Re: double-colon rules and multi-arch builds

[Greg Chicares]

On 2006-3-2 21:51 UTC, Robert P. J. Day wrote:
> 
> http://make.paulandlesley.org/multi-arch.html#advanced
> 
>   part of that makefile are the rules:
> 
>  Makefile : ;
>  %.mk :: ;
> 
>  % :: $(OBJDIR) ; :
> 
> and the question is simply, do either of those last two rules
> *require* the double colon as opposed to a single colon?  i'm not
> interested in issues of efficiency, just in absolute requirement.

Just efficiency, not correctness, unless I overlooked something
in my experiments.

Efficiency has more aspects than just speed. I believe that the
commands in my makefiles run slowly enough (compiling template-
heavy C++ code) to overwhelm any overhead that 'make' adds:
remaking an already-up-to-date system takes only one second.
The greatest benefit I see in the efficiency of double-colon
rules here is that it makes 'make -d' output shorter, hence
easier to read.

Running my build system (based on the multi-arch example) with
a 'show_flags' target whose rules run quickly, and selectively
changing double to single colons, I see different savings for
different double colons. I'm using 'make -d show_flag 2>&1 |wc'
to measure "efficiency" as number of lines of debug output.

>  Makefile : ;

A double colon in my equivalent of this line saves nine lines
of debug output.

>  %.mk :: ;

Nine lines saved here by the double colon.

>  % :: $(OBJDIR) ; :

Here, I measure no difference. I write '; @:' for '; :' here.

The biggest savings--almost two thousand lines--comes from
using a double colon for autodependencies:
  -include *.d
  *.d:: ;

Here's a part of my main makefile; I hope either that it helps
you, or that someone helps me by pointing out any errors.

# Included files that don't need to be remade are given explicit empty
# commands, which significantly reduces the number of lines emitted by
# 'make -d', making debug output easier to read.
#
# Included makefiles are cited by absolute pathname, e.g.
#   include $(src_dir)/included-file
#   $(src_dir)/included-file:: ;
# An '--include-dir' option could make an absolute path unnecessary
# in the 'include' statement itself, but the empty remake command
# would not respect '--include-dir'.
#
# The double-colon rule elicits warnings like
#   Makefile `foo' might loop; not remaking it.
# with 'make -d'. The "stupidly" comment at line 1745 of GNU make's
# 'main.c' version 1.194 seems to suggest that writing such a rule is
# a poor practice, but empty commands ought to be excused from that
# inline comment, and perhaps even from that diagnostic.