Re: c-family PATCH to improve -Wtautological-compare (PR c/81783)

[Marek Polacek <polacek at redhat dot com>]

Ping.

On Wed, Aug 16, 2017 at 05:24:56PM +0200, Marek Polacek wrote:
> On Wed, Aug 16, 2017 at 11:07:36AM -0400, David Malcolm wrote:
> > On Wed, 2017-08-16 at 16:29 +0200, Marek Polacek wrote:
> > > This patch improves -Wtautological-compare so that it also detects
> > > bitwise comparisons involving & and | that are always true or false,
> > > e.g.
> > > 
> > >   if ((a & 16) == 10)
> > >     return 1;
> > > 
> > > can never be true.  Note that e.g. "(a & 9) == 8" is *not* always
> > > false
> > > or true.
> > > 
> > > I think it's pretty straightforward with one snag: we shouldn't warn
> > > if
> > > the constant part of the bitwise operation comes from a macro, but
> > > currently
> > > that's not possible, so I XFAILed this in the new test.
> > 
> > Maybe I'm missing something here, but why shouldn't it warn when the
> > constant comes from a macro?
> 
> Just my past experience.  Sometimes you can't really control the macro
> and then you get annoying warnings.
> 
> E.g. I had to tweak the warning that warns about if (i == i) to not warn about
>   
>   #define N 2
>   if (a[N] == a[2]) {}
> 
> because that gave bogus warning during bootstrap, if I recall well.
> 
> > At the end of your testcase you have this example:
> > 
> > #define N 0x10
> >   if ((a & N) == 10) /* { dg-bogus "bitwise comparison always evaluates to false" "" { xfail *-*-* } } */
> >      return 1;
> >   if ((a | N) == 10) /* { dg-bogus "bitwise comparison always evaluates to false" "" { xfail *-*-* } } */
> >    return 1;
> > 
> > That code looks bogus to me (and if the defn of "N" is further away,
> > it's harder to spot that it's wrong): shouldn't we warn about it?
> 
> I'm glad you think so.  More than happy to make it an expected warning.
> 
> > > This has found one issue in the GCC codebase and it's a genuine bug:
> > > <https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00757.html>.  
> > 
> > In this example GOVD_WRITTEN is from an enum, not a macro, but if
> > GOVD_WRITTEN had been a macro, shouldn't we still issue a warning?
> 
> I feel like we should, but some might feel otherwise.
> 
> Thanks,
> 
> 	Marek

	Marek