--- a/Makefile.in
+++ b/Makefile.in
@@ -204,8 +204,18 @@ deliver: splitsymbols rebase signnss
 
 endif # WINNT
 
 ifneq (,$(wildcard $(DIST)/bin/application.ini))
 BUILDID = $(shell $(PYTHON) $(srcdir)/config/printconfigsetting.py $(DIST)/bin/application.ini App BuildID)
 else
 BUILDID = $(shell $(PYTHON) $(srcdir)/config/printconfigsetting.py $(DIST)/bin/platform.ini Build BuildID)
 endif
+
+#XXX: this is a hack, since we don't want to clobber for MSVC
+# PGO support, but we can't do this test in client.mk
+ifneq ($(OS_ARCH)_$(GNU_CC), WINNT_)
+maybe_clobber_profiledbuild: clobber_all
+else
+maybe_clobber_profiledbuild:
+endif
+
+.PHONY: maybe_clobber_profiledbuild

new file mode 100755
--- /dev/null
+++ b/build/win32/pgomerge.py
@@ -0,0 +1,40 @@
+#!/usr/bin/python
+# Usage: pgomerge.py <binary basename> <dist/bin>
+# Gathers .pgc files from dist/bin and merges them into
+# $PWD/$basename.pgd using pgomgr, then deletes them.
+# No errors if any of these files don't exist.
+
+import sys, os, os.path, subprocess
+if not sys.platform == "win32":
+    raise Exception("This script was only meant for Windows.")
+
+def MergePGOFiles(basename, pgddir, pgcdir):
+  """Merge pgc files produced from an instrumented binary
+     into the pgd file for the second pass of profile-guided optimization
+     with MSVC.  |basename| is the name of the DLL or EXE without the
+     extension.  |pgddir| is the path that contains <basename>.pgd
+     (should be the objdir it was built in).  |pgcdir| is the path
+     containing basename!N.pgc files, which is probably dist/bin.
+     Calls pgomgr to merge each pgc file into the pgd, then deletes
+     the pgc files."""
+  if not os.path.isdir(pgddir) or not os.path.isdir(pgcdir):
+    return
+  pgdfile = os.path.abspath(os.path.join(pgddir, basename + ".pgd"))
+  if not os.path.isfile(pgdfile):
+    return
+  for file in os.listdir(pgcdir):
+    if file.startswith(basename) and file.endswith(".pgc"):
+      try:
+        pgcfile = os.path.normpath(os.path.join(pgcdir, file))
+        subprocess.call(['pgomgr', '-merge',
+                         pgcfile,
+                         pgdfile])
+        os.remove(pgcfile)
+      except OSError:
+        pass
+
+if __name__ == '__main__':
+  if len(sys.argv) != 3:
+      print >>sys.stderr, "Usage: pgomerge.py <binary basename> <dist/bin>"
+      sys.exit(1)
+  MergePGOFiles(sys.argv[1], os.getcwd(), sys.argv[2])

--- a/client.mk
+++ b/client.mk
@@ -1092,19 +1092,18 @@ build::  $(OBJDIR)/Makefile $(OBJDIR)/co
 #    mk_add_options PROFILE_GEN_SCRIPT=/path/to/profile-script
 #
 #  The profile script should exercise the functionality to be included
 #  in the profile feedback.
 
 profiledbuild:: $(OBJDIR)/Makefile $(OBJDIR)/config.status
 	$(MOZ_MAKE) MOZ_PROFILE_GENERATE=1
 	OBJDIR=${OBJDIR} $(PROFILE_GEN_SCRIPT)
-	$(MOZ_MAKE) clobber_all
+	$(MOZ_MAKE) maybe_clobber_profiledbuild
 	$(MOZ_MAKE) MOZ_PROFILE_USE=1
-	find $(OBJDIR) -name "*.da" -exec rm {} \;
 
 ####################################
 # Other targets
 
 # Pass these target onto the real build system
 install export libs clean realclean distclean alldep:: $(OBJDIR)/Makefile $(OBJDIR)/config.status
 	$(MOZ_MAKE) $@
 

--- a/config/config.mk
+++ b/config/config.mk
@@ -351,33 +351,47 @@ DSO_CFLAGS=
 ifeq ($(OS_ARCH)_$(HAVE_GCC3_ABI),Darwin_1)
 DSO_PIC_CFLAGS=-mdynamic-no-pic
 else
 DSO_PIC_CFLAGS=
 endif
 endif
 
 # Enable profile-based feedback
+ifndef NO_PROFILE_GUIDED_OPTIMIZE
 ifdef MOZ_PROFILE_GENERATE
 # No sense in profiling tools
 ifndef INTERNAL_TOOLS
 OS_CFLAGS += $(PROFILE_GEN_CFLAGS)
 OS_CXXFLAGS += $(PROFILE_GEN_CFLAGS)
-endif # INTERNAL_TOOLS
+OS_LDFLAGS += $(PROFILE_GEN_LDFLAGS)
+ifeq (WINNT,$(OS_ARCH))
+AR_FLAGS += -LTCG
+endif
+else
+# We don't want to link internal tools with profiling,
+# but gcc needs to because xpidl will wind up linking with
+# libs that have been built with PROFILE_GEN_CFLAGS.
+ifdef GNU_CC
 OS_LDFLAGS += $(PROFILE_GEN_LDFLAGS)
 endif
+endif # INTERNAL_TOOLS
+endif # MOZ_PROFILE_GENERATE
 
-# We always use the profile-use flags, even in cases where we didn't use the
-# profile-generate flags.  It's harmless, and it saves us from having to
-# answer the question "Would these objects have been built using
-# the profile-generate flags?" which is not trivial.
 ifdef MOZ_PROFILE_USE
+ifndef INTERNAL_TOOLS
 OS_CFLAGS += $(PROFILE_USE_CFLAGS)
 OS_CXXFLAGS += $(PROFILE_USE_CFLAGS)
+OS_LDFLAGS += $(PROFILE_USE_LDFLAGS)
+ifeq (WINNT,$(OS_ARCH))
+AR_FLAGS += -LTCG
 endif
+endif # INTERNAL_TOOLS
+endif # MOZ_PROFILE_USE
+endif # NO_PROFILE_GUIDED_OPTIMIZE
 
 
 # Does the makefile specifies the internal XPCOM API linkage?
 ifneq (,$(MOZILLA_INTERNAL_API)$(LIBXUL_LIBRARY))
 DEFINES += -DMOZILLA_INTERNAL_API
 endif
 
 # Force XPCOM/widget/gfx methods to be _declspec(dllexport) when we're

--- a/config/rules.mk
+++ b/config/rules.mk
@@ -779,16 +779,47 @@ else
 	$(INSTALL) $(IFLAGS1) $(JAVA_LIBRARY) $(FINAL_TARGET)
 endif
 endif # JAVA_LIBRARY
 endif # !NO_DIST_INSTALL
 	+$(LOOP_OVER_DIRS)
 
 ##############################################
 
+ifndef NO_PROFILE_GUIDED_OPTIMIZE
+ifneq (,$(MOZ_PROFILE_GENERATE)$(MOZ_PROFILE_USE))
+ifeq ($(OS_ARCH)_$(GNU_CC)$(INTERNAL_TOOLS), WINNT_)
+# Force re-linking when building with PGO, since
+# the MSVC linker does all the work.  We force re-link
+# in both stages so you can do depend builds with PGO.
+ifdef SHARED_LIBRARY
+$(SHARED_LIBRARY): FORCE
+BINARY_BASENAME = $(SHARED_LIBRARY:$(DLL_SUFFIX)=)
+endif
+ifdef PROGRAM
+$(PROGRAM): FORCE
+BINARY_BASENAME = $(PROGRAM:$(BIN_SUFFIX)=)
+endif
+
+ifdef MOZ_PROFILE_USE
+# In the second pass, we need to merge the pgc files into the pgd file.
+# The compiler would do this for us automatically if they were in the right
+# place, but they're in dist/bin.
+ifdef BINARY_BASENAME
+export::
+	$(PYTHON) $(topsrcdir)/build/win32/pgomerge.py \
+	  $(BINARY_BASENAME) $(DIST)/bin
+endif
+endif # MOZ_PROFILE_USE
+endif # WINNT_
+endif # MOZ_PROFILE_GENERATE || MOZ_PROFILE_USE
+endif # NO_PROFILE_GUIDED_OPTIMIZE
+
+##############################################
+
 checkout:
 	$(MAKE) -C $(topsrcdir) -f client.mk checkout
 
 run_viewer: $(FINAL_TARGET)/viewer
 	cd $(FINAL_TARGET); \
 	MOZILLA_FIVE_HOME=`pwd` \
 	LD_LIBRARY_PATH=".:$(LIBS_PATH):$$LD_LIBRARY_PATH" \
 	viewer

--- a/configure.in
+++ b/configure.in
@@ -1915,16 +1915,22 @@ case "$target" in
         MOZ_FIX_LINK_PATHS=
         DYNAMIC_XPCOM_LIBS='$(LIBXUL_DIST)/lib/xpcom.lib $(LIBXUL_DIST)/lib/xpcom_core.lib'
         XPCOM_FROZEN_LDOPTS='$(LIBXUL_DIST)/lib/xpcom.lib'
         LIBXUL_LIBS='$(LIBXUL_DIST)/lib/xpcom.lib $(LIBXUL_DIST)/lib/xul.lib'
         MOZ_COMPONENT_NSPR_LIBS='$(NSPR_LIBS)'
         MOZ_XPCOM_OBSOLETE_LIBS='$(LIBXUL_DIST)/lib/xpcom_compat.lib'
         if test $_MSC_VER -ge 1400; then
             LDFLAGS="$LDFLAGS -NXCOMPAT -SAFESEH"
+            # For profile-guided optimization
+            PROFILE_GEN_CFLAGS="-GL"
+            PROFILE_GEN_LDFLAGS="-LTCG:PGINSTRUMENT"
+            PROFILE_USE_CFLAGS="-GL"
+            #XXX: should be -LTCG:PGOPTIMIZE, but that fails on libxul
+            PROFILE_USE_LDFLAGS="-LTCG:PGUPDATE"
         fi
     fi
     MOZ_JPEG_LIBS='$(call EXPAND_LIBNAME_PATH,jpeg$(MOZ_BITS)$(VERSION_NUMBER),$(DEPTH)/jpeg)'
     MOZ_PNG_LIBS='$(call EXPAND_LIBNAME_PATH,png,$(DEPTH)/modules/libimg/png)'
     AC_DEFINE(HAVE_SNPRINTF)
     AC_DEFINE(_WINDOWS)
     AC_DEFINE(_WIN32)
     AC_DEFINE(WIN32)
@@ -6509,16 +6515,19 @@ if test "$_IGNORE_LONG_LONG_WARNINGS"; t
      AC_TRY_COMPILE([], [return(0);], 
 	[ _WARNINGS_CFLAGS="${_WARNINGS_CFLAGS} ${_COMPILER_PREFIX}-Wno-long-long"
 	  _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} ${_COMPILER_PREFIX}-Wno-long-long"
 	  result="yes" ], result="no")
      AC_MSG_RESULT([$result])
      CFLAGS="$_SAVE_CFLAGS"
 fi
 
+dnl ========================================================
+dnl Profile guided optimization
+dnl ========================================================
 dnl Test for profiling options
 dnl Under gcc 3.3, use -fprofile-arcs/-fbranch-probabilities
 dnl Under gcc 3.4+, use -fprofile-generate/-fprofile-use
 
 _SAVE_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fprofile-generate"
 
 AC_MSG_CHECKING([whether C compiler supports -fprofile-generate])