The 'define' ... 'endef' directive can describe a recipe that consists
of multiple lines. For example,
all:
@echo hello
@echo world
... can be written as:
define greeting
@echo hello
@echo world
endif
all:
$(greeting)
This is useful to confine a series of shell commands into a single
macro without losing readability.
However, rule_cc_o_c and rule_as_o_S in scripts/Makefile.build are
written like this (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 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@xxxxxxxxxxxxx>
---
Changes in v2:
- Rewrite commit message more precisely, and mention about the
build performance
scripts/Kbuild.include | 6 ++----
scripts/Makefile.build | 22 +++++++++++-----------
2 files changed, 13 insertions(+), 15 deletions(-)
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 4b943f4..978d72b 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -215,7 +215,7 @@ echo-cmd = $(if $($(quiet)cmd_$(1)),\
echo ' $(call escsq,$($(quiet)cmd_$(1)))$(echo-why)';)
# printing commands
-cmd = @$(echo-cmd) $(cmd_$(1))
+cmd = @set -e; $(echo-cmd) $(cmd_$(1))
# Add $(obj)/ for paths that are not absolute
objectify = $(foreach o,$(1),$(if $(filter /%,$(o)),$(o),$(obj)/$(o)))
@@ -268,9 +268,7 @@ cmd_and_fixdep = \
# Usage: $(call if_changed_rule,foo)
# Will check if $(cmd_foo) or any of the prerequisites changed,
# and if so will execute $(rule_foo).
-if_changed_rule = $(if $(strip $(any-prereq) $(arg-check) ), \
- @set -e; \
- $(rule_$(1)), @:)
+if_changed_rule = $(if $(strip $(any-prereq) $(arg-check)), $(rule_$(1)), @:)
###
# why - tell why a target got built
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index c23ee45..e445b3e 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)
--
2.7.4
