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

[Nico Weber]

I gave this a try in chrome. Here's two cases where this warns on that
make me doubtful of this patch.

1.) It warns on code like

        while(YY_CURRENT_BUFFER){ ... }

where YY_CURRENT_BUFFER is something that's defined flex:

./compiler/glslang_lex.cpp:2878:8: warning: implicit conversion of
NULL constant to 'bool' [-Wnull-conversion]
        while(YY_CURRENT_BUFFER){
              ^~~~~~~~~~~~~~~~~
./compiler/glslang_lex.cpp:307:29: note: expanded from macro 'YY_CURRENT_BUFFER'
                          : NULL)

If you use flex, you have to turn off Wnull-conversion because of this
issue. Before the patch, Wnull-conversion was a useful warning.

2.) It warns on this:

../../third_party/skia/src/core/SkScalerContext.cpp:380:9: warning:
implicit conversion from 'int' to 'bool' changes value from 160 to
true [-Wconstant-conversion]
    if (SK_FREETYPE_LCD_LERP) {
        ^~~~~~~~~~~~~~~~~~~~
../../third_party/skia/src/core/SkScalerContext.cpp:372:33: note:
expanded from macro 'SK_FREETYPE_LCD_LERP'
#define SK_FREETYPE_LCD_LERP    160
                                ^~~

This is fairly common in code.

(The warning did find a few cases where we're saying 'return NULL' but
should be saying 'return false', but nothing interesting. I didn't do
a full build of chrome because the build died fairly quickly due to
visibility issues caused by one of espindola's recent patches, so I
tracked that down instead.)

Nico

On Wed, Apr 18, 2012 at 3:42 PM, David Blaikie <dblaikie at gmail.com> wrote:
> On Wed, Apr 18, 2012 at 3:04 PM, Nico Weber <thakis at chromium.org> wrote:
>> 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?
>
> Quite a few, yes. Here's a smattering of examples:
>
> enum X { A, B, COUNT };
> std::vector<bool> b(true, COUNT);
>
> x &= !flag; // in xerces, actually
>
> void log_if(int severity, bool condition);
> log_if(condition, 3);
>
> bool func() { ... return ERROR_CODE_FOO; } // various kinds of error
> codes, often enums
>
> bool b;
> int i;
> ...
> b = 10; // user seems to have jumbled up the variables, or their types
> i = true;
> // similar mistakes to this, except with function calls
> ("set_new_uid(5)" when the flag was really about whether a new uid is
> created, not specifying the uid value itself)
> // a lot of these, admittedly, come up in test code where more
> constants are used - though I'm not sure how much better that makes me
> feel about them
>
> void func(int);
> func(FLAG1 || FLAG2); // should be FLAG1 | FLAG2
>
> if (FLAG1 || FLAG2) // should be "(x == FLAG1 || x == FLAG2)"
>
> bool maxThings = INT_MAX; // fairly clear mistake in the declaration
> of this type
> void func(int);
> func(maxThings);
>
> (x & !(sizeof(void*) - 1)) // probably meant '~' not '!', I believe
>
> if (0 == x && FLAG) // similar to previous examples
>
> bool status;
> ...
> status = -4; // yay, random constants!
>
> while (1729) // I've no idea what this person had in mind... but OK,
> probably working as they intended
>
> if (some_const % other_const) // false positive
>
> bool func() {
>  ...
>    return 0;
>  ...
>    return 1;
>  ...
>    return 2; // aha! :/
> }
>
> Well, that's a rough sample - the enum flag kind of cases seem pretty
> common, or just passing literals of the wrong type to functions or
> constructors (sometimes not as literals, but as constants defined
> elsewhere).
>
>>
>> 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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>>>>>>