[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

[James Dennett]

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.

It's possible that the C++ standard will be revised so that this is no
longer a NPC.

In any case, code that triggers this warning is probably not as its
author intended, and that sounds like a find time to issue a warning.

-- James