| Message ID | CAK7LNATAeMmfwWiZha17mbyWhF6bR5RGjv+gbsQTvL4teoTiPQ@mail.gmail.com |
|---|---|
| State | New |
| Headers | show |
Hi Andy, 2016-03-16 5:48 GMT+09:00 Andy Lutomirski <luto@amacapital.net>: > On Tue, Mar 15, 2016 at 1:45 PM, Michal Marek <mmarek@suse.com> wrote: >> Dne 15.3.2016 v 19:27 Andy Lutomirski napsal(a): >>> Fair enough, although I'm curious why this happens. It might be worth >>> changing the docs to say that .PHONY is *not* an substitute for FORCE >>> in that context, then. >> >> These two are unrelated, except that FORCE is redundant for a .PHONY >> target. FORCE is our idiom to tell make to always remake the target and >> let us handle the dependencies manually. Listing a target as .PHONY >> tells make that the target will not produce a file of the same name >> (typically, "all", "install", etc). >> > > Except that apparently if-changed doesn't work on .PHONY targets that > don't specify FORCE, which confuses me. OK, I will try to explain it. I hope this will help you, not confuse you even more... I think the difference of Kbuild behavior comes down to "$?" behavior of GNU Make. Please see the definition of "if_changed". if_changed = $(if $(strip $(any-prereq) $(arg-check)), \ @set -e; \ $(echo-cmd) $(cmd_$(1)); \ printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd) The "if_changed" does some actions if "any-prereq" or "arg-check" is non-empty. Next, let's take a look at the definition of "any-prereq" any-prereq = $(filter-out $(PHONY),$?) $(filter-out $(PHONY) $(wildcard $^),$^) It seems $? makes the difference between FORCE and PHONY. The GNU Make manual says as follows: -------------------->8----------------------------------- $? The names of all prerequisites that are newer than the target, separated by spaces. --------------------8<----------------------------------- From this statement, it is unclear what happens to $? if the target is a PHONY target. I am not a GNU Make developer, so I have no idea about the detailed implementation, but anyway let's try simple experiments. [Example 3] Let's write a simple Makefile as follows. --------------------->8--------------------- foo: bar FORCE echo $$? is "$?" cp $< $@ bar: touch $@ .PHONY: FORCE FORCE: ----------------------8<-------------------- yamada@beagle:~/workspace/test$ rm -f foo bar yamada@beagle:~/workspace/test$ make touch bar echo $? is "bar FORCE" 0 is bar FORCE cp bar foo yamada@beagle:~/workspace/test$ make echo $? is "" 0 is cp bar foo yamada@beagle:~/workspace/test$ make echo $? is "" 0 is cp bar foo As we expected, $? contains the prerequisite "bar", but it is empty for the second run or later (because "foo" is newer than "bar"). [Example 4] Let's replace the FORCE with PHONY. --------------------->8--------------------- .PHONY: foo foo: bar echo $$? is "$?" cp $< $@ bar: touch $@ ----------------------8<-------------------- yamada@beagle:~/workspace/test2$ rm -f foo bar yamada@beagle:~/workspace/test2$ make touch bar echo $? is "bar" 0 is bar cp bar foo yamada@beagle:~/workspace/test2$ make echo $? is "bar" 0 is bar cp bar foo yamada@beagle:~/workspace/test2$ make echo $? is "bar" 0 is bar cp bar foo This time, $? always contains "bar" nevertheless "foo" is newer than "bar" on the second run or later. So, it looks like GNU Make assumes that all the prerequisites of a PHONY target are always newer than the target. Go back to the definition of "any-prereq", any-prereq = $(filter-out $(PHONY),$?) $(filter-out $(PHONY) $(wildcard $^),$^) If the prerequisite is not a PHONY target (like the "baz" in the example 2 in my former email), it is not filtered out from $?. So, any-prereq is non-empty and if_changed updates the target. -- Best Regards Masahiro Yamada
diff --git a/Makefile b/Makefile index 7b3ecdc..89b7d0d 100644 --- a/Makefile +++ b/Makefile @@ -914,6 +914,22 @@ export KBUILD_ALLDIRS := $(sort $(filter-out arch/%,$(vmlinux- vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) +quiet_cmd_gen_foo = FOO $@ + cmd_gen_foo = (cat $<; echo hello) > $@ + +foo: bar FORCE + $(call if_changed,gen_foo) + +quiet_cmd_gen_bar = BAR $@ + cmd_gen_bar = (cat $<; echo $(GREETING)) > $@ + +bar: baz FORCE + $(call if_changed,gen_bar) + +baz: + @touch $@ + + # Final link of vmlinux cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) quiet_cmd_link-vmlinux = LINK $@ Try the following. yamada@beagle:~/workspace/linux$ git clean -x -f yamada@beagle:~/workspace/linux$ make -s defconfig yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo scripts/kconfig/conf --silentoldconfig Kconfig BAR bar FOO foo yamada@beagle:~/workspace/linux$ make GREETING=GoodMorning foo make: `foo' is up to date. yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo BAR bar FOO foo yamada@beagle:~/workspace/linux$ make GREETING=GoodAfternoon foo make: `foo' is up to date. yamada@beagle:~/workspace/linux$ make GREETING=GoodEvening foo BAR bar FOO foo yamada@beagle:~/workspace/linux$ make GREETING=GoodEvening foo make: `foo' is up to date. Please notice "foo" and "bar" were not rebuilt when I gave the same command line as the previous run. When I changed the command line, "bar" was update and "foo" was also updated because "foo" depends on "bar". It means $(call if_changed,...) is working as expected. Example 2) Add the following to the top Makefile. Please notice that I just replaced "FORCE" with ".PHONY". diff --git a/Makefile b/Makefile index 7b3ecdc..a0899c3 100644 --- a/Makefile +++ b/Makefile @@ -914,6 +914,24 @@ export KBUILD_ALLDIRS := $(sort $(filter-out arch/%,$(vmlinux- vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) +quiet_cmd_gen_foo = FOO $@ + cmd_gen_foo = (cat $<; echo hello) > $@ + +.PHONY: foo +foo: bar + $(call if_changed,gen_foo) + +quiet_cmd_gen_bar = BAR $@ + cmd_gen_bar = (cat $<; echo $(GREETING)) > $@ + +.PHONY: bar +bar: baz + $(call if_changed,gen_bar) + +baz: + @touch $@ + + # Final link of vmlinux cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux)