Re: Wrong order of preprocessor and compiler flags

[Evgeny Grin]

On 24.03.2022 21:37, Nick Bowler wrote:
> On 2022-03-24, Zack Weinberg <zack@owlfolio.org> wrote:
> > On Thu, Mar 24, 2022, at 11:13 AM, Nick Bowler wrote:
> > > However, GNU coding standards state that CFLAGS should be the last
> > > item on compilation commands, so it would appear that this is a case
> > > where traditional "make" behaviour contrasts with GNU standards (which
> > > Automake is following).
> >
> > Huh.  Is there a rationale given in the coding standard?  If not, do you
> > have any idea who might remember the rationale?
>
> The GNU standards just say this[1]:
>
>    "Put CFLAGS last in the compilation command, after other variables
>     containing compiler options, so the user can use CFLAGS to override
>     the others."
>
> When it comes to C(PP)FLAGS the concept of "overriding" options is a
> bit hairy -- many C compiler options do not have direct methods to undo
> their effects -- but whatever.
>
> [1] https://www.gnu.org/prep/standards/standards.html#Command-Variables

Actually example in the GNU standards doesn't use CPPFLAGS, while a few 
lines above it's clearly mentioned "Use CPPFLAGS in any compilation 
command that runs the preprocessor". Compiler runs preprocessor 
internally, so example should have CPPFLAGS with $(CC).

Anyway, I suggest to not read this too literally, but instead see the 
idea behind.
The idea is to give user freedom to override any supplied flags. For 
automake[1] it is supported by AM_CFLAGS, foo_CFLAGS, and CFLAGS.
CFLAGS must be always used after AM_CFLAGS, so user can easily override 
final behaviour without makefiles patching.


To align with make implementations, that never use CPPFLAGS before 
CFLAGS [2] and follow the GNU coding standard, I suggest to use the next 
combination of flags:
$(CC) $(AM_CFLAGS) $(AM_CPPFLAGS) $(CFLAGS) $(CPPFLAGS) -c

CFLAGS and CPPFLAGS are designed to be used by the user. With this 
combination user may override any upstream-supplied AM_CFLAGS and 
AM_CPPFLAGS by CFLAGS (and by CPPFLAGS as well) so GNU standards are 
met. At the same time all CPPFLAGS are used after CFLAGS and this is 
aligned with all 'make' implementations.


[1] 
https://www.gnu.org/software/automake/manual/automake.html#Flag-Variables-Ordering
[2] https://lists.gnu.org/archive/html/autoconf/2022-03/msg00010.html

--
Evgeny

PS Let's move this discussion to the autoconf@gnu.org list

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