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zapcc is a caching C++ compiler based on clang, designed to perform faster compilations
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zapcc is a caching C++ compiler based on clang, designed to perform faster compilations. zapcc uses in-memory compilation cache in client-server architecture, remembering all compilation information between runs. zapcc is the client while zapccs is the server. Each zapcc run will reuse an existing server or if none was available will start a new one.


This open source release is licensed under the LLVM Release License (University of Illinois/NCSA).

Which operating systems and compilers are supported?

zapcc builds on

  • Linux x64 using gcc, clang or zapcc
  • Windows using Visual C++ or mingw-w64, targetting mingw-w64, 32 or 64 bits
  • Targetting Visual C++ binaries or using zapcc-cl is not supported
  • On MacOS using clang

zapcc was thoroughly tested on Linux x64 targetting Linux x64 and minimally on Windows. Rest are experimental, please share your experience.


The prerequisites and build process are identical to building LLVM.

git clone llvm
mkdir build
cd build

How to target mingw-w64 on Windows

You need msys2 and the mingw-builds of mingw-w64. Note there are 32- and 64- bits distribtutions of mingw-w64. To target x86_64:

  • Download the latest MSYS2 installerWebKit and install into the default folder C:\msys64\

  • Download one of the mingw-builds personal distributions, such as x86_64-8.1.0-release-posix-seh-rt_v6-rev0.7z and open into the folder C:\mingw64\

  • Add the bin directories to the PATH, C:\msys64\usr\bin and C:\mingw64\bin

  • Make sure you have just this gcc version available on the PATH. gcc versions outside the PATH are OK.

  • Either Visual C++ or the just-installed mingw-w64 may be used to build zapcc.

  • If building using Visual C++, target x86_64-pc-windows-gnu must be explicitly specified, cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_WARNINGS=OFF -DLLVM_DEFAULT_TARGET_TRIPLE=x86_64-pc-windows-gnu ../llvm

  • zapcc will now target mingw-w64 and ignore Visual C++, even if installed.

To target mingw-builds 32 bits, download the apporpriate 32 bits distribution of mingw-builds and replace x86_64 with i686 in the configuration.

Running the tests

ninja check-all


zapcc command syntax is identical to clang with the command being zapcc.

zapcc ...

Killing the zapccs server

pkill zapcc

To kill the zapccs server to free memory or replace with newly-built zapcc


What is the typical acceleration of Zapcc?

Full builds are 2x-5x faster, see

Typically re-compilation of one modified source file is 10x-50x faster, see Boost.Math.

Acceleration depends on the complexity of the header files vs. the complexity of the source files. It can range from no acceleration at all for plain C projects where caching is disabled to x2-x5 for build-all of heavily templated projects, up to cases of x50 speedups in developer-mode incremental change of one source file.

As a reference number, Zapcc builds LLVM build-all about x2 faster compared to building LLVM using clang.

Is Zapcc Clang compatible?

Yes, zapcc is based on heavily-modified clang code.

Is Zapcc GCC compatible?

Yes, to the extent clang is gcc compatible.

My project does not compile with Zapcc!

Please make sure first your project compiles successfully with Clang. If your project does not compile with Clang, Zapcc, being based on Clang, will not be able to compile any more than clang.

Are the sanitizers supported?


How zapcc is different from precompiled headers?

Precompiled headers requires building your project to the exact precompiled headers rules. Most projects do not bother with using precompiled headers. Even then, precompiled headers do not cache as much as zapcc. Zapcc works within your existing build. See discussion at cfe-dev.

Precompiled headers are currently ignored by Zapcc.

How zapcc is different from C++ modules?

Modules are not yet standard, rarely used and do not support well legacy code and macros found in most existing C++ code, such as Boost. Modules require significant code refactoring in bottom-up approach everything or are slow. Even then, modules do not cache template instantiations and generated code that are specific to your code like zapcc does.

How much memory does Zapcc use?

To avoid killing the server by using endless memory, Zapcc server has a memory limit and will automatically reset after reaching it, restarting with an empty cache and low memory usage. The memory limit is set under [MaxMemory] at bin/zapccs.config, and you can change it to optimize memory usage and the number of servers you plan to use. Usually, you should not set the -j parameter to more than the number of physical cores + 2. This is especially important for Intel CPU with hyper-threading enabled, which report twice the number of physical cores. In such cases, Zapcc may run faster with fewer servers, each using a higher memory limit.

Does it use ccache, distcc, warp or the like?


Where is the zapcc code?

There are patches all around LLVM & clang. Additional zapcc-only code in


When was the source last merged with LLVM trunk?

This open-source release was last merged with LLVM 325000 on 2018-02-13.

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