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+# How to add a new math function to LLVM-libc
+
+A new math function (let call it `func`) to the LLVM-libc can be added as follow:
+
+## Registration
+
+- Add entry points `libc.src.math.func` to the following files:
+```
+  libc/config/linux/<arch>/entrypoints.txt
+  libc/config/windows/entrypoints.txt
+```
+- Add function specs to the file:
+```
+  libc/spec/stdc.td
+```
+
+## Implementation
+
+- Add `add_math_entrypoint_object(func)` to:
+```
+  libc/src/math/CMakeLists.txt
+```
+- Add function declaration (under `__llvm_libc` namespace) to:
+```
+  libc/src/math/func.h
+```
+- Add function definition to:
+```
+  libc/src/math/generic/func.cpp
+```
+- Add the corresponding `add_entrypoint_object` to:
+```
+  libc/src/math/generic/CMakeLists.txt
+```
+- Add architectural specific implementations to:
+```
+  libc/src/math/<arch>/func.cpp
+```
+
+### Floating point utility
+- Floating point utilities and math functions that are also used internally are located at:
+```
+  libc/src/__support/FPUtils
+```
+- These are preferable to be implemented as header-only.
+- To manipulate bits of floating point numbers, use the template class `__llvm_libc::fputil::FPBits<>` in the header file:
+```
+  libc/src/__support/FPUtils/FPBits.h
+```
+
+## Testing
+
+### MPFR utility
+- Add the function enum to `__llvm_libc::testing::mpfr::Operation` in the header file:
+```
+  libc/utils/MPFRWrapper/MPFRUtils.h
+```
+- Add support for `func` in the `MPFRNumber` class and the corresponding between the enum and its call to the file:
+```
+  libc/utils/MPFRWrapper/MPFRUtils.cpp
+```
+
+### Unit tests
+- Add unit test to:
+```
+  libc/test/src/math/func_test.cpp
+```
+- Add the corresponding entry point to:
+```
+  libc/test/src/math/CMakeLists.txt
+```
+
+### Exhaustive tests
+- Add exhaustive test to:
+```
+  libc/test/src/math/exhaustive/func_test.cpp
+```
+- Add the corresponding entry point to:
+```
+  libc/test/src/math/exhaustive/CMakeLists.txt
+```
+
+### Performance tests
+- Add a performance test to:
+```
+  libc/test/src/math/differential_testing/func_perf.cpp
+```
+- Add the corresponding entry point to:
+```
+  libc/test/src/math/differential_testing/CMakeLists.txt
+```
+
+## Build and Run
+
+- Check out the LLVM source tree:
+```
+  $ git clone https://github.com/llvm/llvm-project.git
+```
+
+- Setup projects with CMake:
+```
+  $ cd llvm-project
+  $ mkdir build
+  $ cd build
+  $ cmake ../llvm -G Ninja \
+  -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;llvm;libc" \
+  -DCMAKE_BUILD_TYPE=Debug \
+  -DCMAKE_C_COMPILER=clang \
+  -DCMAKE_CXX_COMPILER=clang++
+```
+
+- Build the whole `libc`:
+```
+  $ ninja llvmlibc
+```
+
+- Build only `libm`:
+```
+  $ ninja llvmlibm
+```
+
+- Run all unit tests:
+```
+  $ ninja check-libc
+```
+
+- Build and Run a specific unit test:
+```
+  $ ninja libc.test.src.math.func_test
+  $ projects/libc/test/src/math/libc.test.src.math.func_test
+```
+
+- Build and Run exhaustive test (might take hours to run):
+```
+  $ ninja libc.test.src.math.exhaustive.func_test
+```
+
+- Build and Run performance test:
+```
+  $ ninja libc.test.src.math.differential_testing.func_perf
+  $ projects/libc/test/src/math/differential_testing/libc.test.src.math.differential_testing.func_perf
+  $ cat func_perf.log
+```
+
+## Code reviews
+
+We follow the code review process of LLVM with Phabricator:
+```
+  https://llvm.org/docs/Phabricator.html
+```
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