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overlay_mode.rst

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Overlay Mode

Table of Contents

One can choose to use LLVM's libc in the overlay mode. In this mode, the link order semantics are exploited to pick symbols from libllvmlibc.a (if they are available in libllvmlibc.a) and the rest are picked from the system libc. The user programs also have to use header files from the system libc. Naturally, only functions which do not depend on implementation specific ABI are included in libllvmlibc.a. Examples of such functions are strlen and round. Functions like fopen and friends are not included as they depend on the implementation specific definition of the FILE data structure.

Building the libc in the overlay mode

There are two different ways in which the libc can be built for use in the overlay mode. In both the ways, we build a static archive named libllvmlibc.a. We use a rather verbose name with a repeated lib to make it clear that it is not the system libc, which is typically named libc.a. Also, if users choose to mix more than one libc with the system libc, then the name libllvmlibc.a makes it absolutely clear that it is the static archive of LLVM's libc.

Building the static archive with libc as a normal LLVM project

We can treat the libc project as any other normal LLVM project and perform the CMake configure step as follows:

$> cd llvm-project  # The llvm-project checkout
$> mkdir build
$> cd build
$> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS="libc"  \
   -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
   -DCMAKE_BUILD_TYPE=<Debug|Release>                    \  # Select build type
   -DCMAKE_INSTALL_PREFIX=<Your prefix of choice>           # Optional

Next, build the libc:

$> ninja libc

The build step will build the static archive the in the directory build/projects/libc/lib. Notice that the above CMake configure step also specified an install prefix. This is optional, but if one uses it, then they can follow up the build step with an install step:

$> ninja install-llvmlibc

Building the static archive as part of the bootstrap build

The bootstrap build is a build mode in which runtime components like libc++, libcxx-abi, libc etc. are built using the ToT clang. The idea is that this build produces an in-sync toolchain of compiler + runtime libraries. Such a synchrony is not essential for the libc but can one still build the overlay static archive as part of the bootstrap build if one wants to. The first step is to configure appropriately:

$> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS="clang" \
   -DLLVM_ENABLE_RUNTIMES="libc"  \  # libc is listed as runtime and not as a project
   -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
   -DCMAKE_BUILD_TYPE=<Debug|Release>                    \  # Select build type
   -DCMAKE_INSTALL_PREFIX=<Your prefix of choice>           # Optional

The build and install steps are similar to the those used when configured as a normal project. Note that the build step takes much longer this time as clang will be built before building libllvmlibc.a.

$> ninja libc
$> ninja install-llvmlibc

Using the overlay static archive

Once built (and optionally installed), the overlay static archive can be linked to your binaries like any other static archive. For example, when building with clang on Linux, one should follow a recipe like:

$> clang <other compiler and/or linker options> <file.o|c(pp)>     \
   -L <path to the directory in which libllvmlibc.a is installed>  \ # Optional
   -lllvmlibc

If you installed libllvmlibc.a in a standard linker lookup path, for example /usr/local/lib on Linux like systems, then specifying the path to the static archive using the -L option is not necessary.

Linking the static archive to other LLVM binaries

Since the libc and other LLVM binaries are developed in the same source tree, linking libllvmlibc.a to those LLVM binaries does not require any special install step or explicitly passing any special linker flags/options. One can simply add llvmlibc as a link library to that binary's target. For example, if you want to link libllvmlibc.a to llvm-objcopy, all you have to do is to add a CMake command as follows:

target_link_libraries(llvm-objcopy PRIVATE llvmlibc)