Makefile: add GNU global tags support

GNU Global is another tags engine which is more like cscope in being
able to support finding both references and definitions. You will be
un-surprised to know it also integrates well with Emacs.

The main benefit of integrating it into find-src-path is it takes less
time to rebuild the database from scratch when you have a lot of build
directories under your source tree.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210114165730.31607-3-alex.bennee@linaro.org>

.gitignore

@@ -7,6 +7,9 @@
 cscope.*
 tags
 TAGS
+GPATH
+GRTAGS
+GTAGS
 *~
 *.ast_raw
 *.depend_raw

Makefile

@@ -253,6 +253,18 @@ ctags:
 	rm -f "$(SRC_PATH)/"tags
 	$(find-src-path) -exec ctags -f "$(SRC_PATH)/"tags --append {} +
 
+.PHONY: gtags
+gtags:
+	$(call quiet-command, 			\
+		rm -f "$(SRC_PATH)/"GTAGS; 	\
+		rm -f "$(SRC_PATH)/"GRTAGS; 	\
+		rm -f "$(SRC_PATH)/"GPATH, 	\
+		"GTAGS", "Remove old $@ files")
+	$(call quiet-command, 				\
+	        (cd $(SRC_PATH) && 			\
+		 $(find-src-path) | gtags -f -), 	\
+		"GTAGS", "Re-index $(SRC_PATH)")
+
 .PHONY: TAGS
 TAGS:
 	rm -f "$(SRC_PATH)/"TAGS
@@ -279,7 +291,7 @@ help:
 	$(call print-help,all,Build all)
 	$(call print-help,dir/file.o,Build specified target only)
 	$(call print-help,install,Install QEMU, documentation and tools)
-	$(call print-help,ctags/TAGS,Generate tags file for editors)
+	$(call print-help,ctags/gtags/TAGS,Generate tags file for editors)
 	$(call print-help,cscope,Generate cscope index)
 	$(call print-help,sparse,Run sparse on the QEMU source)
 	@echo  ''