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
