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<!--===- docs/Overview.md
Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
See https://llvm.org/LICENSE.txt for license information.
SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-->
# Intro
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
```eval_rst
.. contents::
:local:
```
Each phase produces either correct output or fatal errors.
## Prescan and Preprocess
See: [Preprocessing.md](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).
**Output:**
- 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
## Parse
**Input:** Cooked character stream.
**Output:** A parse tree representing a syntactically correct program,
rooted at a `parser::Program`.
See: [Parsing.md](Parsing.md) and [ParserCombinators.md](ParserCombinators.md).
**Entry point:** `parser::Parsing::Parse`
**Command:**
- `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: [LabelResolution.md](LabelResolution.md).
**Entry points:** `semantics::ValidateLabels`, `parser::CanonicalizeDo`
## Resolve Names
**Input:** Parse tree (without `LabelDoStmt`) and `.mod` files from compilation
of USEd modules.
**Output:**
- 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 [ModFiles.md](ModFiles.md).
## 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 [FortranIR.md](FortranIR.md).
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