Re: AC_DEFUN_ONCE semantics

[Eric Blake]

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According to Paolo Bonzini on 1/27/2009 1:03 AM:
> I think that only the following macros must absolutely be defined with
> AC_DEFUN_ONCE: AC_CANONICAL_*, AC_USE_SYSTEM_EXTENSIONS.
> 
> Regarding AC_PROG_* macros, I think most of them pose no problems, but I
> agree that this should be left to a future version N years down the
> road.

OK, on the argument that it is not fixing a bug, but just optimizing
performance (even for things like AC_PROG_MKDIR_P, where I've already
proven that it shrinks the size of coreutils), I will not change any of
the AC_PROG on this go-around.  I'll post my revised patch for review soon.

> Two other points.  First, it's interesting that AC_PROG_LEX is already
> defined with AC_DEFUN_ONCE, and since 2001-08-22 for that matter (I
> don't really understand why).

Yes, I noticed that one myself.

> Second, Eric, I have a doubt: does this idiom work with the new
> AC_DEFUN_ONCE?
> 
>   AC_PROVIDE_IFELSE([AC_PROG_CXX],
>                     [_AM_DEPENDENCIES(CXX)],
>                     [m4_define([AC_PROG_CXX],
>                      m4_defn([AC_PROG_CXX])[_AM_DEPENDENCIES(CXX)])])dnl
> 
> I think so, but I'd rather ask you?

If AC_PROG_CXX has already been expanded, then expand just
_AM_DEPENDENCIES(CXX); otherwise, redefine AC_PROG_CXX so that the next
guy to actually expand it is now expanding the original plus an additional
_AM_DEPENDENCIES(CXX).  I don't see anything wrong with that construct
(other than a potentially missing [] prior to _AM_DEPENDENCIES in case
AC_PROG_CXX originally ended in something that looks like a potential
macro name), or any reason why its semantics would have changed.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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