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

> On Thu, Jan 17, 2013 at 3:00 AM, John Keeping <john@keeping.me.uk> wrote:
>>
>> 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)
>>             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~
>
> The warning seems to be very very wrong, and implies that clang has
> some nasty bug in it.
>
> Since patch->score is 'int', and UNLONG_MAX is 'unsigned long', the
> conversion rules for the comparison is that the int result from the
> assignment is cast to unsigned long. And if you cast (int)-1 to
> unsigned long, you *do* get ULONG_MAX. That's true regardless of
> whether "long" has the same number of bits as "int" or is bigger. The
> implicit cast will be done as a sign-extension (unsigned long is not
> signed, but the source type of 'int' *is* signed, and that is what
> determines the sign extension on casting).
>
> So the "is always false" is pure and utter crap. clang is wrong, and
> it is wrong in a way that implies that it actually generates incorrect
> code. It may well be worth making a clang bug report about this.
>
> That said, clang is certainly understandably confused. The code
> depends on subtle conversion rules and bit patterns, and is clearly
> very confusingly written.
>
> So it would probably be good to rewrite it as
>
>     unsigned long val = strtoul(line, NULL, 10);
>     if (val == ULONG_MAX) ..
>     patch->score = val;
>
> instead. At which point you might as well make the comparison be ">=
> INT_MAX" instead, since anything bigger than that is going to be
> bogus.
>
> So the git code is probably worth cleaning up, but for git it would be
> a cleanup. For clang, this implies a major bug and bad code
> generation.
>
>                    Linus
>                      Linus