Compiler Optimization: LLVM, SSA & JIT

SSA · LLVM IR · passes · JIT

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How a compiler transforms source code into fast machine code through a three-phase pipeline, LLVM IR, SSA form, optimization passes, register allocation, and JIT compilation.

By the end, you can

State the as-if rule and explain why it is the contract governing every compiler optimization.
Describe the three compiler phases and identify which phase is target-neutral.
Explain the narrow-waist argument for a shared IR and compute M+N vs M×N component counts.
Convert a short sequence of reassignments into SSA form with renamed variables.
Explain what a phi function does at a CFG merge point and why it is not a runtime branch.
Trace constant folding and propagation through a three-line example.
Identify when dead-code elimination is legal (dead result and no side effects).
Show how inlining exposes a constant that folding can then collapse.
Describe the interference graph and explain when a value must be spilled.
Compare AOT and JIT compilation on the dimensions of warm-up cost, runtime overhead, and ability to specialize.
Outline the four stages of a tiered JIT (interpret, baseline, optimize, deopt) and explain what triggers deoptimization.

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