Subject: Re: [PATCH 2/2] fix clang -Wtautological-compare with unsigned enum

From: John Keeping <[email-masked]>
Date: Thu, 17 Jan 2013 11:00:08 +0000

On Thu, Jan 17, 2013 at 11:32:39AM +0100, Antoine Pelisse wrote:
> John, could you confirm that you trigger the -Wtautological-compare
> warning with your version of clang ?

Yes, the warning is still there with both 3.2 and a recent trunk build
but this patch squelches it.

> And that this patch makes clang compilation warning-free (with the
> very latest clang) ?

There is one remaining warning on pu which hasn't been discussed in this
thread as far as I can see.  I'll send a patch shortly.

There's also a warning that triggers with clang 3.2 but not clang trunk, which
I think is a legitimate warning - perhaps someone who understands integer type
promotion better than me can explain why the code is OK (patch->score is
declared as 'int'):

builtin/apply.c:1044:47: warning: comparison of constant 18446744073709551615
    with expression of type 'int' is always false
    [-Wtautological-constant-out-of-range-compare]
        if ((patch->score = strtoul(line, NULL, 10)) == ULONG_MAX)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~

> On Wed, Jan 16, 2013 at 11:47 PM, Antoine Pelisse <[email-masked]> wrote:
> > Create a GREP_HEADER_FIELD_MIN so we can check that the field value is
> > sane and silent the clang warning.
> >
> > Clang warning happens because the enum is unsigned (this is
> > implementation-defined, and there is no negative fields) and the check
> > is then tautological.
> >
> > Signed-off-by: Antoine Pelisse <[email-masked]>
> > ---
> > I tried to consider discussion [1] and this [2] discussion on clang's list
> >
> > With these two patches and the patch from Max Horne, I'm finally able to
> > compile with CC=clang CFLAGS=-Werror.
> >
> >  [1]: http://thread.gmane.org/gmane.comp.version-control.git/184908
> >  [2]: http://clang-developers.42468.n3.nabble.com/Possibly-invalid-enum-tautology-warning-td3233140.html
> >
> >  grep.c | 3 ++-
> >  grep.h | 3 ++-
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/grep.c b/grep.c
> > index 4bd1b8b..bb548ca 100644
> > --- a/grep.c
> > +++ b/grep.c
> > @@ -625,7 +625,8 @@ static struct grep_expr *prep_header_patterns(struct grep_opt *opt)
> >         for (p = opt->header_list; p; p = p->next) {
> >                 if (p->token != GREP_PATTERN_HEAD)
> >                         die("bug: a non-header pattern in grep header list.");
> > -               if (p->field < 0 || GREP_HEADER_FIELD_MAX <= p->field)
> > +               if (p->field < GREP_HEADER_FIELD_MIN ||
> > +                   GREP_HEADER_FIELD_MAX <= p->field)
> >                         die("bug: unknown header field %d", p->field);
> >                 compile_regexp(p, opt);
> >         }
> > diff --git a/grep.h b/grep.h
> > index 8fc854f..e4a1df5 100644
> > --- a/grep.h
> > +++ b/grep.h
> > @@ -28,7 +28,8 @@ enum grep_context {
> >  };
> >
> >  enum grep_header_field {
> > -       GREP_HEADER_AUTHOR = 0,
> > +       GREP_HEADER_FIELD_MIN = 0,
> > +       GREP_HEADER_AUTHOR = GREP_HEADER_FIELD_MIN,
> >         GREP_HEADER_COMMITTER,
> >         GREP_HEADER_REFLOG,
> >
> > --
> > 1.8.1.1.435.g20d29be.dirty
> >