--- a/ipc/ipdl/Makefile.in
+++ b/ipc/ipdl/Makefile.in
@@ -107,23 +107,27 @@ GARBAGE += $(CPPSRCS)
 
 LOCAL_INCLUDES += -I$(DEPTH)/ipc/ipdl/_ipdlheaders
 
 
 include $(topsrcdir)/config/config.mk
 include $(topsrcdir)/ipc/chromium/chromium-config.mk
 include $(topsrcdir)/config/rules.mk
 
+-include $(ALL_IPDLSRCS:.ipdl=.ipdl.depends)
 
 # NB: the IPDL compiler manages .ipdl-->.h/.cpp dependencies itself,
 # which is why we don't have explicit .h/.cpp targets here
-export:: $(ALL_IPDLSRCS)
+export:: $(CPPSRCS)
+
+$(CPPSRCS) : $(ALL_IPDLSRCS)
 	$(PYTHON) $(topsrcdir)/config/pythonpath.py	\
 	  -I$(topsrcdir)/other-licenses/ply		\
 	  $(srcdir)/ipdl.py				\
 	  --outheaders-dir=_ipdlheaders			\
 	  --outcpp-dir=.				\
+	  --dependencies				\
 	  $(IPDLDIRS:%=-I$(topsrcdir)/%)		\
 	  $^
 
 # We #include some things in the dom/plugins/ directory that rely on
 # toolkit libraries.
 CXXFLAGS    += $(TK_CFLAGS)

--- a/ipc/ipdl/ipdl.py
+++ b/ipc/ipdl/ipdl.py
@@ -54,22 +54,25 @@ op.add_option('-d', '--outheaders-dir', 
 A protocol Foo in the namespace bar will cause the headers
   dir/bar/Foo.h, dir/bar/FooParent.h, and dir/bar/FooParent.h
 to be generated""")
 op.add_option('-o', '--outcpp-dir', dest='cppdir', default='.',
               help="""Directory into which C++ sources will be generated
 A protocol Foo in the namespace bar will cause the sources
   cppdir/FooParent.cpp, cppdir/FooChild.cpp
 to be generated""")
+op.add_option('-m', '--dependencies', dest='emitdependencies', default=False, action='store_true',
+              help="Emit Makefile dependencies for incremental rebuilds")
 
 
 options, files = op.parse_args()
 _verbosity = options.verbosity
 headersdir = options.headersdir
 cppdir = options.cppdir
+emitdependencies = options.emitdependencies
 includedirs = [ os.path.abspath(incdir) for incdir in options.includedirs ]
 
 if not len(files):
     op.error("No IPDL files specified")
 
 log(2, 'Generated C++ headers will be generated relative to "%s"', headersdir)
 log(2, 'Generated C++ sources will be generated in "%s"', cppdir)
 
@@ -101,23 +104,26 @@ for f in files:
     if not ipdl.typecheck(ast):
         print >>sys.stderr, 'Specification is not well typed.'
         sys.exit(1)
 
     if _verbosity > 2:
         log(3, '  pretty printed code:')
         ipdl.genipdl(ast, codedir)
 
+    if emitdependencies == 1:
+        ipdl.genm(ast, cppdir, normalizedFilename(f))
+
 # Second pass: generate code
 for f in files:
     # Read from parser cache
     filename = normalizedFilename(f)
     ast = ipdl.parse(None, filename, includedirs=includedirs)
     ipdl.gencxx(filename, ast, headersdir, cppdir)
-    
+
     allprotocols.append('%sMsgStart' % ast.protocol.name)
 
 allprotocols.sort()
 
 ipcmsgstart = StringIO()
 
 print >>ipcmsgstart, """
 // CODE GENERATED by ipdl.py. Do not edit.
@@ -135,10 +141,10 @@ print >>ipcmsgstart, """
   LastMsgIndex
 };
 
 COMPILE_ASSERT(LastMsgIndex <= 65536, need_to_update_IPC_MESSAGE_MACRO);
 
 #endif // ifndef IPCMessageStart_h
 """
 
-ipdl.writeifmodified(ipcmsgstart.getvalue(),
+ipdl.writetofile(ipcmsgstart.getvalue(),
                      os.path.join(headersdir, 'IPCMessageStart.h'))

--- a/ipc/ipdl/ipdl/__init__.py
+++ b/ipc/ipdl/ipdl/__init__.py
@@ -25,22 +25,23 @@
 # use your version of this file under the terms of the MPL, indicate your
 # decision by deleting the provisions above and replace them with the notice
 # and other provisions required by the GPL or the LGPL. If you do not delete
 # the provisions above, a recipient may use your version of this file under
 # the terms of any one of the MPL, the GPL or the LGPL.
 #
 # ***** END LICENSE BLOCK *****
 
-__all__ = [ 'gencxx', 'genipdl', 'parse', 'typecheck', 'writeifmodified' ]
+__all__ = [ 'gencxx', 'genipdl', 'parse', 'typecheck', 'writetofile' ]
 
-import os, sys
+import os, sys, errno
 from cStringIO import StringIO
 
 from ipdl.cgen import IPDLCodeGen
+from ipdl.mgen import DependGen
 from ipdl.lower import LowerToCxx
 from ipdl.parser import Parser
 from ipdl.type import TypeCheck
 
 from ipdl.cxx.cgen import CxxCodeGen
 
 
 def parse(specstring, filename='/stdin', includedirs=[ ], errout=sys.stderr):
@@ -67,28 +68,37 @@ def gencxx(ipdlfilename, ast, outheaders
         ]
     def resolveCpp(cpp):
         return [ cpp, os.path.join(outcppdir, cpp.name) ]
 
     for ast, filename in ([ resolveHeader(hdr) for hdr in headers ]
                           + [ resolveCpp(cpp) for cpp in cpps ]):
         tempfile = StringIO()
         CxxCodeGen(tempfile).cgen(ast)
-        writeifmodified(tempfile.getvalue(), filename)
+        writetofile(tempfile.getvalue(), filename)
 
 
 def genipdl(ast, outdir):
     return IPDLCodeGen().cgen(ast)
 
 
-def writeifmodified(contents, file):
+def genm(ast, dir, filename):
+    tempfile = StringIO()
+    DependGen(tempfile).mgen(ast)
+    filename = dir + "/" + os.path.basename(filename) + ".depends"
+    writetofile(tempfile.getvalue(), filename)
+
+
+def writetofile(contents, file):
     dir = os.path.dirname(file)
-    os.path.exists(dir) or os.makedirs(dir)
 
-    oldcontents = None
-    if os.path.exists(file):
-        fd = open(file, 'rb')
-        oldcontents = fd.read()
-        fd.close()
-    if oldcontents != contents:
-        fd = open(file, 'wb')
-        fd.write(contents)
-        fd.close()
+    # Guard against race condition by using Try instead
+    # of |os.path.exists(dir) or os.makedirs(dir)|
+    try:
+        os.makedirs(dir)
+    except OSError, ex:
+        if ex.errno != errno.EEXIST:
+            raise ex
+        # else directory already exists. silently ignore and continue
+
+    fd = open(file, 'wb')
+    fd.write(contents)
+    fd.close()

new file mode 100644
--- /dev/null
+++ b/ipc/ipdl/ipdl/mgen.py
@@ -0,0 +1,54 @@
+# ***** BEGIN LICENSE BLOCK *****
+# Version: MPL 1.1/GPL 2.0/LGPL 2.1
+#
+# The contents of this file are subject to the Mozilla Public License Version
+# 1.1 (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+# http://www.mozilla.org/MPL/
+#
+# Software distributed under the License is distributed on an "AS IS" basis,
+# WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+# for the specific language governing rights and limitations under the
+# License.
+#
+# The Original Code is mozilla.org code.
+#
+# Contributor(s):
+#   Benoit Girard <bgirard@mozilla.com>
+#
+# Alternatively, the contents of this file may be used under the terms of
+# either of the GNU General Public License Version 2 or later (the "GPL"),
+# or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+# in which case the provisions of the GPL or the LGPL are applicable instead
+# of those above. If you wish to allow use of your version of this file only
+# under the terms of either the GPL or the LGPL, and not to allow others to
+# use your version of this file under the terms of the MPL, indicate your
+# decision by deleting the provisions above and replace them with the notice
+# and other provisions required by the GPL or the LGPL. If you do not delete
+# the provisions above, a recipient may use your version of this file under
+# the terms of any one of the MPL, the GPL or the LGPL.
+#
+# ***** END LICENSE BLOCK *****
+
+import sys
+
+from ipdl.cgen import CodePrinter
+from ipdl.cxx.ast import TypeArray, Visitor
+
+class DependGen(CodePrinter, Visitor):
+    def __init__(self, outf=sys.stdout, indentCols=4):
+        CodePrinter.__init__(self, outf, indentCols)
+
+    def mgen(self, cxxfile):
+        cxxfile.accept(self)
+
+    def visitTranslationUnit(self, tu):
+        self.write(tu.filename)
+        self.write(": ")
+
+        for pinc in tu.protocolIncludes:
+            self.write(pinc.file)
+            self.write(" ")
+
+        self.println();
+