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This patch removes `f18`, a.k.a. the old driver. It is being replaced with the new driver, `flang-new`, which has reached feature parity with `f18` a while ago. This was discussed in [1] and also in [2]. With this change, `FLANG_BUILD_NEW_DRIVER` is no longer needed and is also deleted. This means that we are making the dependency on Clang permanent (i.e. it cannot be disabled with a CMake flag). LIT set-up is updated accordingly. All references to `f18` or `f18.cpp` are either updated or removed. The `F18_FC` variable from the `flang` bash script is replaced with `FLANG_FC`. The former is still supported for backwards compatibility. [1] https://lists.llvm.org/pipermail/flang-dev/2021-June/000742.html [2] https://reviews.llvm.org/D103177 Differential Revision: https://reviews.llvm.org/D105811
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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
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Each phase produces either correct output or fatal errors.
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).
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 and ParserCombinators.md.
Entry point: parser::Parsing::Parse
Command:
flang-new -fc1 -fdebug-dump-parse-tree src.f90dumps the parse treeflang-new -fc1 -fdebug-unparse src.f90converts 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.
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::symbolfield points to one of the symbols - each
parser::TypeSpec::declTypeSpecfield points to one of the types - array element references that were parsed as function references or statement functions are corrected
- each
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.
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.