[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 19, 2012, at 2:38 PM, David Blaikie wrote:
> 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?

Nothing in particular;  I'm just saying that this is part of the mandate for
this warning.

static_cast<uintptr_t>(0) is actually not an unreasonable idiom:
notably, it's actually portable across all compilers, platforms, and
language dialects (assuming only the existence of the uintptr_t typedef),
which none of the other idioms you've noted are.  Of course, we can
decide that it's uncommon enough to not be worth white-listing —
a reasonable default assumption — but that's an empirical claim
that can run up hard against reality.

John.