[cfe-commits] Support <x>-to-bool warnings for more contexts
David Blaikie
dblaikie at gmail.com
Wed Feb 4 12:51:40 PST 2015
Sean - here's the thread on that old patch (that might've caught Kostya's
"bool NumFoo = 10" case), in case you're interested.
On Tue, May 15, 2012 at 9:59 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 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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