[cfe-commits] r161501 - in /cfe/trunk: include/clang/AST/Expr.h include/clang/Basic/DiagnosticGroups.td include/clang/Basic/DiagnosticSemaKinds.td lib/AST/Expr.cpp lib/Sema/SemaExpr.cpp lib/Sema/SemaOverload.cpp test/SemaCXX/constant-expression-c

[David Blaikie]

On Sun, Aug 19, 2012 at 2:35 PM, John McCall <rjmccall at apple.com> wrote:
> On Aug 17, 2012, at 10:16 PM, James Dennett wrote:
>> On Fri, Aug 17, 2012 at 4:00 PM, Nico Weber <thakis at chromium.org> wrote:
>>> On Fri, Aug 17, 2012 at 3:54 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>>>>
>>>> On Aug 17, 2012, at 15:43 , David Blaikie <dblaikie at gmail.com> wrote:
>>>>
>>>>> On Fri, Aug 17, 2012 at 3:33 PM, Nico Weber <thakis at chromium.org> wrote:
>>>>>> Should this really be on by default? On chrome, this triggers a single
>>>>>> time (linux-only):
>>>>>>
>>>>>> ../../third_party/tcmalloc/chromium/src/stack_trace_table.cc:138:16:
>>>>>> warning: expression which evaluates to zero treated as a null pointer
>>>>>> constant of type 'void *' [-Wnon-literal-null-conversion]
>>>>>> out[idx++] = static_cast<uintptr_t>(0);
>>>>>>              ^~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>>
>>>>>> out is declared as `void** out = new void*[out_len];`. The warning
>>>>>> isn't wrong, but it looks rather pedantic to me. Should this be only
>>>>>> in -Wall (or maybe even in -pedantic)?
>>>>>
>>>>> Might be a fair candidate for -Wall, though it did find some
>>>>> reasonable stuff in google. 18 cases overall with some fairly
>>>>> interesting ones (see b/6954211 for the ones that've been committed so
>>>>> far, or cl/32692314 for some of the remaining ones.
>>>>>
>>>>> The worst offenders are integer constants with value 0 that aren't at
>>>>> all intended to be pointers. (most easily occurred in function calls
>>>>> where the caller thought the argument was of one type but it's
>>>>> actually of a pointer type)
>>>>>
>>>>> I have some more once this warning opens up to cover comparisons,
>>>>> conditional operands, and return statements - there's a lot of
>>>>> confusing "cstr == '\0'" code where the user probably meant to deref
>>>>> the lhs but didn't.
>>>>
>>>> IMHO, this should remain on by default. The Chromium example clearly shows an impedance mismatch between the array and the value being stored. I would say it's not unlikely that at one point the array was a uintptr_t*, but was changed, and this part of the code wasn't updated to match because it didn't warn. But I can see the argument that "because this isn't harmful, we shouldn't warn unless asked to".
>>>
>>> I don't feel strongly either way. This code is in one of the
>>> third-party libraries we use. We build those without -Wall because the
>>> warning policy is up to the library (not everybody believes in
>>> -Wall-clean), but we do build with the default warnings enabled so
>>> that clang can point out obvious bugs. It's easy for me to just
>>> disable this warning for the third-party library where it fires, but
>>> the warning felt like it's mostly pedantry. It sounds like it caught
>>> real bugs in google's internal code though, so *shrug* :-)
>>
>> FWIW, this seems a perfectly reasonable "on by default" warning to me,
>> and I'm struggling to see the pedantry.
>>
>> I think many users would be surprised that
>>  static_cast<uintptr_t>(0)
>> is a null pointer constant, and I doubt that its author meant it that
>> way.  I could be wrong.
>
> I agree;  this should be on-by-default as long as we're properly
> suppressing it in cases where the expression is a reasonable idiom
> for creating a pointer-sized null constant.  (Ensuring that a null constant
> is pointer-sized is important when passing it to a variadic function).

I believe NULL (which (well, GNUNull/__null does) seems to have the
right target-dependent tweaks for size), nullptr, 0, and 0l, (0ul,
0u), etc should all work just fine. Did you have some other idiom(s)
in mind for that particular purpose?

- David