Wrong order of preprocessor and compiler flags

[Evgeny Grin]

Hello,

This discussion was started initially in the autoconf list. [1]
Automake and autoconf use compiler and preprocessor flags in different 
order.
Within 'configure' scripts, compile checks/tests are performed as [2]:
$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&AS_MESSAGE_LOG_FD
but resulting flags are used in another order in automake makefiles:
$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) 
$(AM_CFLAGS) $(CFLAGS)

Automake uses CPPFLAGS before CFLAGS [3].

In the following research I found than no 'make' implementation uses 
CPPFLAGS before CFLAGS [4]. Almost all 'make' implementations (GNU, 
OpenBSD, NetBSD, Solaris) put CPPFLAGS after CFLAGS, the only exception 
is FreeBSD version, which doesn't use CPPFLAGS at all.

glibc uses CFLAGS before CPPFLAGS [5][6].

While usage of CPPFLAGS before CFLAGS looks more logical, seems that 
majority of the software tools use flags in other order.

GNU Coding Standards recommends to put CFLAGS at the end of command line 
parameters [7] to give user ability to override upstream-supplied flags.

I think that automake should be aligned with other tools for several 
reasons:
* automated build systems are using the same CFLAGS, CPPFLAGS for 
packages built with autotools and pure Makefiles. Currently if may give 
different results.
* 'configure' results are based on CFLAGS CPPFLAGS, but automake's 
makefiles use CPPFLAGS CFLAGS which may produce result different from 
expected.

I see several ways to implement it in automake:
1. Use AM_CFLAGS AM_CPPFLAGS CFLAGS CPPFLAGS. I think this is the best 
option. As required by GNU Standards, CFLAGS still override all 
upstream-defined flags.
2. Use AM_CFLAGS CFLAGS AM_CPPFLAGS CPPFLAGS. This is more aligned with 
current flags grouping, but CFLAGS will not override definitions in 
AM_CPPFLAGS (less aligned with GNU Standards).
3. Use AM_CFLAGS AM_CPPFLAGS CPPFLAGS CFLAGS.
4. Use AM_CPPFLAGS AM_CFLAGS CFLAGS CPPFLAGS.
Although I can find arguments for the last two options, I don't think 
they make any real sense.

To avoid any possible breakage of existing packages, some new automake 
option could be introduced, with name like "right-flags-order" with 
warning that flag will be default soon.

I may work on patches, if my proposal will be accepted.


[1] https://lists.gnu.org/archive/html/autoconf/2022-03/msg00004.html
[2] 
https://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=blob;f=lib/autoconf/c.m4;hb=00358457d09c19ff6b5ec7ed98708540d1994a5f#l64
[3] 
https://git.savannah.gnu.org/cgit/automake.git/tree/bin/automake.in?id=fee9a828bcc968656edfc89e38b157c28d6335f0#n700
[4] https://lists.gnu.org/archive/html/autoconf/2022-03/msg00010.html
[5] 
https://sourceware.org/git/?p=glibc.git;a=blob;f=Makefile;hb=305769b2a15c2e96f9e1b5195d3c4e0d6f0f4b68#l528
[6] https://lists.gnu.org/archive/html/autoconf/2022-03/msg00008.html
[7] https://www.gnu.org/prep/standards/standards.html#Command-Variables

--
Evgeny

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