Re: Always use -c -o when compiling C

[Stefano Lattarini]

On 05/30/2013 12:14 AM, Peter Rosin wrote:
> Hi!
> 
> I just tried the testsuite on maint with MSYS/MSVC and I have one
> failure, namely in silent-many-languages.sh. The reason it fails is
> that while I have specified MSVC as the C and C++ compiler, I didn't
> bother to say that I wasn't interested in fortran. The many-lang
> test then mixes the output from the GCC fortran compiler with the
> output from MSVC, which is perhaps not very wise but appears to work
> for the trivial code in the testcase once you fix the automake
> machinery to allow it.
> 
> The problem with the automake machinery in this case is that GCC
> uses foo.o, while MSVC uses foo.obj, and it so happens that GCC
> "wins" and OBJEXT is set to "o". This, coupled with the fact that
> automake tries not to use the -o option unless it's really needed
> causes make rules to be created w/o -o, but MSVC will then create
> foo.obj, which fails badly during linking when automake expects
> foo.$OBJEXT, i.e. foo.o.
> 
> So, since the maint branch already assumes that -c -o works, how
> about the below change? It makes the test PASS for me, and I see
> no regression in any other test (but I have a lot of SKIPs, so
> I'm not really confident that the change will not cause some
> spurious FAILs in any of those).
> 
> The concept seems sound to me, but it's perhaps too dangerous?
> 
> Cheers,
> Peter
> 
> diff --git a/bin/automake.in b/bin/automake.in
> index 24ff2a6..40b3181 100644
> --- a/bin/automake.in
> +++ b/bin/automake.in
> @@ -632,6 +632,7 @@ register_language ('name' => 'c',
>                    'linker' => 'LINK',
>                    'link' => '$(CCLD) $(AM_CFLAGS) $(CFLAGS) $(AM_LDFLAGS) 
> $(LDF
>                    'compile_flag' => '-c',
> +                  'output_flag' => '-o',
>                    'libtool_tag' => 'CC',
>                    'extensions' => ['.c']);
> 
> @@ -1313,14 +1314,6 @@ sub handle_languages ()
>         if (((! option 'no-dependencies') && $lang->autodep ne 'no')
>             || defined $lang->compile)
>         {
> -           # Some C compilers don't support -c -o.  Use it only if really
> -           # needed.
> -           my $output_flag = $lang->output_flag || '';
> -           $output_flag = '-o'
> -             if (! $output_flag
> -                 && $lang->name eq 'c'
> -                 && option 'subdir-objects');
> -
>             # Compute a possible derived extension.
>             # This is not used by depend2.am.
>             my $der_ext = ($lang->output_extensions->($ext))[0];
> @@ -1364,7 +1357,7 @@ sub handle_languages ()
> 
>                              COMPILE   => '$(' . $lang->compiler . ')',
>                              LTCOMPILE => '$(LT' . $lang->compiler . ')',
> -                            -o        => $output_flag,
> +                            -o        => $lang->output_flag,
>                              SUBDIROBJ => !! option 'subdir-objects');
>         }
> 
> 
I think we already have something similar in master, in fact.  Not sure
why I didn't apply that directly to maint, since all the changes necessary
for such a simplification are in maint as well.  Perhaps because I did
this simplification in a middle of another master-based patch series, so
that it didn't apply cleanly to maint?  Probably so.

In short, I agree this change goes in the right direction.  Thank you.
Extra kudos if you can prepare a patch with the complete explanation,
and maybe even a reference to the similar change already landed in
master.  I will give a fresh round of tests on Fedora, Debian, BSD
and Solaris tomorrow.

Regards and thanks,
  Stefano