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

From: Paolo Bonzini
Subject: Re: [PATCH v2 1/3] autoconf: prefer an unrolled loop for trivial AC_CHECK_FUNCS
Date: Thu, 3 Nov 2016 00:41:04 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

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 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
> 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

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


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