Study LLVM IR: Compile C Code and Read the .ll Output

LLVM IR in Production

LLVM IR is the interchange format behind Clang, Rust, Swift, Julia, and many GPU compilers. It is typed, SSA-based, and infinite-register. Reading .ll output at -O0 vs -O2 is how engineers diagnose missed optimizations and undefined-behavior surprises.

Module Structure

A module contains functions. Each function has basic blocks of instructions. Types are explicit: i32, float, pointers. The phi instruction merges SSA values. alloca, load, and store model stack memory.

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• clang -S -emit-llvm foo.c produces .ll text
• SSA names use % prefix; globals use @
• Optimizations run as pass pipelines on IR
• IR is both human-readable text and bitcode binary

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 study LLVM IR by compiling C code with Clang and reading the .ll output. This exercise asks you to trace how C constructs map to LLVM instructions and compare unoptimized vs optimized IR for the same source.