Is the semantic analysis step in Clang an essential part of the compiler?

Question

I'm trying to understand the ins and outs of Clang, and I'm not really sure about the "Sema" library. Is the semantic analysis in the path the compiler takes to compile a program? Or is it only used by the programmer to analyze his/her code?

From what I gather, the parser builds an AST, then there are "AST consumers" that use the AST to do different things. So, the code generation library turns the AST into IR. And the semantic analysis library uses the AST to analyze the code. Is this understanding correct, or is the semantic analyzer also used for compiling?

Answer 1

Semantic analysis is part of compile analysis process, usually coming after lexical and syntax analysis. Semantic analyzer checks validity of used data types, does type casting etc, and reports errors if there are some.

In other words, when it comes to semantic analysis, compiler is already sure that valid words are used in program (lexical), and that sentences are built correctly, according to given grammar of language(syntax). There is only left to check if those sentences make sense - checking data types, return values, size boundaries, uninitialized variables, etc.

My knowledge of compile process is more general rather than specific about Clang, but I think semantic analysis is definitely present in the code analysis.