diff --git a/pkgs/development/compilers/llvm/git/default.nix b/pkgs/development/compilers/llvm/git/default.nix
index eb068f6d4722..38de0f70cf2e 100644
--- a/pkgs/development/compilers/llvm/git/default.nix
+++ b/pkgs/development/compilers/llvm/git/default.nix
@@ -21,13 +21,14 @@ let
   release_version = "12.0.0";
   candidate = ""; # empty or "rcN"
   dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
+  rev = ""; # When using a Git commit
   version = "${release_version}${dash-candidate}"; # differentiating these (variables) is important for RCs
   targetConfig = stdenv.targetPlatform.config;
 
   src = fetchFromGitHub {
     owner = "llvm";
     repo = "llvm-project";
-    rev = "llvmorg-${version}";
+    rev = if rev != "" then rev else "llvmorg-${version}";
     sha256 = "07jz8pywc2qqa1srdnqg5p2y4lx3ki1inpigarzgxc3j20r4gb58";
   };
 
diff --git a/pkgs/development/compilers/llvm/update-git.py b/pkgs/development/compilers/llvm/update-git.py
new file mode 100755
index 000000000000..119a050e4701
--- /dev/null
+++ b/pkgs/development/compilers/llvm/update-git.py
@@ -0,0 +1,70 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i python3 -p python3 nix
+
+import csv
+import fileinput
+import json
+import os
+import re
+import subprocess
+
+from codecs import iterdecode
+from datetime import datetime
+from urllib.request import urlopen, Request
+
+
+def get_latest_chromium_build():
+    HISTORY_URL = 'https://omahaproxy.appspot.com/history?os=linux'
+    print(f'GET {HISTORY_URL}')
+    with urlopen(HISTORY_URL) as resp:
+        builds = csv.DictReader(iterdecode(resp, 'utf-8'))
+        for build in builds:
+            if build['channel'] != 'dev':
+                continue
+            return build
+
+
+def get_file_revision(revision, file_path):
+    """Fetches the requested Git revision of the given Chromium file."""
+    url = f'https://raw.githubusercontent.com/chromium/chromium/{revision}/{file_path}'
+    with urlopen(url) as http_response:
+        return http_response.read().decode()
+
+
+def get_commit(ref):
+    url = f'https://api.github.com/repos/llvm/llvm-project/commits/{ref}'
+    headers = {'Accept': 'application/vnd.github.v3+json'}
+    request = Request(url, headers=headers)
+    with urlopen(request) as http_response:
+        return json.loads(http_response.read().decode())
+
+
+def nix_prefetch_url(url, algo='sha256'):
+    """Prefetches the content of the given URL."""
+    print(f'nix-prefetch-url {url}')
+    out = subprocess.check_output(['nix-prefetch-url', '--type', algo, '--unpack', url])
+    return out.decode('utf-8').rstrip()
+
+
+chromium_build = get_latest_chromium_build()
+chromium_version = chromium_build['version']
+print(f'chromiumDev version: {chromium_version}')
+print('Getting LLVM commit...')
+clang_update_script = get_file_revision(chromium_version, 'tools/clang/scripts/update.py')
+clang_revision = re.search(r"^CLANG_REVISION = '(.+)'$", clang_update_script, re.MULTILINE).group(1)
+clang_commit_short = re.search(r"llvmorg-[0-9]+-init-[0-9]+-g([0-9a-f]{8})", clang_revision).group(1)
+release_version = re.search(r"^RELEASE_VERSION = '(.+)'$", clang_update_script, re.MULTILINE).group(1)
+commit = get_commit(clang_commit_short)
+date = datetime.fromisoformat(commit['commit']['committer']['date'].rstrip('Z')).date().isoformat()
+version = f'unstable-{date}'
+print('Prefetching source tarball...')
+hash = nix_prefetch_url(f'https://github.com/llvm/llvm-project/archive/{commit["sha"]}.tar.gz')
+print('Updating default.nix...')
+default_nix = os.path.join(os.path.dirname(os.path.abspath(__file__)), 'git/default.nix')
+with fileinput.FileInput(default_nix, inplace=True) as f:
+    for line in f:
+        result = re.sub(r'^  release_version = ".+";', f'  release_version = "{release_version}";', line)
+        result = re.sub(r'^  version = ".+";', f'  version = "{version}";', line)
+        result = re.sub(r'^  rev = ".*";', f'  rev = "{commit["sha"]}";', result)
+        result = re.sub(r'^    sha256 = ".+";', f'    sha256 = "{hash}";', result)
+        print(result, end='')