Register Allocation Basics

Linear Scan and Simple Allocation

Before full graph coloring, linear scan allocation sorts live intervals by start point and assigns registers greedily. LLVM's fast -O0 path and many JIT compilers use simpler schemes. Understanding basics clarifies why spills happen.

Interval-Based Assignment

Compute live interval for each temporary. Sort by start. Maintain active set of intervals currently in registers. When registers are full, spill the interval ending latest.

cCopy
Loading…

• Live interval: first def to last use
• Spill slot in stack frame for spilled temps
• Reload before each use of spilled value
• Simpler than graph coloring; good enough for learning

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 basic register allocation with live intervals. This exercise requires tracking when temporaries are live and assigning a fixed number of physical registers with spill fallback.