lecture 4:compilerII

Semantic Analysis
What Is Semantic Analysis
Parsing only verifies that the program consists of tokens arranged in a syntactically valid combination. Now we’ll move forward to semantic analysis, where we delve even deeper to check whether they form a sensible set of instructions in the programming language. For a program to be semantically valid, all variables, functions, classes, etc. must be properly defined, expressions and variables must be used in ways that respect the type system, access control must be respected, and so forth.
Semantic analysis is the front end’s penultimate phase and the compiler’s last chance to weed out incorrect programs. We need to ensure the program is sound enough to carry on to code generation.
A large part of semantic analysis consists of tracking variable/function/type declarations and type checking. In many languages, identifiers have to be declared before they’re used. As the compiler encounters a new declaration, it records the type information assigned to that identifier. Then, as it continues examining the rest of the program, it verifies that the type of an identifier is respected in terms of the operations being performed.
For example, the type of the right side expression of an assignment statement should match the type of the left side, and the left side needs to be a properly declared and assignable identifier. The parameters of a function should match the arguments of a function call in both number and type. The language may require that identifiers be unique, thereby forbidding two global declarations from sharing the same name. Arithmetic operands will need to be of numeric—perhaps even the exact same type (no automatic int-to-double conversion, for instance). These are examples of the things checked in the semantic analysis phase.
Some semantic analysis might be done right in the middle of parsing. As a particular construct is recognized, say an addition expression, the parser action could check the two operands and verify they are of numeric type and compatible for this operation.