[cfe-commits] Support <x>-to-bool warnings for more contexts
David Blaikie
dblaikie at gmail.com
Sat Apr 21 21:54:15 PDT 2012
On Sat, Apr 21, 2012 at 9:16 PM, Nico Weber <thakis at chromium.org> wrote:
> I gave this a try in chrome. Here's two cases where this warns on that
> make me doubtful of this patch.
I agree in its current state it'll need some tweaking to improve the
accuracy of the cases it opens up. Or are you saying you think it's
non-viable on principle/beyond correction?
> 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.
Hmm - wonder what the right fix for this is...
I wouldn't mind seeing the full definition of YY_CURRENT_BUFFER if you
have a chance to send it to me. It /sounds/ like the conditional
operator being used there isn't doing what the author thinks it's
doing (it's probably got a bool argument on the LHS & so the NULL on
the rhs is always being converted to 'false' & should just be written
that way).
> 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.
Yep - my thinking was that we could reduce the -Wconstant-conversion
cases that convert to bool could be limited to literals rather than
arbitrary expressions (though we'd have to skip the macro/constant
cases too - but that might miss a lot of really good cases... )
> (The warning did find a few cases where we're saying 'return NULL' but
> should be saying 'return false', but nothing interesting.
Curious - given all the fun things I found I'm surprised it didn't hit
other fun things in chromium. Thanks for giving it a go, though.
> 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.)
Fair enough,
- David
>
> 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?
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> cfe-commits mailing list
>>>>>>> cfe-commits at cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>>>>>>>
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