Re: Question about format.c check for __GNUC_MINOR__ of 6

[Eric Blake]

On Wed, Sep 06, 2023 at 03:08:33PM -0700, Mike Fulton wrote:
> Hello,
> 
> The m4 dev line ( git://git.savannah.gnu.org/m4.git )
> has the following code:
> 
> ```
> #if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
> # pragma GCC diagnostic push
> # pragma GCC diagnostic ignored "-Wformat-nonliteral"
> #endif
> ```

That expression appears to be designed to reject gcc 3.5 and older,
while accepting 3.6 and newer.

I also wonder if modern gnulib provides a better way to test things
than just open-coding it (as Bruno has done a lot of work testing
features of newer compilers).  This may be one case where ignoring the
diagnostic for older builds is desirable - there comes a point where
you can't please both old and new compilers simultaneously, but the
target that should be warning-free is the newer compiler.

That is, maybe it's time to just rewrite the check to something as
simple as #if 8 <= __GNUC__ (that is, only add the pragmas on a much
newer, albeit arbitrary, point in time)?

Conversely, I see that m4 uses gnulib's lib/c-stack.c, which also has:

/* Pacify GCC 9.3.1, which otherwise would complain about segv_handler.  */
# if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
#  pragma GCC diagnostic ignored "-Wsuggest-attribute=pure"
# endif

so it's not like m4 is doing anything radically different from gnulib
when it comes to pragmas for ignoring certain diagnostics; but thd
difference may be which diagnostic is being ignored.

> 
> This produces an error on the clang compiler I am using on z/OS, which is
> at 4.2 with the xasprintf code that follows, e.g.
> 
> ```
> str = xasprintf (fstart, width, ARG_INT(argc, argv));
> ```
> 
> If I change the check to:
> 
> ```
> #if 4 < __GNUC__ + (2 <= __GNUC_MINOR__)
> # pragma GCC diagnostic push
> # pragma GCC diagnostic ignored "-Wformat-nonliteral"
> #endif
> ```
> 
> all is good - it compiles clean. The question is whether the check for 6 is
> too high or if the clang compiler on z/OS has a bug (or something else)?

What does clang claim to be?  That is, what do you get for
  print '__GNUC__ __GNUC_MINOR__' | clang -E - | tail -n1

and what is the actual compiler error you got when the build failed on
the original source?

> I haven't been able to determine what level of gcc provides this diagnostic.
> 
> thanks, mike

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
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