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Toolchain.rst 12.61 KiB

Assembling a Complete Toolchain

Introduction

Clang is only one component in a complete tool chain for C family programming languages. In order to assemble a complete toolchain, additional tools and runtime libraries are required. Clang is designed to interoperate with existing tools and libraries for its target platforms, and the LLVM project provides alternatives for a number of these components.

This document describes the required and optional components in a complete toolchain, where to find them, and the supported versions and limitations of each option.

Warning

This document currently describes Clang configurations on POSIX-like operating systems with the GCC-compatible clang driver. When targeting Windows with the MSVC-compatible clang-cl driver, some of the details are different.

Tools

A complete compilation of C family programming languages typically involves the following pipeline of tools, some of which are omitted in some compilations:

  • Preprocessor: This performs the actions of the C preprocessor: expanding #includes and #defines. The -E flag instructs Clang to stop after this step.
  • Parsing: This parses and semantically analyzes the source language and builds a source-level intermediate representation ("AST"), producing a :ref:`precompiled header (PCH) <usersmanual-precompiled-headers>`, preamble, or :doc:`precompiled module file (PCM) <Modules>`, depending on the input. The -precompile flag instructs Clang to stop after this step. This is the default when the input is a header file.
  • IR generation: This converts the source-level intermediate representation into an optimizer-specific intermediate representation (IR); for Clang, this is LLVM IR. The -emit-llvm flag instructs Clang to stop after this step. If combined with -S, Clang will produce textual LLVM IR; otherwise, it will produce LLVM IR bitcode.
  • Compiler backend: This converts the intermediate representation into target-specific assembly code. The -S flag instructs Clang to stop after this step.
  • Assembler: This converts target-specific assembly code into target-specific machine code object files. The -c flag instructs Clang to stop after this step.
  • Linker: This combines multiple object files into a single image (either a shared object or an executable).

Clang provides all of these pieces other than the linker. When multiple steps are performed by the same tool, it is common for the steps to be fused together to avoid creating intermediate files.

When given an output of one of the above steps as an input, earlier steps are skipped (for instance, a .s file input will be assembled and linked).

The Clang driver can be invoked with the -### flag (this argument will need to be escaped under most shells) to see which commands it would run for the above steps, without running them. The -v (verbose) flag will print the commands in addition to running them.

Clang frontend

The Clang frontend (clang -cc1) is used to compile C family languages. The command-line interface of the frontend is considered to be an implementation detail, intentionally has no external documentation, and is subject to change without notice.

Language frontends for other languages

Clang can be provided with inputs written in non-C-family languages. In such cases, an external tool will be used to compile the input. The currently-supported languages are: