Add a make target for generating Ctags file

Makefile

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 414 matching lines @@
         rm -rf $(OUTDIR)/$(basename $@).release
         rm -rf $(OUTDIR)/$(basename $@).debug
         rm -rf $(OUTDIR)/$(basename $@).optdebug
         find $(OUTDIR) -regex '.*\(host\|target\)\.$(basename $@).*\.mk' -delete
 
 native.clean:
         rm -f $(OUTDIR)/Makefile.native
         rm -rf $(OUTDIR)/native
         find $(OUTDIR) -regex '.*\(host\|target\)\.native\.mk' -delete
 
-clean: $(addsuffix .clean, $(ARCHES) $(ANDROID_ARCHES) $(NACL_ARCHES)) native.clean gtags.clean
+clean: $(addsuffix .clean, $(ARCHES) $(ANDROID_ARCHES) $(NACL_ARCHES)) native.clean gtags.clean tags.clean
 
 # GYP file generation targets.
 OUT_MAKEFILES = $(addprefix $(OUTDIR)/Makefile.,$(BUILDS))
 $(OUT_MAKEFILES): $(GYPFILES) $(ENVFILE)
         $(eval CXX_TARGET_ARCH:=$(shell $(CXX) -v 2>&1 | grep ^Target: | \
                 cut -f 2 -d " " | cut -f 1 -d "-" ))
         $(eval CXX_TARGET_ARCH:=$(subst aarch64,arm64,$(CXX_TARGET_ARCH)))
         $(eval CXX_TARGET_ARCH:=$(subst x86_64,x64,$(CXX_TARGET_ARCH)))
         $(eval V8_TARGET_ARCH:=$(subst .,,$(suffix $(basename $@))))
         PYTHONPATH="$(shell pwd)/tools/generate_shim_headers:$(shell pwd)/build:$(PYTHONPATH):$(shell pwd)/build/gyp/pylib:$(PYTHONPATH)" \
@@ skipping 44 matching lines @@
 grokdump: ia32.release
         @cat $(DUMP_FILE).tmpl > $(DUMP_FILE)
         @$(OUTDIR)/ia32.release/d8 --dump-heap-constants >> $(DUMP_FILE)
 
 # Support for the GNU GLOBAL Source Code Tag System.
 gtags.files: $(GYPFILES) $(ENVFILE)
         @find include src test -name '*.h' -o -name '*.cc' -o -name '*.c' > $@
 
 # We need to manually set the stack limit here, to work around bugs in
 # gmake-3.81 and global-5.7.1 on recent 64-bit Linux systems.
-GPATH GRTAGS GSYMS GTAGS: gtags.files $(shell cat gtags.files 2> /dev/null)
+# Using $(wildcard ...) gracefully ignores non-existing files, so that stale
+# gtags.files after switching branches don't cause recipe failures.
+GPATH GRTAGS GSYMS GTAGS: gtags.files $(wildcard $(shell cat gtags.files 2> /dev/null))
         @bash -c 'ulimit -s 10240 && GTAGSFORCECPP=yes gtags -i -q -f $<'
 
 gtags.clean:
         rm -f gtags.files GPATH GRTAGS GSYMS GTAGS
 
+tags: gtags.files $(wildcard $(shell cat gtags.files 2> /dev/null))
+        @(ctags --version | grep 'Exuberant Ctags' >/dev/null) || \
+                (echo "Please install Exuberant Ctags (check 'ctags --version')" >&2; false)
+        ctags --fields=+l -L $<
+
+tags.clean:
+        rm -r tags
+
 dependencies builddeps:
         $(error Use 'gclient sync' instead)