Overview of Compiler Phases

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The Flang compiler transforms Fortran source code into an executable file. This transformation proceeds in three high level phases -- analysis, lowering, and code generation/linking.

The first high level phase (analysis) transforms Fortran source code into a decorated parse tree and a symbol table. During this phase, all user related errors are detected and reported.

The second high level phase (lowering), changes the decorated parse tree and symbol table into the Fortran Intermediate Representation (FIR), which is a dialect of LLVM‘s Multi-Level Intermediate Representation or MLIR. It then runs a series of passes on the FIR code which verify its validity, perform a series of optimizations, and finally transform it into LLVM’s Intermediate Representation, or LLVM IR

The third high level phase generates machine code and invokes a linker to produce an executable file.

This document describes the first two high level phases. Each of these is described in more detailed phases.

Each detailed phase is described -- its inputs and outputs along with how to produce a readable version of the outputs.

Each detailed phase produces either correct output or fatal errors.


This high level phase validates that the program is correct and creates all of the information needed for lowering.

Prescan and Preprocess

See Preprocessing.md.

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. Also, directives are processed and macros expanded.
  • Provenance information mapping each character back to the source it came from. This is used in subsequent phases that need source locations. This includes error messages, optimization reports, and debugging information.

Entry point: parser::Parsing::Prescan


  • flang-new -fc1 -E src.f90 dumps the cooked character stream
  • flang-new -fc1 -fdebug-dump-provenance src.f90 dumps provenance information


Input: Cooked character stream

Output: A parse tree for each Fortran program unit in the source code representing a syntactically correct program, rooted at the program unit. See: Parsing.md and ParserCombinators.md.

Entry point: parser::Parsing::Parse


  • flang-new -fc1 -fdebug-dump-parse-tree-no-sema src.f90 dumps the parse tree
  • flang-new -fc1 -fdebug-unparse src.f90 converts the parse tree to normalized Fortran
  • flang-new -fc1 -fdebug-dump-parsing-log src.f90 runs an instrumented parse and dumps the log
  • flang-new -fc1 -fdebug-measure-parse-tree src.f90 measures the parse tree

Semantic processing

Input: the parse tree, the cooked character stream, and provenance information


  • a symbol table
  • modified parse tree
  • module files, (see: ModFiles.md)
  • the intrinsic procedure table
  • the target characteristics
  • the runtime derived type derived type tables (see: RuntimeTypeInfo.md)

Entry point: semantics::Semantics::Perform

For more detail on semantic analysis, see: Semantics.md. Semantic processing performs several tasks:

  • validates labels, see: LabelResolution.md.
  • canonicalizes DO statements,
  • canonicalizes OpenACC and OpenMP code
  • resolves names, building a tree of scopes and symbols
  • rewrites the parse tree to correct parsing mistakes (when needed) once semantic information is available to clarify the program's meaning
  • checks the validity of declarations
  • analyzes expressions and statements, emitting error messages where appropriate
  • creates module files if the source code contains modules, see ModFiles.md.

In the course of semantic analysis, the compiler:

  • creates the symbol table
  • decorates the parse tree with semantic information (such as pointers into the symbol table)
  • creates the intrinsic procedure table
  • folds constant expressions

At the end of semantic processing, all validation of the user's program is complete. This is the last detailed phase of analysis processing.


  • flang-new -fc1 -fdebug-dump-parse-tree src.f90 dumps the parse tree after semantic analysis
  • flang-new -fc1 -fdebug-dump-symbols src.f90 dumps the symbol table
  • flang-new -fc1 -fdebug-dump-all src.f90 dumps both the parse tree and the symbol table


Lowering takes the parse tree and symbol table produced by analysis and produces LLVM IR.

Create the lowering bridge


  • the parse tree
  • the symbol table
  • The default KINDs for intrinsic types (specified by default or command line option)
  • The intrinsic procedure table (created in semantics processing)
  • The target characteristics (created during semantics processing)
  • The cooked character stream
  • The target triple -- CPU type, vendor, operating system
  • The mapping between Fortran KIND values to FIR KIND values

The lowering bridge is a container that holds all of the information needed for lowering.

Output: A container with all of the information needed for lowering

Entry point: lower::LoweringBridge::create

Initial lowering

Input: the lowering bridge

Output: A Fortran IR (FIR) representation of the program.

Entry point: lower::LoweringBridge::lower

The compiler then takes the information in the lowering bridge and creates a pre-FIR tree or PFT. The PFT is a list of programs and modules. The programs and modules contain lists of function-like units. The function-like units contain a list of evaluations. All of these contain pointers back into the parse tree. The compiler walks the PFT generating FIR.


  • flang-new -fc1 -fdebug-dump-pft src.f90 dumps the pre-FIR tree
  • flang-new -fc1 -emit-mlir src.f90 dumps the FIR to the files src.mlir

Transformation passes

Input: initial version of the FIR code

Output: An LLVM IR representation of the program

Entry point: mlir::PassManager::run

The compiler then runs a series of passes over the FIR code. The first is a verification pass. It's followed by a series of transformation passes that perform various optimizations and transformations. The final pass creates an LLVM IR representation of the program.


  • flang-new -mmlir --mlir-print-ir-after-all -S src.f90 dumps the FIR code after each pass to standard error
  • flang-new -fc1 -emit-llvm src.f90 dumps the LLVM IR to src.ll

Object code generation and linking

After the LLVM IR is created, the flang driver invokes LLVM's existing infrastructure to generate object code and invoke a linker to create the executable file.