diff --git a/core/cleanbuild.mk b/core/cleanbuild.mk
index b76e7a5..c2386a4 100644
--- a/core/cleanbuild.mk
+++ b/core/cleanbuild.mk
@@ -105,11 +105,10 @@
   _crs_new_cmd :=
   steps :=
 endif
-CURRENT_CLEAN_BUILD_VERSION :=
-CURRENT_CLEAN_STEPS :=
 
 # Write the new state to the file.
 #
+ifneq ($(CURRENT_CLEAN_BUILD_VERSION)-$(CURRENT_CLEAN_STEPS),$(INTERNAL_CLEAN_BUILD_VERSION)-$(INTERNAL_CLEAN_STEPS))
 $(shell \
   mkdir -p $(dir $(clean_steps_file)) && \
   echo "CURRENT_CLEAN_BUILD_VERSION := $(INTERNAL_CLEAN_BUILD_VERSION)" > \
@@ -117,7 +116,10 @@
   echo "CURRENT_CLEAN_STEPS := $(INTERNAL_CLEAN_STEPS)" >> \
       $(clean_steps_file) \
  )
+endif
 
+CURRENT_CLEAN_BUILD_VERSION :=
+CURRENT_CLEAN_STEPS :=
 clean_steps_file :=
 INTERNAL_CLEAN_STEPS :=
 INTERNAL_CLEAN_BUILD_VERSION :=
@@ -164,7 +166,6 @@
     endif
   endif
 endif  # else, this is the first build, so no need to clean.
-PREVIOUS_BUILD_CONFIG :=
 
 ifdef PREVIOUS_SANITIZE_TARGET
   ifneq "$(current_sanitize_target)" "$(PREVIOUS_SANITIZE_TARGET)"
@@ -172,10 +173,10 @@
     force_objclean := true
   endif
 endif  # else, this is the first build, so no need to clean.
-PREVIOUS_SANITIZE_TARGET :=
 
 # Write the new state to the file.
 #
+ifneq ($(PREVIOUS_BUILD_CONFIG)-$(PREVIOUS_SANITIZE_TARGET),$(current_build_config)-$(current_sanitize_target))
 $(shell \
   mkdir -p $(dir $(previous_build_config_file)) && \
   echo "PREVIOUS_BUILD_CONFIG := $(current_build_config)" > \
@@ -183,6 +184,9 @@
   echo "PREVIOUS_SANITIZE_TARGET := $(current_sanitize_target)" >> \
       $(previous_build_config_file) \
  )
+endif
+PREVIOUS_BUILD_CONFIG :=
+PREVIOUS_SANITIZE_TARGET :=
 previous_build_config_file :=
 current_build_config :=
 
@@ -286,6 +290,50 @@
 endif
 force_objclean :=
 
+###########################################################
+# Clean build tools when swithcing between prebuilt host tools (such as in
+# apps_only build) and tools built from source (platform build).
+previous_prebuilt_tools_config_file := $(HOST_OUT)/previous_prebuilt_tools_config.mk
+ifneq (,$(TARGET_BUILD_APPS)$(filter true,$(TARGET_BUILD_PDK)))
+current_prebuilt_tools := true
+else
+current_prebuilt_tools := false
+endif
+PREVIOUS_PREBUILT_TOOLS :=
+-include $(previous_prebuilt_tools_config_file)
+force_tools_clean :=
+ifdef PREVIOUS_PREBUILT_TOOLS
+ifneq ($(PREVIOUS_PREBUILT_TOOLS),$(current_prebuilt_tools))
+force_tools_clean := true
+endif
+endif # else, this is the first build, so no need to clean.
+
+# Write the new state to the file.
+ifneq ($(PREVIOUS_PREBUILT_TOOLS),$(current_prebuilt_tools))
+$(shell \
+  mkdir -p $(dir $(previous_prebuilt_tools_config_file)) && \
+  echo "PREVIOUS_PREBUILT_TOOLS:=$(current_prebuilt_tools)" > \
+    $(previous_prebuilt_tools_config_file))
+endif
+
+ifeq ($(force_tools_clean),true)
+# For this list of prebuilt tools, see prebuilts/sdk/tools/Android.mk.
+tools_clean_files := \
+  $(HOST_OUT_COMMON_INTERMEDIATES)/JAVA_LIBRARIES/signapk_intermediates \
+  $(HOST_OUT_COMMON_INTERMEDIATES)/JAVA_LIBRARIES/dx_intermediates \
+  $(HOST_OUT_COMMON_INTERMEDIATES)/JAVA_LIBRARIES/shrinkedAndroid_intermediates \
+  $(HOST_OUT)/obj*/EXECUTABLES/aapt_intermediates \
+  $(HOST_OUT)/obj*/EXECUTABLES/aidl_intermediates \
+  $(HOST_OUT)/obj*/EXECUTABLES/zipalign_intermediates \
+  $(HOST_OUT)/obj*/lib/libc++$(HOST_SHLIB_SUFFIX) \
+
+$(info *** build type changed, clean host tools...)
+$(info *** rm -rf $(tools_clean_files))
+$(shell rm -rf $(tools_clean_files))
+endif
+
+###########################################################
+
 .PHONY: clean-jack-files
 clean-jack-files: clean-dex-files
 	$(hide) find $(OUT_DIR) -name "*.jack" | xargs rm -f
@@ -303,4 +351,3 @@
 clean-jack-incremental:
 	$(hide) find $(OUT_DIR) -name "jack-incremental" -type d | xargs rm -rf
 	@echo "All jack incremental dirs have been removed."
-
diff --git a/core/config_sanitizers.mk b/core/config_sanitizers.mk
index c28fff1..f71996d 100644
--- a/core/config_sanitizers.mk
+++ b/core/config_sanitizers.mk
@@ -45,6 +45,10 @@
         my_allow_undefined_symbols := true
       endif
     endif
+    # Workaround for a bug in AddressSanitizer that breaks stack unwinding.
+    # https://code.google.com/p/address-sanitizer/issues/detail?id=387
+    # Revert when external/compiler-rt is updated past r236014.
+    LOCAL_PACK_MODULE_RELOCATIONS := false
   endif
 endif
 
diff --git a/core/cxx_stl_setup.mk b/core/cxx_stl_setup.mk
index c59cd34..3941edf 100644
--- a/core/cxx_stl_setup.mk
+++ b/core/cxx_stl_setup.mk
@@ -70,6 +70,7 @@
     else
         ifeq (arm,$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH))
             my_static_libraries += libunwind_llvm
+            my_ldflags += -Wl,--exclude-libs,libunwind_llvm.a
         endif
 
         ifeq ($(my_link_type),static)
diff --git a/core/post_clean.mk b/core/post_clean.mk
index 0273ff2..7fafe4a 100644
--- a/core/post_clean.mk
+++ b/core/post_clean.mk
@@ -88,7 +88,7 @@
 $(shell echo 'MODULES_WITH_AIDL_FILES := $(MODULES_WITH_AIDL_FILES)' >> $(current_aidl_config))
 
 # Now current becomes previous.
-$(shell mv -f $(current_aidl_config) $(previous_aidl_config))
+$(shell cmp $(current_aidl_config) $(previous_aidl_config) > /dev/null 2>&1 || mv -f $(current_aidl_config) $(previous_aidl_config))
 
 MODULES_WITH_AIDL_FILES :=
 modules_with_aidl_files :=
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index f31d416..bc2667d 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -135,10 +135,6 @@
     image_props["fs_config"] = fs_config
   if block_list:
     image_props["block_list"] = block_list
-  if image_props.get("system_root_image") == "true":
-    image_props["ramdisk_dir"] = os.path.join(input_dir, "BOOT/RAMDISK")
-    image_props["ramdisk_fs_config"] = os.path.join(
-        input_dir, "META/boot_filesystem_config.txt")
 
   succ = build_image.BuildImage(os.path.join(input_dir, what),
                                 image_props, img)
diff --git a/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index 8934c77..74a5452 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -413,6 +413,7 @@
     copy_prop("system_verity_block_device", "verity_block_device")
     copy_prop("system_root_image", "system_root_image")
     copy_prop("ramdisk_dir", "ramdisk_dir")
+    copy_prop("ramdisk_fs_config", "ramdisk_fs_config")
     copy_prop("has_ext4_reserved_blocks", "has_ext4_reserved_blocks")
   elif mount_point == "data":
     # Copy the generic fs type first, override with specific one if available.
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index cffb4bd..0d8ca34 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -145,15 +145,20 @@
   if "fstab_version" not in d:
     d["fstab_version"] = "1"
 
-  # During building, we use the "file_contexts" in the out/ directory tree.
-  # It is no longer available when (re)generating from target_files zip. So
-  # when generating from target_files zip, we look for a copy under META/
-  # first, if not available search under BOOT/RAMDISK/. Note that we may need
-  # a different file_contexts to build images than the one running on device,
-  # such as when enabling system_root_image. In that case, we must have the
-  # one for building copied to META/.
+  # A few properties are stored as links to the files in the out/ directory.
+  # It works fine with the build system. However, they are no longer available
+  # when (re)generating from target_files zip. If input_dir is not None, we
+  # are doing repacking. Redirect those properties to the actual files in the
+  # unzipped directory.
   if input_dir is not None:
+    # We carry a copy of file_contexts under META/. If not available, search
+    # BOOT/RAMDISK/. Note that sometimes we may need a different file_contexts
+    # to build images than the one running on device, such as when enabling
+    # system_root_image. In that case, we must have the one for image
+    # generation copied to META/.
     fc_config = os.path.join(input_dir, "META", "file_contexts")
+    if d.get("system_root_image") == "true":
+      assert os.path.exists(fc_config)
     if not os.path.exists(fc_config):
       fc_config = os.path.join(input_dir, "BOOT", "RAMDISK", "file_contexts")
       if not os.path.exists(fc_config):
@@ -162,6 +167,12 @@
     if fc_config:
       d["selinux_fc"] = fc_config
 
+    # Similarly we need to redirect "ramdisk_dir" and "ramdisk_fs_config".
+    if d.get("system_root_image") == "true":
+      d["ramdisk_dir"] = os.path.join(input_dir, "ROOT")
+      d["ramdisk_fs_config"] = os.path.join(
+          input_dir, "META", "root_filesystem_config.txt")
+
   try:
     data = read_helper("META/imagesizes.txt")
     for line in data.split("\n"):