This document goes briefly over compiler phases in Flang. It focuses on the internal implementation and as such, it is intended for Flang developers rather than end-users.

Overview of Compiler Phases

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Each phase produces either correct output or fatal errors.

Prescan and Preprocess


Input: Fortran source and header files, command line macro definitions, set of enabled compiler directives (to be treated as directives rather than comments).


  • A “cooked” character stream: the entire program as a contiguous stream of normalized Fortran source. Extraneous whitespace and comments are removed (except comments that are compiler directives that are not disabled) and case is normalized.
  • Provenance information mapping each character back to the source it came from. This is used in subsequent phases to issue errors messages that refer to source locations.

Entry point: parser::Parsing::Prescan

Command: flang-new -fc1 -E src.f90 dumps the cooked character stream


Input: Cooked character stream.

Output: A parse tree representing a syntactically correct program, rooted at a parser::Program. See: and

Entry point: parser::Parsing::Parse


  • flang-new -fc1 -fdebug-dump-parse-tree src.f90 dumps the parse tree
  • flang-new -fc1 -fdebug-unparse src.f90 converts the parse tree to normalized Fortran

Validate Labels and Canonicalize Do Statements

Input: Parse tree.

Output: The parse tree with label constraints and construct names checked, and each LabelDoStmt converted to a NonLabelDoStmt. See:

Entry points: semantics::ValidateLabels, parser::CanonicalizeDo

Resolve Names

Input: Parse tree (without LabelDoStmt) and .mod files from compilation of USEd modules.


  • Tree of scopes populated with symbols and types
  • Parse tree with some refinements:
    • each parser::Name::symbol field points to one of the symbols
    • each parser::TypeSpec::declTypeSpec field points to one of the types
    • array element references that were parsed as function references or statement functions are corrected

Entry points: semantics::ResolveNames, semantics::RewriteParseTree

Command: flang-new -fc1 -fdebug-dump-symbols src.f90 dumps the tree of scopes and symbols in each scope

Check DO CONCURRENT Constraints

Input: Parse tree with names resolved.

Output: Parse tree with semantically correct DO CONCURRENT loops.

Write Module Files

Input: Parse tree with names resolved.

Output: For each module and submodule, a .mod file containing a minimal Fortran representation suitable for compiling program units that depend on it. See

Analyze Expressions and Assignments

Input: Parse tree with names resolved.

Output: Parse tree with parser::Expr::typedExpr filled in and semantic checks performed on all expressions and assignment statements.

Entry points: semantics::AnalyzeExpressions, semantics::AnalyzeAssignments

Produce the Intermediate Representation

Input: Parse tree with names and labels resolved.

Output: An intermediate representation of the executable program. See