[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

[John McCall]

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).

John.