linux-kernel - Re: [PATCH 5/8] kbuild: change if_changed

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Message-ID: <6ee62b52-5d6d-c540-5f01-f522b6986c4d@rasmusvillemoes.dk>
Date:   Thu, 15 Nov 2018 10:12:18 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kbuild@...r.kernel.org
Cc:     Sam Ravnborg <sam@...nborg.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] kbuild: change if_changed_rule to accept multi-line
 recipe

On 15/11/2018 09.27, Masahiro Yamada wrote:
> GNU Make supports 'define' ... 'endef' directive, which can describe
> a recipe that consists of multiple lines.
> 
>   endef
> 
> This does not actually exploit the benefits of 'define' ... 'endef'
> form. All shell commands must be concatenated with '; \' so that it
> looks like a single command from the Makefile point of view. '@' can
> only appear before the first action.
> 
> The root cause of this misfortune is the '@set -e;' in if_changed_rule.
> It is easily solvable 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
> were replaced with $(call cmd,*). The tailing back-slashes went away.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
>  
>  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

Does this mean that Make now spawns a new shell for each of these
commands, and if so, what's the performance impact? Or am I just
misreading things? If this does change the semantics (one shell instance
versus many), I think that's worth mentioning explicitly in the
changelog, regardless of whether there's no measuarable performance impact.

Rasmus