--- a/make/InitSupport.gmk	Fri Nov 06 11:11:51 2015 -0800
+++ b/make/InitSupport.gmk	Tue Nov 10 11:41:21 2015 +0100
@@ -321,6 +321,8 @@
   # If any value contains "+", it will be replaced by space.
   define ParseCompareBuild
     ifneq ($$(COMPARE_BUILD), )
+      COMPARE_BUILD_OUTPUT_ROOT := $(TOPDIR)/build/compare-build/$(CONF_NAME)
+
       ifneq ($$(findstring :, $$(COMPARE_BUILD)), )
         $$(foreach part, $$(subst :, , $$(COMPARE_BUILD)), \
           $$(if $$(filter PATCH=%, $$(part)), \
@@ -364,6 +366,73 @@
     endif
   endef
 
+  # Prepare for a comparison rebuild
+  define PrepareCompareBuild
+	$(ECHO) "Preparing for comparison rebuild"
+        # Apply patch, if any
+	$(if $(COMPARE_BUILD_PATCH), $(PATCH) -p1 < $(COMPARE_BUILD_PATCH))
+        # Move the first build away temporarily
+	$(RM) -r $(TOPDIR)/build/.compare-build-temp
+	$(MKDIR) -p $(TOPDIR)/build/.compare-build-temp
+	$(MV) $(OUTPUT_ROOT) $(TOPDIR)/build/.compare-build-temp
+        # Restore an old compare-build, or create a new compare-build directory.
+	if test -d $(COMPARE_BUILD_OUTPUT_ROOT); then \
+	  $(MV) $(COMPARE_BUILD_OUTPUT_ROOT) $(OUTPUT_ROOT); \
+	else \
+	  $(MKDIR) -p $(OUTPUT_ROOT); \
+	fi
+        # Re-run configure with the same arguments (and possibly some additional),
+        # must be done after patching.
+	( cd $(OUTPUT_ROOT) && PATH="$(ORIGINAL_PATH)" \
+	    $(BASH) $(TOPDIR)/configure $(CONFIGURE_COMMAND_LINE) $(COMPARE_BUILD_CONF))
+  endef
+
+  # Cleanup after a compare build
+  define CleanupCompareBuild
+        # If running with a COMPARE_BUILD patch, reverse-apply it
+	$(if $(COMPARE_BUILD_PATCH), $(PATCH) -R -p1 < $(COMPARE_BUILD_PATCH))
+        # Move this build away and restore the original build
+	$(MKDIR) -p $(TOPDIR)/build/compare-build
+	$(MV) $(OUTPUT_ROOT) $(COMPARE_BUILD_OUTPUT_ROOT)
+	$(MV) $(TOPDIR)/build/.compare-build-temp/$(CONF_NAME) $(OUTPUT_ROOT)
+	$(RM) -r $(TOPDIR)/build/.compare-build-temp
+  endef
+
+  # Do the actual comparison of two builds
+  define CompareBuildDoComparison
+        # Compare first and second build. Ignore any error code from compare.sh.
+	$(ECHO) "Comparing between comparison rebuild (this/new) and baseline (other/old)"
+	$(if $(COMPARE_BUILD_COMP_DIR), \
+	  +(cd $(COMPARE_BUILD_OUTPUT_ROOT) && ./compare.sh $(COMPARE_BUILD_COMP_OPTS) \
+	      -2dirs $(COMPARE_BUILD_OUTPUT_ROOT)/$(COMPARE_BUILD_COMP_DIR) $(OUTPUT_ROOT)/$(COMPARE_BUILD_COMP_DIR) || true), \
+	  +(cd $(COMPARE_BUILD_OUTPUT_ROOT) && ./compare.sh $(COMPARE_BUILD_COMP_OPTS) \
+	      -o $(OUTPUT_ROOT) || true) \
+	)
+  endef
+
+  define PrintFailureReports
+	$(if $(wildcard $(MAKESUPPORT_OUTPUTDIR)/failure-logs/*), \
+	  $(PRINTF) "=== Output from failing command(s) repeated here ===\n" $(NEWLINE) \
+	  $(foreach logfile, $(sort $(wildcard $(MAKESUPPORT_OUTPUTDIR)/failure-logs/*)), \
+	      $(PRINTF) "* For target $(notdir $(basename $(logfile))):\n" $(NEWLINE) \
+	      $(CAT) $(logfile) | $(GREP) -v -e "^Note: including file:" $(NEWLINE) \
+	  ) \
+	  $(PRINTF) "=== End of repeated output ===\n" \
+	)
+  endef
+
+  define PrintBuildLogFailures
+	if $(GREP) -q "recipe for target .* failed" $(BUILD_LOG) 2> /dev/null; then  \
+	  $(PRINTF) "=== Make failure sequence repeated here ===\n" ; \
+	  $(GREP) "recipe for target .* failed" $(BUILD_LOG) ; \
+	  $(PRINTF) "=== End of repeated output ===\n" ; \
+	  $(PRINTF) "Hint: Try searching the build log for the name of the first failed target.\n" ; \
+	else \
+	  $(PRINTF) "No indication of failed target found.\n" ; \
+	  $(PRINTF) "Hint: Try searching the build log for '] Error'.\n" ; \
+	fi
+  endef
+
   define RotateLogFiles
 	$(RM) $(BUILD_LOG).old 2> /dev/null
 	$(MV) $(BUILD_LOG) $(BUILD_LOG).old 2> /dev/null || true