The front end of the P4 compiler translates P4 source code into the P4 Intermediate Representation (P4C-IR), performs type-checking, and ensures that the P4 program complies with the P4 specification. The front end also applies architecture-independent optimizations such as Strength Reduction, Constant Folding, Inlining, or Dead-Code Elimination. This is a non-exhaustive list of passes:

• Program Parsing: Converts the P4 source code into an Abstract Syntax Tree (AST).
• Validation: Checks the program against P4 language rules and constraints to ensure it is valid.
• Name Resolution: Resolves all identifiers, such as variables and functions, to their declarations.
• Type Checking/Inference: Uses the Hindley-Milner type inference algorithm to ensure that all types are correctly assigned and compatible.
• Making Semantics Explicit: Ensures the program's semantics, such as the order of side effects, are clearly defined and explicit.

Optimization Tasks

The front-end also performs some optimization-related tasks, including:

• Inlining: Replaces function calls with the function body to reduce overhead.
• Compile-Time Evaluation and Specialization: Evaluates expressions and specializes code at compile time for efficiency.
• Conversion to P4 Source: Converts the AST back to P4 source code if needed.
• Deparser Inference (for P4_14 programs): Automatically infers deparser logic for P4_14 programs.

After completing these passes, the front-end generates the control-plane API, which is essential for the interaction between the control plane and the data plane.