Blue Swirl <blauwir...@gmail.com> writes:
> Add logic to detect changes in generated files. If the old
> and new files are identical, don't touch the generated file.
> This avoids a lot of churn since many files depend on trace.h.
>
> Signed-off-by: Blue Swirl <blauwir...@gmail.com>
> ---
> Makefile | 18 ++++++++++++++++--
> 1 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index ff39025..085e8ed 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -107,10 +107,24 @@ ui/vnc.o: QEMU_CFLAGS += $(VNC_TLS_CFLAGS)
> bt-host.o: QEMU_CFLAGS += $(BLUEZ_CFLAGS)
>
> trace.h: $(SRC_PATH)/trace-events config-host.mak
> - $(call quiet-command,sh $(SRC_PATH)/tracetool --$(TRACE_BACKEND) -h
> < $< > $@," GEN $@")
> + $(call quiet-command,sh $(SRC_PATH)/tracetool --$(TRACE_BACKEND) -h
> < $< > $...@.tmp," GEN $@")
> + @if test -f $@; then \
> + if ! cmp -s $@ $...@.tmp; then \
> + mv $...@.tmp $@; \
> + fi; \
> + else \
> + mv $...@.tmp $@; \
> + fi
Why the conditional? cmp -s fails fine when argument files don't exist.
Note that common versions of make including GNU Make do not stat a
rule's target to check whether the rule changed it. Instead, they
assume it changed, and remake everything depending on it.
arm...@blackfin:~/tmp$ cat Makefile
foo: bar
echo "Remaking foo"
bar:
[ -f $@ ] || touch $@ && echo "Touched bar"
arm...@blackfin:~/tmp$ rm -f foo
arm...@blackfin:~/tmp$ make
[ -f bar ] || touch bar && echo "Touched bar"
Touched bar
echo "Remaking foo"
Remaking foo
arm...@blackfin:~/tmp$ make
echo "Remaking foo"
Remaking foo
I doubt your patch avoids churn as advertized with such makes.
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