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A compiler back end translates an intermediate representation into operations suitable for a target machine. Typical tasks include instruction selection, register allocation, instruction scheduling and emission of assembly or object code. It uses facts about the target's instructions, registers, calling conventions and data layout.^[ref]

For example, a load from base + 4*i might use one indexed-addressing instruction when the target supports that addressing mode, or require separate address arithmetic otherwise. If more values are live than registers are available, Register allocation may spill some values to memory. Those choices change performance while having to preserve the source program's specified observable behavior. A target-independent intermediate form is therefore useful, but does not erase target-specific constraints.

A compiler front end reads a source language and checks or translates its meaning into a representation used by later compilation stages. lexical analysis groups characters into tokens; parsing builds structured syntax; name resolution and type checking establish which declarations and operations the program denotes. These responsibilities need not be implemented as completely separate passes.

For example, x + 2 * y must be grouped as x + (2 * y) under the usual precedence rules. The parser establishes that grouping, while semantic analysis determines whether x and y exist and support the requested operations. An abstract syntax tree can retain source locations for precise diagnostics. The LLVM Kaleidoscope tutorial shows this transition from parsed syntax to LLVM intermediate code.^[ref] The front end's language-facing checks complement the machine-facing decisions of a compiler back end.

LLVM is a collection of reusable compiler infrastructure built around a typed intermediate representation. Its static single-assignment form gives each register-like value one definition, making data dependencies explicit for compiler optimization. The same intermediate representation has an in-memory form, a compact bitcode form and a readable textual form.^[ref]

A compiler front end translates a source programming language into the intermediate representation; optimization passes transform it while preserving the required semantics; a compiler back end produces code for a target computer architecture. Sharing the middle stages lets several source programming languages reuse the same analyses and target support. For a small textual example, %y = add i32 %x, 1 defines a new 32-bit integer value from an earlier one. The value %y is defined once even if the source program repeatedly assigns to a source variable.