kbuild: change if_changed_rule for multi-line recipe

The 'define' ... 'endef' directive is useful to confine a series of
shell commands into a single macro:

  define foo
          [action1]
          [action2]
          [action3]
  endif

Each action is executed in a separate subshell.

However, rule_cc_o_c and rule_as_o_S in scripts/Makefile.build are
written as follows (with a trailing semicolon in each cmd_*):

  define rule_cc_o_c
          [action1] ; \
          [action2] ; \
          [action3] ;
  endef

All shell commands are concatenated with '; \' so that it looks like
a single command from the Makefile point of view. This does not
exploit the benefits of 'define' ... 'endef' form because a single
shell command can be more simply written, like this:

  rule_cc_o_c = \
          [action1] ; \
          [action2] ; \
          [action3] ;

I guess the intention for the command concatenation was to let the
'@set -e' in if_changed_rule cover all the commands.

We can improve the readability by moving '@set -e' to the 'cmd' macro.
The combo of $(call echo-cmd,*) $(cmd_*) in rule_cc_o_c and rule_as_o_S
have been replaced with $(call cmd,*). The trailing back-slashes have
been removed.

Here is a note about the performance: the commands in rule_cc_o_c and
rule_as_o_S were previously executed all together in a single subshell,
but now each line in a separate subshell. This means Make will spawn
extra subshells [1]. I measured the build performance for
  x86_64_defconfig + CONFIG_MODVERSIONS + CONFIG_TRIM_UNUSED_KSYMS
and I saw slight performance regression, but I believe code readability
and maintainability wins.

[1] Precisely, GNU Make may optimize this by executing the command
    directly instead of forking a subshell, if no shell special
    characters are found in the command line and omitting the subshell
    will not change the behavior.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

1 files changed, 11 insertions, 11 deletions

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 23ebf2508234..a0002246366b 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -260,20 +260,20 @@ cmd_gen_ksymdeps = \
 endif
 
 define rule_cc_o_c
-	$(call echo-cmd,checksrc) $(cmd_checksrc)			  \
-	$(call cmd_and_fixdep,cc_o_c)					  \
-	$(cmd_gen_ksymdeps)						  \
-	$(cmd_checkdoc)							  \
-	$(call echo-cmd,objtool) $(cmd_objtool)				  \
-	$(cmd_modversions_c)						  \
-	$(call echo-cmd,record_mcount) $(cmd_record_mcount)
+	$(call cmd,checksrc)
+	$(call cmd_and_fixdep,cc_o_c)
+	$(call cmd,gen_ksymdeps)
+	$(call cmd,checkdoc)
+	$(call cmd,objtool)
+	$(call cmd,modversions_c)
+	$(call cmd,record_mcount)
 endef
 
 define rule_as_o_S
-	$(call cmd_and_fixdep,as_o_S)					  \
-	$(cmd_gen_ksymdeps)						  \
-	$(call echo-cmd,objtool) $(cmd_objtool)				  \
-	$(cmd_modversions_S)
+	$(call cmd_and_fixdep,as_o_S)
+	$(call cmd,gen_ksymdeps)
+	$(call cmd,objtool)
+	$(call cmd,modversions_S)
 endef
 
 # List module undefined symbols (or empty line if not enabled)