[cfe-commits] Support <x>-to-bool warnings for more contexts

[Nico Weber]

On Wed, Apr 18, 2012 at 11:36 AM, David Blaikie <dblaikie at gmail.com> wrote:
> On Fri, Apr 6, 2012 at 4:42 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> On Mon, Apr 2, 2012 at 9:44 PM, Nico Weber <thakis at chromium.org> wrote:
>>> Do you have any numbers on bug / false positive ratios before and
>>> after this change?
>>
>> I'm surprised this didn't catch more - but I found only 2 cases where
>> this diagnostic fired (on the same function call, no less) & they seem
>> like perfectly reasonable true positives. Something like:
>>
>> void func(bool, bool);
>> func(0.7, 0.3);
>>
>> I'm not really sure what the author intended, but I'm fairly certain
>> they didn't get it (unless their intent was to confuse future
>> readers).
>
> So this was a little more positive than it looks - these were the new
> warnings for -Wliteral-conversion that were found by this patch. The
> new warnings for -Wconstant-conversion (these were the vast majority
> of the new warnings for my change  - though we don't use
> -Wnull-conversion at the moment, so I haven't measured the increase in
> that warning, for example) are a bit more difficult.
>
> While a lot of cases were legitimate, there are a few major false
> positive cases:

This sounds to me like "more trouble than it's worth". Did you find
any interesting bugs with this?

Nico

>
> * in the /existing/ warning, we have a 'false'-ish positive involving
> code like this: int i = std::string::npos; ... if (i ==
> std::string::npos) - npos is actually, say, LONG_MAX, so when stored
> in an int it truncates to -1, but it compares == to -1 just fine.
> Perhaps we could subcategorize -Wconstant-conversion to allow these
> particular cases that happen to map back/forth non-destructively?
>
> * The major case of false positives with my improved warning amounts
> to a use case like this: #define MY_ALLOC(Type, Count)
> malloc(sizeof(Type) * ((Count) ? Count : 1)) // the actual code is a
> bit more involved, but it's in Python's PyMem_NEW macro
>  The problem is that when you pass a compile-time constant count, now
> we appear to be truncating an integer (stuffing that big count into
> zero or one of a boolean). It would be nice if we could somehow detect
> the case where a macro parameter is used inside a constant expression
> & flag that constant expression as "not so constant". This logic will
> be necessary for improvements to Clang's unreachable code diagnostic
> anyway (we need to know when constant expressions might still vary
> depending on the build settings (or 'call' sites in the case of
> macros))
>  * equally, improvements to allow for sizeof expressions to trigger
> similar "not quite constant" flags would be good. While "if
> (sizeof(X))" is silly & we can happily warn on that, "if (sizeof(X) -
> 3)" might be less clear cut (or sizeof in some other part of a
> constant expression) - though I haven't seen (m)any false positives
> like this.
>
> * Template parameters - this leads to code a lot like macros:
> template<int N> void func() { ... if (N) { ... } }; I've currently
> worked around this by having "IgnoreParenImpCasts" not ignore
> SubstNonTypeTemplateParmExprs - this is a bit of a dirty hack (both
> because this code was presumably written this way for a reason -
> though removing it doesn't regress any test cases - and because I
> don't think it falls down as soon as N is a subexpression such as "if
> (N - 3)")
>
> Any thoughts on whether or not these are reasonable goals and how best
> to achieve them would be most welcome,
>
> - David
>
>>
>> - David
>>
>>> On Mon, Apr 2, 2012 at 6:03 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>> SemaChecking.cpp:3989 currently returns early from checking implicit
>>>> conversions after it tests some specific X-to-boolean cases (including
>>>> string and funciton literals) but before checking various other cases
>>>> later on like NULL-to-X and wide integer literal to narrow integer.
>>>>
>>>> This change removes the early return, fixes the diagnostic (to
>>>> correctly emit the fact that non-zero literals produce a "true"
>>>> boolean value rather than simply truncating the larger integer
>>>> literal), and updates the tests. In some cases the test cases were
>>>> fixed or updated (//expected-warning), in others I simply suppressed
>>>> the diagnostic because there adding the expected-warnings would've
>>>> added a lot of noise to the test cases*.
>>>>
>>>> * This last case is a little bit questionable: in one specific case we
>>>> produce a really good diagnostic about constant integer literals used
>>>> in boolean contexts:
>>>>
>>>> int f1();
>>>> bool f2() {
>>>>  return f1() && 42;
>>>> }
>>>>
>>>> we produce:
>>>>
>>>> conv.cpp:3:15: warning: use of logical '&&' with constant operand
>>>> [-Wconstant-logical-operand]
>>>>  return f1() && 42;
>>>>              ^  ~~
>>>> conv.cpp:3:15: note: use '&' for a bitwise operation
>>>>  return f1() && 42;
>>>>              ^~
>>>>              &
>>>> conv.cpp:3:15: note: remove constant to silence this warning
>>>>  return f1() && 42;
>>>>             ~^~~~~
>>>>
>>>> But then with my patch we get an extra diagnostic after the above warning/notes:
>>>>
>>>> conv.cpp:3:18: warning: implicit conversion from 'int' to 'bool'
>>>> changes value from 42 to true [-Wconstant-conversion]
>>>>  return f1() && 42;
>>>>              ~~ ^~
>>>>
>>>> which isn't great - since we already gave a much more specific
>>>> diagnosis of the problem in the first warning. If there's some nice
>>>> way that we could suppress the second one whenever the first one is
>>>> provided (unless the first one is only a warning and the second is
>>>> -Werror'd?) I'd be happy to implement that.
>>>>
>>>>
>>>>
>>>> Another thing I noticed as I was exploring this. We have a warning for
>>>> float-literal-to-int such as:
>>>>
>>>> conv.cpp:2:9: warning: implicit conversion turns literal
>>>> floating-point number into integer: 'double' to 'int'
>>>> [-Wliteral-conversion]
>>>> int i = 3.1415;
>>>>    ~   ^~~~~~
>>>>
>>>> But this warning is off-by-default. Why is that? It's already
>>>> relatively conservative (allowing things like : "int i = 3.0" because
>>>> 3.0 converts to an int without loss of precision) - though it's not a
>>>> DiagnoseRuntimeBehavior, which it could be changed to (to be
>>>> consistent with similar things for integers like "unsigned char c =
>>>> 256").
>>>>
>>>> Or is it really that common to deliberately use floating point
>>>> literals to initialize integer values?
>>>>
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