- [Contributing to PyTorch](#contributing-to-pytorch)
- [Developing PyTorch](#developing-pytorch)
- [Codebase structure](#codebase-structure)
- [Unit testing](#unit-testing)
  * [Better local unit tests with pytest](#better-local-unit-tests-with-pytest)
- [Writing documentation](#writing-documentation)
  * [Building documentation](#building-documentation)
    + [Tips](#tips)
    + [Building C++ Documentation](#building-c---documentation)
  * [Previewing changes](#previewing-changes)
    + [Submitting changes for review](#submitting-changes-for-review)
  * [Adding documentation tests](#adding-documentation-tests)
- [Profiling with `py-spy`](#profiling-with-py-spy)
- [Managing multiple build trees](#managing-multiple-build-trees)
- [C++ development tips](#c---development-tips)
  * [Build only what you need](#build-only-what-you-need)
  * [Code completion and IDE support](#code-completion-and-ide-support)
  * [Make no-op build fast](#make-no-op-build-fast)
    + [Use Ninja](#use-ninja)
    + [Use CCache](#use-ccache)
    + [Use a faster linker](#use-a-faster-linker)
  * [C++ frontend development tips](#c---frontend-development-tips)
- [CUDA development tips](#cuda-development-tips)
- [Windows development tips](#windows-development-tips)
  * [Known MSVC (and MSVC with NVCC) bugs](#known-msvc--and-msvc-with-nvcc--bugs)
  * [Running clang-tidy](#running-clang-tidy)
  * [Pre-commit tidy/linting hook](#pre-commit-tidy-linting-hook)
  * [Building PyTorch with ASAN](#building-pytorch-with-asan)
    + [Getting `ccache` to work](#getting--ccache--to-work)
    + [Why this stuff with `LD_PRELOAD` and `LIBASAN_RT`?](#why-this-stuff-with--ld-preload--and--libasan-rt--)
    + [Why LD_PRELOAD in the build function?](#why-ld-preload-in-the-build-function-)
    + [Why no leak detection?](#why-no-leak-detection-)
- [Caffe2 notes](#caffe2-notes)

## Contributing to PyTorch

If you are interested in contributing to PyTorch, your contributions will fall
into two categories:

1. You want to propose a new feature and implement it.
    - Post about your intended feature, and we shall discuss the design and
    implementation. Once we agree that the plan looks good, go ahead and implement it.
2. You want to implement a feature or bug-fix for an outstanding issue.
    - Search for your issue here: https://github.com/pytorch/pytorch/issues
    - Pick an issue and comment on the task that you want to work on this feature.
    - If you need more context on a particular issue, please ask and we shall provide.

Once you finish implementing a feature or bug-fix, please send a Pull Request to
https://github.com/pytorch/pytorch

This document covers some of the more technical aspects of contributing
to PyTorch.  For more non-technical guidance about how to contribute to
PyTorch, see the [Contributing Guide](docs/source/community/contribution_guide.rst).

## Developing PyTorch

To develop PyTorch on your machine, here are some tips:

1. Uninstall all existing PyTorch installs:
```bash
conda uninstall pytorch
pip uninstall torch
pip uninstall torch # run this command twice
```

2. Clone a copy of PyTorch from source:

```bash
git clone https://github.com/pytorch/pytorch
cd pytorch
```

2.1. If you already have PyTorch from source, update it:

```bash
git pull --rebase
git submodule sync --recursive
git submodule update --init --recursive
```

If you want to have no-op incremental rebuilds (which are fast), see the section below titled "Make no-op build fast."


3. Install PyTorch in `develop` mode:

A full set of instructions on installing PyTorch from source is here:
https://github.com/pytorch/pytorch#from-source

The change you have to make is to replace

```bash
python setup.py install
```

with

```bash
python setup.py develop
```

This mode will symlink the Python files from the current local source
tree into the Python install.  Hence, if you modify a Python file, you
do not need to reinstall PyTorch again and again.  This is especially
useful if you are only changing Python files.

For example:
- Install local PyTorch in `develop` mode
- modify your Python file `torch/__init__.py` (for example)
- test functionality
- modify your Python file `torch/__init__.py`
- test functionality
- modify your Python file `torch/__init__.py`
- test functionality

You do not need to repeatedly install after modifying Python files.

In case you want to reinstall, make sure that you uninstall PyTorch first by running `pip uninstall torch`
and `python setup.py clean`. Then you can install in `develop` mode again.

## Codebase structure

* [c10](c10) - Core library files that work everywhere, both server
  and mobile. We are slowly moving pieces from [ATen/core](aten/src/ATen/core)
  here. This library is intended only to contain essential functionality,
  and appropriate to use in settings where binary size matters. (But
  you'll have a lot of missing functionality if you try to use it
  directly.)
* [aten](aten) - C++ tensor library for PyTorch (no autograd support)
  * [src](aten/src)
    * [TH](aten/src/TH)
      [THC](aten/src/THC)
      [THNN](aten/src/THNN)
      [THCUNN](aten/src/THCUNN) - Legacy library code from the original
      Torch. Try not to add things here; we're slowly porting these to
      [native](aten/src/ATen/native).
      * generic - Contains actual implementations of operators,
        parametrized over `scalar_t`. Files here get compiled N times
        per supported scalar type in PyTorch.
    * [ATen](aten/src/ATen)
      * [core](aten/src/ATen/core) - Core functionality of ATen. This
        is migrating to top-level c10 folder.
      * [native](aten/src/ATen/native) - Modern implementations of
        operators. If you want to write a new operator, here is where
        it should go. Most CPU operators go in the top level directory,
        except for operators which need to be compiled specially; see
        cpu below.
        * [cpu](aten/src/ATen/native/cpu) - Not actually CPU
          implementations of operators, but specifically implementations
          which are compiled with processor-specific instructions, like
          AVX. See the [README](aten/src/ATen/native/cpu/README.md) for more
          details.
        * [cuda](aten/src/ATen/native/cuda) - CUDA implementations of
          operators.
        * [sparse](aten/src/ATen/native/sparse) - CPU and CUDA
          implementations of COO sparse tensor operations
        * [mkl](aten/src/ATen/native/mkl) [mkldnn](aten/src/ATen/native/mkldnn)
          [miopen](aten/src/ATen/native/miopen) [cudnn](aten/src/ATen/native/cudnn)
          - implementations of operators which simply bind to some
            backend library.
* [torch](torch) - The actual PyTorch library. Everything that is not
  in [csrc](torch/csrc) is a Python module, following the PyTorch Python
  frontend module structure.
  * [csrc](torch/csrc) - C++ files composing the PyTorch library. Files
    in this directory tree are a mix of Python binding code, and C++
    heavy lifting. Consult `setup.py` for the canonical list of Python
    binding files; conventionally, they are often prefixed with
    `python_`.
    * [jit](torch/csrc/jit) - Compiler and frontend for TorchScript JIT
      frontend.
    * [autograd](torch/csrc/autograd) - Implementation of reverse-mode automatic
      differentiation.
    * [api](torch/csrc/api) - The PyTorch C++ frontend.
    * [distributed](torch/csrc/distributed) - Distributed training
      support for PyTorch.
* [tools](tools) - Code generation scripts for the PyTorch library.
  See [README](tools/README.md) of this directory for more details.
* [test](tests) - Python unit tests for PyTorch Python frontend.
  * [test_torch.py](test/test_torch.py) - Basic tests for PyTorch
    functionality.
  * [test_autograd.py](test/test_autograd.py) - Tests for non-NN
    automatic differentiation support.
  * [test_nn.py](test/test_nn.py) - Tests for NN operators and
    their automatic differentiation.
  * [test_jit.py](test/test_jit.py) - Tests for the JIT compiler
    and TorchScript.
  * ...
  * [cpp](test/cpp) - C++ unit tests for PyTorch C++ frontend.
  * [expect](test/expect) - Automatically generated "expect" files
    which are used to compare against expected output.
  * [onnx](test/onnx) - Tests for ONNX export functionality,
    using both PyTorch and Caffe2.
* [caffe2](caffe2) - The Caffe2 library.
  * [core](caffe2/core) - Core files of Caffe2, e.g., tensor, workspace,
    blobs, etc.
  * [operators](caffe2/operators) - Operators of Caffe2.
  * [python](caffe2/python) - Python bindings to Caffe2.
  * ...

## Unit testing

PyTorch's testing is located under `test/`. Run the entire test suite with

```bash
python test/run_test.py
```

or run individual test files, like `python test/test_nn.py`, for individual test suites.

### Better local unit tests with pytest
We don't officially support `pytest`, but it works well with our `unittest` tests and offers
a number of useful features for local developing. Install it via `pip install pytest`.

If you want to just run tests that contain a specific substring, you can use the `-k` flag:

```bash
pytest test/test_nn.py -k Loss -v
```

The above is an example of testing a change to Loss functions: this command runs tests such as
`TestNN.test_BCELoss` and `TestNN.test_MSELoss` and can be useful to save keystrokes.

## Writing documentation

PyTorch uses [Google style](http://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html)
for formatting docstrings. Length of line inside docstrings block must be limited to 80 characters to
fit into Jupyter documentation popups.

### Building documentation

To build the documentation:

1. Build and install PyTorch

2. Install the prequesities

```bash
cd docs
pip install -r requirements.txt
# `katex` must also be available in your PATH.
# You can either install katex globally if you have properly configured npm:
# npm install -g katex
# Or if you prefer an uncontaminated global executable environment or do not want to go through the node configuration:
# npm install katex && export PATH="$PATH:$(pwd)/node_modules/.bin"
```

3. Generate the documentation HTML files. The generated files will be in `docs/build/html`.

```bash
cd docs
make html
```

#### Tips

The `.rst` source files live in [docs/source](docs/source). Some of the `.rst`
files pull in docstrings from PyTorch Python code (for example, via
the `autofunction` or `autoclass` directives). To vastly shorten doc build times,
it is helpful to remove the files you are not working on, only keeping the base
`index.rst` file and the files you are editing. The Sphinx build will produce
missing file warnings but will still complete. For example, to work on `jit.rst`:

```bash
cd docs/source
ls | grep rst | grep -v index | grep -v jit | xargs rm

# Make your changes, build the docs, etc.

# Don't commit the deletions!
git add index.rst jit.rst
...
```

#### Building C++ Documentation
For C++ documentation (https://pytorch.org/cppdocs), we use
[Doxygen](http://www.doxygen.nl/) and then convert it to
[Sphinx](http://www.sphinx-doc.org/) via
[Breathe](https://github.com/michaeljones/breathe) and
[Exhale](https://github.com/svenevs/exhale). Check the [Doxygen
reference](http://www.stack.nl/~dimitri/doxygen/manual/index.html) for more
information on the documentation syntax.

We run Doxygen in CI (Travis) to verify that you do not use invalid Doxygen
commands. To run this check locally, run `./check-doxygen.sh` from inside
`docs/cpp`.

To build the documentation, follow the same steps as above, but run them from
`docs/cpp` instead of `docs`.

### Previewing changes

To view HTML files locally, you can open the files in your web browser. For example,
navigate to `file:///your_pytorch_folder/docs/build/html/index.html` in a web
browser.

If you are developing on a remote machine, you can set up an SSH tunnel so that
you can access the HTTP server on the remote machine from your local machine. To map
remote port 8000 to local port 8000, use either of the following commands.

```bash
# For SSH
ssh my_machine -L 8000:my_machine:8000

# For Eternal Terminal
et my_machine -t="8000:8000"
```

Then navigate to `localhost:8000` in your web browser.

#### Submitting changes for review

It is helpful when submitting a PR that changes the docs to provide a rendered
version of the result. If your change is small, you can add a screenshot of the
changed docs to your PR.

If your change to the docs is large and affects multiple pages, you can host
the docs yourself with the following steps, then add a link to the output in your
PR. These instructions use GitHub pages to host the docs
you have built. To do so, follow [these steps](https://guides.github.com/features/pages/)
to make a repo to host your changed documentation.

GitHub pages expects to be hosting a Jekyll generated website which does not work
well with the static resource paths used in the PyTorch documentation. To get around
this, you must add an empty file called `.nojekyll` to your repo.

```bash
cd your_github_pages_repo
touch .nojekyll
git add .
git commit
git push
```

Then, copy built documentation and push the changes:

```bash
cd your_github_pages_repo
cp -r ~/my_pytorch_path/docs/build/html/* .
git add .
git commit
git push
```

Then you should be able to see the changes at your_github_username.github.com/your_github_pages_repo.


### Adding documentation tests

It is easy for code snippets in docstrings and `.rst` files to get out of date. The docs
build includes the [Sphinx Doctest Extension](https://www.sphinx-doc.org/en/master/usage/extensions/doctest.html),
which can run code in documentation as a unit test. To use the extension, use
the `.. testcode::` directive in your `.rst` and docstrings.

To manually run these tests, follow steps 1 and 2 above, then run:

```bash
cd docs
make doctest
```

## Profiling with `py-spy`

Evaluating the performance impact of code changes in PyTorch can be complicated,
particularly if code changes happen in compiled code. One simple way to profile
both Python and C++ code in PyTorch is to use
[`py-spy`](https://github.com/benfred/py-spy), a sampling profiler for Python
that has the ability to profile native code and Python code in the same session.

`py-spy` can be installed via `pip`:

```bash
$ pip install py-spy
```

To use `py-spy`, first write a Python test script that exercises the
functionality you would like to profile. For example, this script profiles
`torch.add`:

```python
import torch

t1 = torch.tensor([[1, 1], [1, 1.]])
t2 = torch.tensor([[0, 0], [0, 0.]])

for _ in range(1000000):
    torch.add(t1, t2)
```

Since the `torch.add` operation happens in microseconds, we repeat it a large
number of times to get good statistics. The most straightforward way to use
`py-spy` with such a script is to generate a [flame
graph](http://www.brendangregg.com/flamegraphs.html):

```bash
$ py-spy record -o profile.svg --native -- python test_tensor_tensor_add.py
```

This will output a file named `profile.svg` containing a flame graph you can
view in a web browser or SVG viewer. Individual stack frame entries in the graph
can be selected interactively with your mouse to zoom in on a particular part of
the program execution timeline. The `--native` command-line option tells
`py-spy` to record stack frame entries for PyTorch C++ code. To get line numbers
for C++ code it may be necessary to compile PyTorch in debug mode by prepending
your `setup.py develop` call to compile PyTorch with `DEBUG=1`. Depending on
your operating system it may also be necessary to run `py-spy` with root
privileges.

`py-spy` can also work in an `htop`-like "live profiling" mode and can be
tweaked to adjust the stack sampling rate, see the `py-spy` readme for more
details.

## Managing multiple build trees

One downside to using `python setup.py develop` is that your development
version of PyTorch will be installed globally on your account (e.g., if
you run `import torch` anywhere else, the development version will be
used.

If you want to manage multiple builds of PyTorch, you can make use of
[conda environments](https://conda.io/docs/using/envs.html) to maintain
separate Python package environments, each of which can be tied to a
specific build of PyTorch. To set one up:

```bash
conda create -n pytorch-myfeature
source activate pytorch-myfeature
# if you run python now, torch will NOT be installed
python setup.py develop
```

## C++ development tips

If you are working on the C++ code, there are a few important things that you
will want to keep in mind:

1. How to rebuild only the code you are working on.
2. How to make rebuilds in the absence of changes go faster.

### Build only what you need

`python setup.py build` will build everything by default, but sometimes you are
only interested in a specific component.

- Working on a test binary? Run `(cd build && ninja bin/test_binary_name)` to
  rebuild only that test binary (without rerunning cmake). (Replace `ninja` with
  `make` if you don't have ninja installed).
- Don't need Caffe2?  Pass `BUILD_CAFFE2_OPS=0` to disable build of
  Caffe2 operators.

On the initial build, you can also speed things up with the environment
variables `DEBUG`, `USE_DISTRIBUTED`, `USE_MKLDNN`, `USE_CUDA`, `BUILD_TEST`, `USE_FBGEMM`, `USE_NNPACK` and `USE_QNNPACK`.

- `DEBUG=1` will enable debug builds (-g -O0)
- `REL_WITH_DEB_INFO=1` will enable debug symbols with optimizations (-g -O3)
- `USE_DISTRIBUTED=0` will disable distributed (c10d, gloo, mpi, etc.) build.
- `USE_MKLDNN=0` will disable using MKL-DNN.
- `USE_CUDA=0` will disable compiling CUDA (in case you are developing on something not CUDA related), to save compile time.
- `BUILD_TEST=0` will disable building C++ test binaries.
- `USE_FBGEMM=0` will disable using FBGEMM (quantized 8-bit server operators).
- `USE_NNPACK=0` will disable compiling with NNPACK.
- `USE_QNNPACK=0` will disable QNNPACK build (quantized 8-bit operators).

For example:
```bash
DEBUG=1 USE_DISTRIBUTED=0 USE_MKLDNN=0 USE_CUDA=0 BUILD_TEST=0 USE_FBGEMM=0 USE_NNPACK=0 USE_QNNPACK=0 python setup.py develop
```

For subsequent builds (i.e., when `build/CMakeCache.txt` exists), the build
options passed for the first time will persist; please run `ccmake build/`, run
`cmake-gui build/`, or directly edit `build/CMakeCache.txt` to adapt build
options.

### Code completion and IDE support

When using `python setup.py develop`, PyTorch will generate
a `compile_commands.json` file that can be used by many editors
to provide command completion and error highlighting for PyTorch's
C++ code. You need to `pip install ninja` to generate accurate
information for the code in `torch/csrc`. More information at:
- https://sarcasm.github.io/notes/dev/compilation-database.html

### Make no-op build fast

#### Use Ninja

By default, cmake will use its Makefile generator to generate your build
system.  You can get faster builds if you install the ninja build system
with `pip install ninja`.  If PyTorch was already built, you will need
to run `python setup.py clean` once after installing ninja for builds to
succeed.

#### Use CCache

Even when dependencies are tracked with file modification,
there are many situations where files get rebuilt when a previous
compilation was exactly the same.

Using ccache in a situation like this is a real time-saver. The ccache manual
describes [two ways to use ccache](https://ccache.samba.org/manual/latest.html#_run_modes).
In the PyTorch project, currently only the latter method of masquerading as
the compiler via symlinks works for CUDA compilation.

Here are the instructions for installing ccache from source (tested at commit
`7abac8f` of the `ccache` repo):

```bash
# install and export ccache
if ! ls ~/ccache/bin/ccache
then
    sudo apt-get update
    sudo apt-get install -y automake autoconf
    sudo apt-get install -y asciidoc
    mkdir -p ~/ccache
    pushd /tmp
    rm -rf ccache
    git clone https://github.com/ccache/ccache.git
    pushd ccache
    ./autogen.sh
    ./configure
    make install prefix=~/ccache
    popd
    popd

    mkdir -p ~/ccache/lib
    mkdir -p ~/ccache/cuda
    ln -s ~/ccache/bin/ccache ~/ccache/lib/cc
    ln -s ~/ccache/bin/ccache ~/ccache/lib/c++
    ln -s ~/ccache/bin/ccache ~/ccache/lib/gcc
    ln -s ~/ccache/bin/ccache ~/ccache/lib/g++
    ln -s ~/ccache/bin/ccache ~/ccache/cuda/nvcc

    ~/ccache/bin/ccache -M 25Gi
fi

export PATH=~/ccache/lib:$PATH
export CUDA_NVCC_EXECUTABLE=~/ccache/cuda/nvcc
```

Alternatively, `ccache` provided by newer Linux distributions (e.g. Debian/sid)
also works, but the `nvcc` symlink to `ccache` as described above is still required.

Note that the original `nvcc` binary (typically at `/usr/local/cuda/bin`) must
be on your `PATH`, otherwise `ccache` will emit the following error:

    ccache: error: Could not find compiler "nvcc" in PATH

For example, here is how to install/configure `ccache` on Ubuntu:

```bash
# install ccache
sudo apt install ccache

# update symlinks and create/re-create nvcc link
sudo /usr/sbin/update-ccache-symlinks
sudo ln -s /usr/bin/ccache /usr/lib/ccache/nvcc

# config: cache dir is ~/.ccache, conf file ~/.ccache/ccache.conf
# max size of cache
ccache -M 25Gi  # -M 0 for unlimited
# unlimited number of files
ccache -F 0

# deploy (and add to ~/.bashrc for later)
export PATH="/usr/lib/ccache:$PATH"
```

It is also possible to install `ccache` via `conda` by installing it from the
community-maintained `conda-forge` channel. Here is how to set up `ccache` this
way:

```bash
# install ccache
conda install -c conda-forge ccache

# set up ccache compiler symlinks
mkdir ~/ccache
mkdir ~/ccache/lib
mkdir ~/ccache/cuda
ln -s $CONDA_PREFIX/bin/ccache ~/ccache/lib/cc
ln -s $CONDA_PREFIX/bin/ccache ~/ccache/lib/c++
ln -s $CONDA_PREFIX/bin/ccache ~/ccache/lib/gcc
ln -s $CONDA_PREFIX/bin/ccache ~/ccache/lib/g++
ln -s $CONDA_PREFIX/bin/ccache ~/ccache/cuda/nvcc

# update PATH to reflect symlink locations, consider
# adding this to your .bashrc
export PATH=~/ccache/lib:$PATH
export CUDA_NVCC_EXECUTABLE=~/ccache/cuda/nvcc

# increase ccache cache size to 25 GiB
ccache -M 25Gi
```

To check this is working, do two clean builds of pytorch in a row. The second
build should be substantially and noticeably faster than the first build.


#### Use a faster linker
If you are editing a single file and rebuilding in a tight loop, the time spent
linking will dominate. The system linker available in most Linux distributions
(GNU `ld`) is quite slow. Use a faster linker, like [lld](https://lld.llvm.org/).

The easiest way to use `lld` this is download the
[latest LLVM binaries](http://releases.llvm.org/download.html#8.0.0) and run:
```
ln -s /path/to/downloaded/ld.lld /usr/local/bin/ld
```

### C++ frontend development tips

We have very extensive tests in the [test/cpp/api](test/cpp/api) folder. The
tests are a great way to see how certain components are intended to be used.
When compiling PyTorch from source, the test runner binary will be written to
`build/bin/test_api`. The tests use the [GoogleTest](https://github.com/google/googletest/blob/master/googletest)
framework, which you can read up about to learn how to configure the test runner. When
submitting a new feature, we care very much that you write appropriate tests.
Please follow the lead of the other tests to see how to write a new test case.

## CUDA development tips

If you are working on the CUDA code, here are some useful CUDA debugging tips:

1. `CUDA_DEVICE_DEBUG=1` will enable CUDA device function debug symbols (`-g -G`).
    This will be particularly helpful in debugging device code. However, it will
    slow down the build process for about 50% (compared to only `DEBUG=1`), so use wisely.
2. `cuda-gdb` and `cuda-memcheck` are your best CUDA debugging friends. Unlike`gdb`,
   `cuda-gdb` can display actual values in a CUDA tensor (rather than all zeros).
3. CUDA supports a lot of C++11/14 features such as, `std::numeric_limits`, `std::nextafter`,
   `std::tuple` etc. in device code. Many of such features are possible because of the
   [--expt-relaxed-constexpr](https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#constexpr-functions)
   nvcc flag. There is a known [issue](https://github.com/ROCm-Developer-Tools/HIP/issues/374)
   that ROCm errors out on device code, which uses such stl functions.
4. A good performance metric for a CUDA kernel is the
   [Effective Memory Bandwidth](https://devblogs.nvidia.com/how-implement-performance-metrics-cuda-cc/).
   It is useful for you to measure this metric whenever you are writing/optimizing a CUDA
   kernel. Following script shows how we can measure the effective bandwidth of CUDA `uniform_`
   kernel.
   ```python
   import torch
   import time
   size = 128*512
   nrep = 100
   nbytes_read_write = 4 # this is number of bytes read + written by a kernel. Change this to fit your kernel.

   for i in range(10):
       a=torch.Tensor(size).cuda().uniform_()
       torch.cuda.synchronize()
       start = time.time()
       # dry run to alloc
       out = a.uniform_()
       torch.cuda.synchronize()
       start = time.time()
       for i in range(nrep):
         out = a.uniform_()
       torch.cuda.synchronize()
       end = time.time()
       timec = (end-start)/nrep
       print("uniform, size, elements", size, "forward", timec, "bandwidth (GB/s)", size*(nbytes_read_write)*1e-9/timec)
       size *=2
   ```


Hope this helps, and thanks for considering to contribute.

## Windows development tips

For building from source on Windows, consult
[our documentation](https://pytorch.org/docs/stable/notes/windows.html) on it.

Occasionally, you will write a patch which works on Linux, but fails CI on Windows.
There are a few aspects in which MSVC (the Windows compiler toolchain we use) is stricter
than Linux, which are worth keeping in mind when fixing these problems.

1. Symbols are NOT exported by default on Windows; instead, you have to explicitly
   mark a symbol as exported/imported in a header file with `__declspec(dllexport)` /
   `__declspec(dllimport)`. We have codified this pattern into a set of macros
   which follow the convention `*_API`, e.g., `CAFFE2_API` inside Caffe2 and ATen.
   (Every separate shared library needs a unique macro name, because symbol visibility
   is on a per shared library basis. See c10/macros/Macros.h for more details.)

   The upshot is if you see an "unresolved external" error in your Windows build, this
   is probably because you forgot to mark a function with `*_API`. However, there is
   one important counterexample to this principle: if you want a *templated* function
   to be instantiated at the call site, do NOT mark it with `*_API` (if you do mark it,
   you'll have to explicitly instantiate all of the specializations used by the call
   sites.)

2. If you link against a library, this does not make its dependencies transitively
   visible. You must explicitly specify a link dependency against every library whose
   symbols you use. (This is different from Linux where in most environments,
   transitive dependencies can be used to fulfill unresolved symbols.)

3. If you have a Windows box (we have a few on EC2 which you can request access to) and
   you want to run the build, the easiest way is to just run `.jenkins/pytorch/win-build.sh`.
   If you need to rebuild, run `REBUILD=1 .jenkins/pytorch/win-build.sh` (this will avoid
   blowing away your Conda environment.)

Even if you don't know anything about MSVC, you can use cmake to build simple programs on
Windows; this can be helpful if you want to learn more about some peculiar linking behavior
by reproducing it on a small example. Here's a simple example cmake file that defines
two dynamic libraries, one linking with the other:

```CMake
project(myproject CXX)
set(CMAKE_CXX_STANDARD 14)
add_library(foo SHARED foo.cpp)
add_library(bar SHARED bar.cpp)
# NB: don't forget to __declspec(dllexport) at least one symbol from foo,
# otherwise foo.lib will not be created.
target_link_libraries(bar PUBLIC foo)
```

You can build it with:

```bash
mkdir build
cd build
cmake ..
cmake --build .
```

### Known MSVC (and MSVC with NVCC) bugs

The PyTorch codebase sometimes likes to use exciting C++ features, and
these exciting features lead to exciting bugs in Windows compilers.
To add insult to injury, the error messages will often not tell you
which line of code actually induced the erroring template instantiation.

We've found the most effective way to debug these problems is to
carefully read over diffs, keeping in mind known bugs in MSVC/NVCC.
Here are a few well known pitfalls and workarounds:

* This is not actually a bug per se, but in general, code generated by MSVC
  is more sensitive to memory errors; you may have written some code
  that does a use-after-free or stack overflows; on Linux the code
  might work, but on Windows your program will crash. ASAN may not
  catch all of these problems: stay vigilant to the possibility that
  your crash is due to a real memory problem.

* (NVCC) `c10::optional` does not work when used from device code. Don't use
  it from kernels. Upstream issue: https://github.com/akrzemi1/Optional/issues/58
  and our local issue #10329.

* `constexpr` generally works less well on MSVC.

  * The idiom `static_assert(f() == f())` to test if `f` is constexpr
    does not work; you'll get "error C2131: expression did not evaluate
    to a constant". Don't use these asserts on Windows.
    (Example: `c10/util/intrusive_ptr.h`)

* (NVCC) Code you access inside a `static_assert` will eagerly be
  evaluated as if it were device code, and so you might get an error
  that the code is "not accessible".

```cpp
class A {
  static A singleton_;
  static constexpr inline A* singleton() {
    return &singleton_;
  }
};
static_assert(std::is_same(A*, decltype(A::singleton()))::value, "hmm");
```

* The compiler will run out of heap space if you attempt to compile files that
  are too large. Splitting such files into separate files helps.
  (Example: `THTensorMath`, `THTensorMoreMath`, `THTensorEvenMoreMath`.)

* MSVC's preprocessor (but not the standard compiler) has a bug
  where it incorrectly tokenizes raw string literals, ending when it sees a `"`.
  This causes preprocessor tokens inside the literal like an`#endif`  to be incorrectly
  treated as preprocessor directives. See https://godbolt.org/z/eVTIJq as an example.

* Either MSVC or the Windows headers have a PURE macro defined and will replace
  any occurrences of the PURE token in code with an empty string. This is why
  we have AliasAnalysisKind::PURE_FUNCTION and not AliasAnalysisKind::PURE.
  The same is likely true for other identifiers that we just didn't try to use yet.

## Running clang-tidy

[Clang-Tidy](https://clang.llvm.org/extra/clang-tidy/index.html) is a C++
linter and static analysis tool based on the clang compiler. We run clang-tidy
in our CI to make sure that new C++ code is safe, sane and efficient. See our
[.travis.yml](https://github.com/pytorch/pytorch/blob/master/.travis.yml) file
for the simple commands we use for this.

To run clang-tidy locally, follow these steps:

1. Install clang-tidy. First, check if you already have clang-tidy by simply
writing `clang-tidy` in your terminal. If you don't yet have clang-tidy, you
should be able to install it easily with your package manager, e.g. by writing
`apt-get install clang-tidy` on Ubuntu. See https://apt.llvm.org for details on
how to install the latest version. Note that newer versions of clang-tidy will
have more checks than older versions. In our CI, we run clang-tidy-6.0.

2. Use our driver script to run clang-tidy over any changes relative to some
   git revision (you may want to replace `HEAD~1` with `HEAD` to pick up
   uncommitted changes). Changes are picked up based on a `git diff` with the
   given revision:
  ```bash
  python tools/clang_tidy.py -d build -p torch/csrc --diff 'HEAD~1'
  ```

Above, it is assumed you are in the PyTorch root folder. `path/to/build` should
be the path to where you built PyTorch from source, e.g. `build` in the PyTorch
root folder if you used `setup.py build`. You can use `-c <clang-tidy-binary>`
to change the clang-tidy this script uses. Make sure you have PyYaml installed,
which is in PyTorch's `requirements.txt`.

## Pre-commit tidy/linting hook

We use clang-tidy and flake8 (installed with flake8-bugbear,
flake8-comprehensions, flake8-mypy, and flake8-pyi) to perform additional
formatting and semantic checking of code. We provide a pre-commit git hook for
performing these checks, before a commit is created:

  ```bash
  ln -s ../../tools/git-pre-commit .git/hooks/pre-commit
  ```

You'll need to install an appropriately configured flake8; see
[Lint as you type](https://github.com/pytorch/pytorch/wiki/Lint-as-you-type)
for documentation on how to do this.

## Building PyTorch with ASAN

[ASAN](https://github.com/google/sanitizers/wiki/AddressSanitizer) is very
useful for debugging memory errors in C++. We run it in CI, but here's how to
get the same thing to run on your local machine.

First, install LLVM 8. The easiest way is to get [prebuilt
binaries](http://releases.llvm.org/download.html#8.0.0) and extract them to
folder (later called `$LLVM_ROOT`).

Then set up the appropriate scripts. You can put this in your `.bashrc`:

```
LLVM_ROOT=<wherever your llvm install is>
PYTORCH_ROOT=<wherever your pytorch checkout is>

LIBASAN_RT="$LLVM_ROOT/lib/clang/8.0.0/lib/linux/libclang_rt.asan-x86_64.so"
build_with_asan()
{
  LD_PRELOAD=${LIBASAN_RT} \
  CC="$LLVM_ROOT/bin/clang" \
  CXX="$LLVM_ROOT/bin/clang++" \
  LDSHARED="clang --shared" \
  LDFLAGS="-stdlib=libstdc++" \
  CFLAGS="-fsanitize=address -fno-sanitize-recover=all -shared-libasan -pthread" \
  CXX_FLAGS="-pthread" \
  NO_CUDA=1 USE_OPENMP=0 BUILD_CAFFE2_OPS=0 NO_DISTRIBUTED=1 DEBUG=1 \
  python setup.py develop
}

run_with_asan()
{
  LD_PRELOAD=${LIBASAN_RT} $@
}

# you can look at build-asan.sh to find the latest options the CI uses
export ASAN_OPTIONS=detect_leaks=0:symbolize=1:strict_init_order=true
export UBSAN_OPTIONS=print_stacktrace=1:suppressions=$PYTORCH_ROOT/ubsan.supp
export ASAN_SYMBOLIZER_PATH=$LLVM_ROOT/bin/llvm-symbolizer
```

Then you can use the scripts like:

```
suo-devfair ~/pytorch ❯ build_with_asan
suo-devfair ~/pytorch ❯ run_with_asan python test/test_jit.py
```

### Getting `ccache` to work

The scripts above specify the `clang` and `clang++` binaries directly, which
bypasses `ccache`. Here's how to get `ccache` to work:

1. Make sure the ccache symlinks for `clang` and `clang++` are set up (see
   CONTRIBUTING.md)
2. Make sure `$LLVM_ROOT/bin` is available on your `$PATH`.
3. Change the `CC` and `CXX` variables in `build_with_asan()` to point
   directly to `clang` and `clang++`.

### Why this stuff with `LD_PRELOAD` and `LIBASAN_RT`?

The “standard” workflow for ASAN assumes you have a standalone binary:

1. Recompile your binary with `-fsanitize=address`.
2. Run the binary, and ASAN will report whatever errors it find.

Unfortunately, PyTorch is a distributed as a shared library that is loaded by
a third-party executable (Python). It’s too much of a hassle to recompile all
of Python every time we want to use ASAN. Luckily, the ASAN folks have a
workaround for cases like this:

1. Recompile your library with `-fsanitize=address -shared-libasan`. The
   extra `-shared-libasan` tells the compiler to ask for the shared ASAN
   runtime library.
2. Use `LD_PRELOAD` to tell the dynamic linker to load the ASAN runtime
   library before anything else.

More information can be found
[here](https://github.com/google/sanitizers/wiki/AddressSanitizerAsDso).

### Why LD_PRELOAD in the build function?

We need `LD_PRELOAD` because there is a cmake check that ensures that a
simple program builds and runs. If we are building with ASAN as a shared
library, we need to `LD_PRELOAD` the runtime library, otherwise there will
dynamic linker errors and the check will fail.

We don’t actually need either of these if we fix the cmake checks.

### Why no leak detection?

Python leaks a lot of memory. Possibly we could configure a suppression file,
but we haven’t gotten around to it.

## Caffe2 notes

In 2018, we merged Caffe2 into the PyTorch source repository. While the
steady state aspiration is that Caffe2 and PyTorch share code freely,
in the meantime there will be some separation.

If you submit a PR to only PyTorch or only Caffe2 code, CI will only
run for the project you edited. The logic for this is implemented
in `.jenkins/pytorch/dirty.sh` and `.jenkins/caffe2/dirty.sh`; you
can look at this to see what path prefixes constitute changes.
This also means if you ADD a new top-level path, or you start
sharing code between projects, you need to modify these files.

There are a few "unusual" directories which, for historical reasons,
are Caffe2/PyTorch specific. Here they are:

- `CMakeLists.txt`, `Makefile`, `binaries`, `cmake`, `conda`, `modules`,
  `scripts` are Caffe2-specific. Don't put PyTorch code in them without
  extra coordination.

- `mypy*`, `requirements.txt`, `setup.py`, `test`, `tools` are
  PyTorch-specific. Don't put Caffe2 code in them without extra
  coordination.
