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 <[email protected]> wrote: > On Sat, Apr 21, 2012 at 9:54 PM, David Blaikie <[email protected]> wrote: >> On Sat, Apr 21, 2012 at 9:16 PM, Nico Weber <[email protected]> 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 <[email protected]> wrote: >>>> On Wed, Apr 18, 2012 at 3:04 PM, Nico Weber <[email protected]> wrote: >>>>> On Wed, Apr 18, 2012 at 11:36 AM, David Blaikie <[email protected]> >>>>> wrote: >>>>>> On Fri, Apr 6, 2012 at 4:42 PM, David Blaikie <[email protected]> wrote: >>>>>>> On Mon, Apr 2, 2012 at 9:44 PM, Nico Weber <[email protected]> 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 <[email protected]> >>>>>>>> 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 >>>>>>>>> [email protected] >>>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits >>>>>>>>> _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
