[cfe-commits] Support <x>-to-bool warnings for more contexts
David Blaikie
dblaikie at gmail.com
Tue May 15 09:59:16 PDT 2012
Committed some of the easier parts of this separately in r156826 -
providing (floating) literal-to-bool and NULL-to-bool, but not
exposing the problems with constant-to-bool until I can iron out the
false positives.
On Mon, Apr 23, 2012 at 4:35 PM, Nico Weber <thakis at chromium.org> wrote:
> On Sat, Apr 21, 2012 at 9:54 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> 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?
>
> I was just commenting on the patch as-is.
>
>>
>>> 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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