions in C. Stop before division by zero (may be runtime error) or side-effecting calls.

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• Work on IR or AST after constant propagation
• Re-run after other opts expose new constants
• Float folding respects IEEE rounding mode flags
• Conditional folds enable unreachable code elimination

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 implement constant folding on your IR. This exercise requires detecting instructions whose operands are all constants, replacing them with a single constant result, and enabling follow-on optimizations.