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+LLVM Dependency
+===============
+
+Overview
+--------
+
+SwiftShader's [Reactor](Reactor.md) library uses LLVM
+as one of its JIT-compiler backends. This page contains notes about building and
+upgrading LLVM.
+
+Directory structure
+-------------------
+
+The current version of LLVM we use is 10, and can be found in
+`third_party/llvm-10.0`.
+
+In this folder you will find the following directories:
+
+*   configs : Contains per-platform headers that LLVM sources include to
+    configure the build. These are generated by running `scripts/update.py`
+    (more on that below).
+*   llvm : Contains a subset of the LLVM source code needed to build the JIT
+    support required by SwiftShader.
+*   scripts : Contains `update.py`, which is used to update the files in the
+    `configs` folder. More on that below.
+
+Updating the current version of LLVM to latest
+----------------------------------------------
+
+Updating to the latest version of LLVM can be tricky to do manually, especially
+because the [llvm-project repo](https://github.com/llvm/llvm-project) includes
+much more than just LLVM (e.g. it includes all the Clang source). Furthermore,
+we may have local changes to our copy of LLVM that must be maintained, or at
+least considered across updates.
+
+To ease this pain, run the script `third_party/update-llvm-10.sh` on Linux. This
+script works by updating a separate branch of SwiftShader, `llvm10-clean`, on
+which the latest snapshot of LLVM is fetched and committed, and then this branch
+is merged back into `master`. During the merge, if there are conflicts to
+resolve because of local changes we've made, these can be resolved in the usual
+manner, and the merge can be resumed.
+
+The script is configured to fetch from the branch in `LLVM_REPO_BRANCH`, and
+will automatically grab the latest commit on that branch.
+
+Although not always necessary, if there were new configuration variables added
+or modified, you may need to run `update.py` as described below. Otherwise, if
+all goes well, the update to LLVM can be committed and pushed.
+
+Updating LLVM configuration files
+---------------------------------
+
+The script `third_party/llvm-10.0/scripts/update.py` is used to update the
+config files in `third_party/llvm-10.0/configs`.
+
+Before running this script, you must make sure to update two variables in it
+(and commit this change):
+
+```
+# LLVM_BRANCH must match the value of the same variable in third_party/update-llvm-10.sh
+LLVM_BRANCH = "release/10.x"
+
+# LLVM_COMMIT must be set to the commit hash that we last updated to when running third_party/update-llvm-10.sh.
+# Run 'git show -s origin/llvm10-clean' and look for 'llvm-10-update: <hash>' to retrieve it.
+LLVM_COMMIT = "d32170dbd5b0d54436537b6b75beaf44324e0c28"
+```
+
+The script takes a platform as argument, and extra CMake args. For example, to
+update the Linux configs, run:
+
+```
+python3 update.py linux -j 200
+```
+
+This script does the following:
+
+*   Clones the LLVM repo and checks out `LLVM_COMMIT` from `LLVM_BRANCH`.
+*   Builds LLVM specifically for the target architectures specified in
+    `LLVM_TRIPLES` dictionary.
+*   Copies the specified platform config files to
+    `third_party/llvm-10.0/configs`, applying certain transformations to the
+    files, such as undefining macros listed in `LLVM_UNDEF_MACROS` (see the
+    `copy_platform_file` function).
+
+Note that certain configuration options depend on the host OS, you will need to
+run the script on the right host OS. See the `LLVM_PLATFORM_TO_HOST_SYSTEM`
+dictionary for the mapping, which looks like this at the time of this writing:
+
+```
+# Mapping of target platform to the host it must be built on
+LLVM_PLATFORM_TO_HOST_SYSTEM = {
+    'android': 'Linux',
+    'darwin': 'Darwin',
+    'linux': 'Linux',
+    'windows': 'Windows',
+    'fuchsia': 'Linux'
+}
+```
+
+Generally, Windows to build Window, Darwin to build Darwin (MacOS), and Linux
+for everything else. Also note that for android and fuchsia, the config is
+closest to that of Linux, but you will likely have to manually tweak the configs
+(in particular, `configs/<platform>/include/llvm/Config/config.h`).
+
+Supported platforms, architectures, and build systems
+-----------------------------------------------------
+
+SwiftShader is used by many products on many architectures:
+
+*   OS: Windows, Linux, MacOS, Android, Fuchsia
+*   Architecture: x64, x86, ARM, ARM64, MIPS, MIPS64
+*   Build systems: CMake, GN, Soong, Blaze
+
+Upgrading/updating LLVM usually entails making sure it builds for all of these.
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