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350 lines
12 KiB
ReStructuredText
===============================
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Assembling a Complete Toolchain
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===============================
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.. contents::
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:local:
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:depth: 2
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Introduction
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============
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Clang is only one component in a complete tool chain for C family
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programming languages. In order to assemble a complete toolchain,
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additional tools and runtime libraries are required. Clang is designed
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to interoperate with existing tools and libraries for its target
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platforms, and the LLVM project provides alternatives for a number
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of these components.
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This document describes the required and optional components in a
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complete toolchain, where to find them, and the supported versions
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and limitations of each option.
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.. warning::
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This document currently describes Clang configurations on POSIX-like
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operating systems with the GCC-compatible ``clang`` driver. When
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targeting Windows with the MSVC-compatible ``clang-cl`` driver, some
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of the details are different.
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Tools
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=====
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.. FIXME: Describe DWARF-related tools
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A complete compilation of C family programming languages typically
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involves the following pipeline of tools, some of which are omitted
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in some compilations:
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* **Preprocessor**: This performs the actions of the C preprocessor:
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expanding #includes and #defines.
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The ``-E`` flag instructs Clang to stop after this step.
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* **Parsing**: This parses and semantically analyzes the source language and
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builds a source-level intermediate representation ("AST"), producing a
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:ref:`precompiled header (PCH) <usersmanual-precompiled-headers>`,
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preamble, or
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:doc:`precompiled module file (PCM) <Modules>`,
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depending on the input.
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The ``-precompile`` flag instructs Clang to stop after this step. This is
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the default when the input is a header file.
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* **IR generation**: This converts the source-level intermediate representation
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into an optimizer-specific intermediate representation (IR); for Clang, this
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is LLVM IR.
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The ``-emit-llvm`` flag instructs Clang to stop after this step. If combined
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with ``-S``, Clang will produce textual LLVM IR; otherwise, it will produce
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LLVM IR bitcode.
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* **Compiler backend**: This converts the intermediate representation
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into target-specific assembly code.
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The ``-S`` flag instructs Clang to stop after this step.
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* **Assembler**: This converts target-specific assembly code into
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target-specific machine code object files.
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The ``-c`` flag instructs Clang to stop after this step.
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* **Linker**: This combines multiple object files into a single image
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(either a shared object or an executable).
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Clang provides all of these pieces other than the linker. When multiple
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steps are performed by the same tool, it is common for the steps to be
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fused together to avoid creating intermediate files.
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When given an output of one of the above steps as an input, earlier steps
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are skipped (for instance, a ``.s`` file input will be assembled and linked).
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The Clang driver can be invoked with the ``-###`` flag (this argument will need
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to be escaped under most shells) to see which commands it would run for the
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above steps, without running them. The ``-v`` (verbose) flag will print the
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commands in addition to running them.
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Clang frontend
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--------------
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The Clang frontend (``clang -cc1``) is used to compile C family languages. The
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command-line interface of the frontend is considered to be an implementation
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detail, intentionally has no external documentation, and is subject to change
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without notice.
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Language frontends for other languages
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--------------------------------------
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Clang can be provided with inputs written in non-C-family languages. In such
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cases, an external tool will be used to compile the input. The
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currently-supported languages are:
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* Ada (``-x ada``, ``.ad[bs]``)
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* Fortran (``-x f95``, ``.f``, ``.f9[05]``, ``.for``, ``.fpp``, case-insensitive)
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* Java (``-x java``)
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In each case, GCC will be invoked to compile the input.
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Assembler
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---------
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Clang can either use LLVM's integrated assembler or an external system-specific
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tool (for instance, the GNU Assembler on GNU OSes) to produce machine code from
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assembly.
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By default, Clang uses LLVM's integrated assembler on all targets where it is
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supported. If you wish to use the system assembler instead, use the
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``-fno-integrated-as`` option.
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Linker
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------
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Clang can be configured to use one of several different linkers:
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* GNU ld
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* GNU gold
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* LLVM's `lld <https://lld.llvm.org>`_
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* MSVC's link.exe
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Link-time optimization is natively supported by lld, and supported via
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a `linker plugin <https://llvm.org/docs/GoldPlugin.html>`_ when using gold.
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The default linker varies between targets, and can be overridden via the
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``-fuse-ld=<linker name>`` flag.
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Runtime libraries
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=================
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A number of different runtime libraries are required to provide different
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layers of support for C family programs. Clang will implicitly link an
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appropriate implementation of each runtime library, selected based on
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target defaults or explicitly selected by the ``--rtlib=`` and ``--stdlib=``
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flags.
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The set of implicitly-linked libraries depend on the language mode. As a
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consequence, you should use ``clang++`` when linking C++ programs in order
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to ensure the C++ runtimes are provided.
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.. note::
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There may exist other implementations for these components not described
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below. Please let us know how well those other implementations work with
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Clang so they can be added to this list!
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.. FIXME: Describe Objective-C runtime libraries
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.. FIXME: Describe profiling runtime library
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.. FIXME: Describe cuda/openmp/opencl/... runtime libraries
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Compiler runtime
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----------------
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The compiler runtime library provides definitions of functions implicitly
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invoked by the compiler to support operations not natively supported by
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the underlying hardware (for instance, 128-bit integer multiplications),
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and where inline expansion of the operation is deemed unsuitable.
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The default runtime library is target-specific. For targets where GCC is
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the dominant compiler, Clang currently defaults to using libgcc_s. On most
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other targets, compiler-rt is used by default.
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compiler-rt (LLVM)
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^^^^^^^^^^^^^^^^^^
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`LLVM's compiler runtime library <https://compiler-rt.llvm.org/>`_ provides a
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complete set of runtime library functions containing all functions that
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Clang will implicitly call, in ``libclang_rt.builtins.<arch>.a``.
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You can instruct Clang to use compiler-rt with the ``--rtlib=compiler-rt`` flag.
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This is not supported on every platform.
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If using libc++ and/or libc++abi, you may need to configure them to use
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compiler-rt rather than libgcc_s by passing ``-DLIBCXX_USE_COMPILER_RT=YES``
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and/or ``-DLIBCXXABI_USE_COMPILER_RT=YES`` to ``cmake``. Otherwise, you
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may end up with both runtime libraries linked into your program (this is
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typically harmless, but wasteful).
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libgcc_s (GNU)
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^^^^^^^^^^^^^^
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`GCC's runtime library <https://gcc.gnu.org/onlinedocs/gccint/Libgcc.html>`_
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can be used in place of compiler-rt. However, it lacks several functions
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that LLVM may emit references to, particularly when using Clang's
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``__builtin_*_overflow`` family of intrinsics.
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You can instruct Clang to use libgcc_s with the ``--rtlib=libgcc`` flag.
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This is not supported on every platform.
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Atomics library
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---------------
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If your program makes use of atomic operations and the compiler is not able
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to lower them all directly to machine instructions (because there either is
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no known suitable machine instruction or the operand is not known to be
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suitably aligned), a call to a runtime library ``__atomic_*`` function
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will be generated. A runtime library containing these atomics functions is
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necessary for such programs.
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compiler-rt (LLVM)
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^^^^^^^^^^^^^^^^^^
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compiler-rt contains an implementation of an atomics library.
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libatomic (GNU)
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^^^^^^^^^^^^^^^
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libgcc_s does not provide an implementation of an atomics library. Instead,
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`GCC's libatomic library <https://gcc.gnu.org/wiki/Atomic/GCCMM>`_ can be
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used to supply these when using libgcc_s.
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.. note::
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Clang does not currently automatically link against libatomic when using
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libgcc_s. You may need to manually add ``-latomic`` to support this
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configuration when using non-native atomic operations (if you see link errors
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referring to ``__atomic_*`` functions).
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Unwind library
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--------------
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The unwind library provides a family of ``_Unwind_*`` functions implementing
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the language-neutral stack unwinding portion of the Itanium C++ ABI
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(`Level I <https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#base-abi>`_).
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It is a dependency of the C++ ABI library, and sometimes is a dependency
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of other runtimes.
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libunwind (LLVM)
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^^^^^^^^^^^^^^^^
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LLVM's unwinder library is part of the llvm-project git repository. To
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build it, pass ``-DLLVM_ENABLE_PROJECTS=libunwind`` to the cmake invocation.
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If using libc++abi, you may need to configure it to use libunwind
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rather than libgcc_s by passing ``-DLIBCXXABI_USE_LLVM_UNWINDER=YES``
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to ``cmake``. If libc++abi is configured to use some version of
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libunwind, that library will be implicitly linked into binaries that
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link to libc++abi.
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libgcc_s (GNU)
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^^^^^^^^^^^^^^
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libgcc_s has an integrated unwinder, and does not need an external unwind
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library to be provided.
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libunwind (nongnu.org)
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^^^^^^^^^^^^^^^^^^^^^^
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This is another implementation of the libunwind specification.
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See `libunwind (nongnu.org) <https://www.nongnu.org/libunwind>`_.
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libunwind (PathScale)
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^^^^^^^^^^^^^^^^^^^^^
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This is another implementation of the libunwind specification.
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See `libunwind (pathscale) <https://github.com/pathscale/libunwind>`_.
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Sanitizer runtime
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-----------------
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The instrumentation added by Clang's sanitizers (``-fsanitize=...``) implicitly
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makes calls to a runtime library, in order to maintain side state about the
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execution of the program and to issue diagnostic messages when a problem is
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detected.
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The only supported implementation of these runtimes is provided by LLVM's
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compiler-rt, and the relevant portion of that library
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(``libclang_rt.<sanitizer>.<arch>.a``)
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will be implicitly linked when linking with a ``-fsanitize=...`` flag.
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C standard library
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------------------
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Clang supports a wide variety of
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`C standard library <https://en.cppreference.com/w/c>`_
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implementations.
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C++ ABI library
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---------------
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The C++ ABI library provides an implementation of the library portion of
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the Itanium C++ ABI, covering both the
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`support functionality in the main Itanium C++ ABI document
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<https://itanium-cxx-abi.github.io/cxx-abi/abi.html>`_ and
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`Level II of the exception handling support
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<https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#cxx-abi>`_.
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References to the functions and objects in this library are implicitly
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generated by Clang when compiling C++ code.
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While it is possible to link C++ code using libstdc++ and code using libc++
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together into the same program (so long as you do not attempt to pass C++
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standard library objects across the boundary), it is not generally possible
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to have more than one C++ ABI library in a program.
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The version of the C++ ABI library used by Clang will be the one that the
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chosen C++ standard library was linked against. Several implementations are
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available:
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libc++abi (LLVM)
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^^^^^^^^^^^^^^^^
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`libc++abi <https://libcxxabi.llvm.org/>`_ is LLVM's implementation of this
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specification.
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libsupc++ (GNU)
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^^^^^^^^^^^^^^^
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libsupc++ is GCC's implementation of this specification. However, this
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library is only used when libstdc++ is linked statically. The dynamic
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library version of libstdc++ contains a copy of libsupc++.
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.. note::
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Clang does not currently automatically link against libatomic when statically
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linking libstdc++. You may need to manually add ``-lsupc++`` to support this
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configuration when using ``-static`` or ``-static-libstdc++``.
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libcxxrt (PathScale)
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^^^^^^^^^^^^^^^^^^^^
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This is another implementation of the Itanium C++ ABI specification.
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See `libcxxrt <https://github.com/pathscale/libcxxrt>`_.
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C++ standard library
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--------------------
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Clang supports use of either LLVM's libc++ or GCC's libstdc++ implementation
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of the `C++ standard library <https://en.cppreference.com/w/cpp>`_.
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libc++ (LLVM)
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^^^^^^^^^^^^^
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`libc++ <https://libcxx.llvm.org/>`_ is LLVM's implementation of the C++
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standard library, aimed at being a complete implementation of the C++
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standards from C++11 onwards.
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You can instruct Clang to use libc++ with the ``-stdlib=libc++`` flag.
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libstdc++ (GNU)
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^^^^^^^^^^^^^^^
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`libstdc++ <https://gcc.gnu.org/onlinedocs/libstdc++/>`_ is GCC's
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implementation of the C++ standard library. Clang supports libstdc++
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4.8.3 (released 2014-05-22) and later. Historically Clang implemented
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workarounds for issues discovered in libstdc++, and these are removed
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as fixed libstdc++ becomes sufficiently old.
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You can instruct Clang to use libstdc++ with the ``-stdlib=libstdc++`` flag.
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