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77 lines
2.8 KiB
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77 lines
2.8 KiB
ReStructuredText
.. _build_concepts:
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==============
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Build Concepts
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==============
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Most people don't need to build their own C library — the one provided by their
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system works well. However, LLVM-libc's **Overlay Mode** can provide key updates
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like faster or more consistent math functions for projects that need them.
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For those who do need a full C library, LLVM-libc supports several build
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configurations depending on your target environment and intended usage.
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The Five Build Scenarios
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========================
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1. Overlay Mode (Augmenting the System Libc)
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--------------------------------------------
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In Overlay Mode, LLVM-libc functions are compiled alongside the host's existing
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system library (like ``glibc``). Only the functions explicitly implemented in
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LLVM-libc are used; the rest "fall back" to the system library. This is the
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preferred method for most contributors as it is the fastest to build and test.
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To configure for an overlay build, point CMake to the ``runtimes`` directory
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and set ``LLVM_LIBC_FULL_BUILD=OFF`` (which is the default). This will build a
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static archive named ``libllvmlibc.a``:
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.. code-block:: sh
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cmake -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libc" \
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-DLLVM_LIBC_FULL_BUILD=OFF ...
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2. Full Build Mode (Standalone Library)
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---------------------------------------
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In Full Build Mode, LLVM-libc is a complete replacement for the system library.
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This is used to build standalone ``libc.a`` and ``libm.a`` (with separate CMake
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targets) for a new operating system or to generate a sysroot for a specific target.
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To configure for a full build, set ``LLVM_LIBC_FULL_BUILD=ON``:
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.. code-block:: sh
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cmake -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" \
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-DLLVM_LIBC_FULL_BUILD=ON ...
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3. Bootstrap Build
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------------------
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A bootstrap build first builds the compiler (Clang) and other LLVM tools using
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the host compiler, and then uses that newly-built Clang to build the libc.
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This ensures you are using a matched toolchain where the compiler and
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the library are built for each other.
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To configure a bootstrap build, you point CMake to the ``llvm`` directory:
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.. code-block:: sh
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cmake -S llvm -B build -DLLVM_ENABLE_PROJECTS="clang" -DLLVM_ENABLE_RUNTIMES="libc;compiler-rt" ...
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4. Cross-compiler Build (Targeting Other Architectures)
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-------------------------------------------------------
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Used when you want to build LLVM-libc for a different architecture than you are
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currently running on (e.g., building on x86_64 for an aarch64 target).
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This requires a cross-compiler or a toolchain file.
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5. Bootstrap Cross-compiler (New Environment)
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---------------------------------------------
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For users who are starting from scratch (e.g., with only Linux kernel headers)
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and want to generate a full C compiler and sysroot for their target. This is
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the most common path for those building entire environments to tinker in.
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