Re: [PATCH v2 1/3] autoconf: prefer an unrolled loop for trivial AC_CHECK_FUNCS

[Paolo Bonzini]

On 02/11/2016 23:14, Eric Blake wrote:
> Our use of AC_REQUIRE hoists the check outside of any AS_IF or similar
> code.  However, while I don't think any well-written configure.ac script
> will be checking $ac_cv_func_foo prior to calling
> AC_CHECK_FUNC_ONCE(foo), I _am_ a bit worried that poorly written
> scripts that do:
> 
> if condition
>   AC_CHECK_FUNC_ONCE(foo)
> fi
> test $ac_cv_func_foo

I'm not worried about this.  However, now that I think more about it,
I'm a bit more worried about something that has a deep chain of
AC_REQUIREs, and ultimately ends up expanding AC_CHECK_FUNC_ONCE inside
an "if".

> Potential solution: collect the list of AC_CHECK_FUNC_ONCE functions in
> an m4 list, and unroll that list where we used to do the AS_FOR, so that
> we aren't changing the semantics of hoisting all the checks up front
> early during configure.

How would you do that?  You would have to expand before the first
AC_CHECK_FUNC_ONCE, but you do not have a diversion there (you might
need one per language, in fact).

Perhaps you can put the whole code for the tests in a variable (it's
just a couple lines of shell if you add ac_fn_simple_define) in
INIT_PREPARE or SHELL_FN, and then eval it in _AC_FUNCS_EXPANSION.

That said, what I'm doing now is actually already done by
AC_CHECK_DECLS_ONCE, so we might just document it as a possible
incompatibility.

Paolo