shortcut of ":" but not /bin/true

[David Boyce]

Just wondering why GNU make (apparently) has logic built in to skip 
creating a new process for the ":" shell builtin but not for the "true" and 
"false" commands. Running truss on Solaris shows that with the following 
Makefile:

% cat Makefile
.PHONY: all
all:
        true
        :
        true
GNU make 3.80 execs the following processes:
12220:  execve("/opt/sfw/bin/gmake", 0xFFBFF32C, 0xFFBFF334)  argc = 1
12222:  execve("/usr/bin/true", 0x0004C980, 0x00052E70)  argc = 2
12224:  execve("/usr/bin/true", 0x0004C980, 0x00052E70)  argc = 2
Is this just an accident of history or is there a reason why it's ok to 
shortcut ":" but not "true"?

This comes up because it's not uncommon for Makefiles to use macro-ized 
commands such as

        RANLIB := true
I recently observed a build system which ran "true" twice for every source 
file it compiled. Of course the workaround is obvious but in many cases 
Makefiles are generated by configure. For instance I just checked the 
Solaris build of Perl 5.8.3 under gmake with truss - it runs /usr/bin/true 
561 times!

Looks like the code to deal with ":" is around job.c:1016, BTW.
Thanks,
David Boyce

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