Index: INSTALL.txt
===================================================================
--- INSTALL.txt
+++ INSTALL.txt
@@ -7,6 +7,11 @@
 on your system to either build lldb or debug using lldb.  Please see the code
 signing documentation in docs/code-signing.txt for more detailed directions.
 
+If you are building on Mac OS X and LLVM is not present in llvm/, then LLDB
+will check it out automatically.  The files in scripts/Xcode/repos determine
+which branches of LLVM/Clang are checked out, depending on the current
+LLDB branch, according to the algorithm in scripts/Xcode/repo.py.
+
 For instructions to build LLDB on Linux, or more details about supported
 compiler versions, other dependencies, and build flags, see:
 
Index: scripts/Xcode/build-llvm.py
===================================================================
--- scripts/Xcode/build-llvm.py
+++ scripts/Xcode/build-llvm.py
@@ -6,6 +6,7 @@
 import os
 import platform
 import re
+import repo
 import subprocess
 import sys
 
@@ -17,44 +18,38 @@
 def LLVM_HASH_INCLUDES_DIFFS():
     return False
 
-# The use of "x = "..."; return x" here is important because tooling looks for
-# it with regexps.  Only change how this works if you know what you are doing.
+# For use with Xcode-style builds
 
+def process_vcs(vcs):
+    return {
+        "svn": VCS.svn,
+        "git": VCS.git
+    }[vcs]
 
-def LLVM_REF():
-    llvm_ref = "master"
-    return llvm_ref
+def process_root(name):
+    return {
+        "llvm": llvm_source_path(),
+        "clang": clang_source_path(),
+        "ninja": ninja_source_path()
+    }[name]
 
+def process_repo(r):
+    return {
+        'name': r["name"],
+        'vcs': process_vcs(r["vcs"]),
+        'root': process_root(r["name"]),
+        'url': r["url"],
+        'ref': r["ref"]
+    }
 
-def CLANG_REF():
-    clang_ref = "master"
-    return clang_ref
-
-# For use with Xcode-style builds
-
-
 def XCODE_REPOSITORIES():
-    return [
-        {'name': "llvm",
-         'vcs': VCS.git,
-         'root': llvm_source_path(),
-         'url': "http://llvm.org/git/llvm.git",
-         'ref': LLVM_REF()},
+    identifier = repo.identifier()
+    if identifier == None:
+        identifier = "<invalid>" # repo.find will just use the fallback file
+    set = repo.find(identifier)
+    return [process_repo(r) for r in set]
 
-        {'name': "clang",
-         'vcs': VCS.git,
-         'root': clang_source_path(),
-         'url': "http://llvm.org/git/clang.git",
-         'ref': CLANG_REF()},
 
-        {'name': "ninja",
-         'vcs': VCS.git,
-         'root': ninja_source_path(),
-         'url': "https://github.com/ninja-build/ninja.git",
-         'ref': "master"}
-    ]
-
-
 def get_c_compiler():
     return subprocess.check_output([
         'xcrun',
Index: scripts/Xcode/repo.py
===================================================================
--- scripts/Xcode/repo.py
+++ scripts/Xcode/repo.py
@@ -0,0 +1,42 @@
+import json
+import os
+import re
+import shutil
+import subprocess
+
+def identifier():
+	try:
+		svn_output = subprocess.check_output(["svn", "info", "--show-item", "url"], stderr=subprocess.STDOUT).rstrip()
+		return svn_output
+	except:
+		pass
+	try:
+		git_remote_and_branch = subprocess.check_output(["git", "rev-parse", "--abbrev-ref", "--symbolic-full-name", "@{u}"]).rstrip()
+		git_remote = git_remote_and_branch.split("/")[0]
+		git_branch = "/".join(git_remote_and_branch.split("/")[1:])
+		git_url = subprocess.check_output(["git", "remote", "get-url", git_remote]).rstrip()
+		return git_url + ":" + git_branch
+	except:
+		pass
+	return None
+
+def find(identifier):
+	dir = os.path.dirname(os.path.realpath(__file__))
+	repos_dir = os.path.join(dir, "repos")
+	json_regex = re.compile(r"^.*.json$")
+	override_path = os.path.join(repos_dir, "OVERRIDE")
+	if os.path.isfile(override_path):
+		override_set = json.load(open(override_path))
+		return override_set["repos"]
+	fallback_path = os.path.join(repos_dir, "FALLBACK")
+	for path in [os.path.join(repos_dir, f) for f in filter(json_regex.match, os.listdir(repos_dir))]:
+		fd = open(path)
+		set = json.load(fd)
+		fd.close()
+		if any(re.match(set_regex, identifier) for set_regex in set["regexs"]):
+			shutil.copyfile(path, fallback_path)
+			return set["repos"]
+	if os.path.isfile(fallback_path):
+		fallback_set = json.load(open(fallback_path))
+		return fallback_set["repos"]
+	sys.exit("Couldn't find a branch configuration for " + identifier + " and there was no " + fallback_path)
Index: scripts/Xcode/repos/FALLBACK
===================================================================
--- scripts/Xcode/repos/FALLBACK
+++ scripts/Xcode/repos/FALLBACK
@@ -0,0 +1,19 @@
+{
+    "regexs" : [".*llvm\\.org.*"],
+    "repos" : [
+        {"name": "llvm",
+         "vcs": "git",
+         "url": "http://llvm.org/git/llvm.git",
+         "ref": "master"},
+
+        {"name": "clang",
+         "vcs": "git",
+         "url": "http://llvm.org/git/clang.git",
+         "ref": "master"},
+
+        {"name": "ninja",
+         "vcs": "git",
+         "url": "https://github.com/ninja-build/ninja.git",
+         "ref": "master"}
+    ]
+}
Index: scripts/Xcode/repos/svn-trunk.json
===================================================================
--- scripts/Xcode/repos/svn-trunk.json
+++ scripts/Xcode/repos/svn-trunk.json
@@ -0,0 +1,19 @@
+{
+    "regexs" : [".*llvm\\.org.*"],
+    "repos" : [
+        {"name": "llvm",
+         "vcs": "git",
+         "url": "http://llvm.org/git/llvm.git",
+         "ref": "master"},
+
+        {"name": "clang",
+         "vcs": "git",
+         "url": "http://llvm.org/git/clang.git",
+         "ref": "master"},
+
+        {"name": "ninja",
+         "vcs": "git",
+         "url": "https://github.com/ninja-build/ninja.git",
+         "ref": "master"}
+    ]
+}