r226870 - Make the ?: precedence warning handle pointers to the left of ?

[David Blaikie]

On Tue, Jan 27, 2015 at 9:29 AM, Hans Wennborg <hans at chromium.org> wrote:

> On Thu, Jan 22, 2015 at 4:55 PM, Hans Wennborg <hans at chromium.org> wrote:
> > On Thu, Jan 22, 2015 at 2:43 PM, David Blaikie <dblaikie at gmail.com>
> wrote:
> >>
> >>
> >> On Thu, Jan 22, 2015 at 2:11 PM, Hans Wennborg <hans at hanshq.net> wrote:
> >>>
> >>> Author: hans
> >>> Date: Thu Jan 22 16:11:56 2015
> >>> New Revision: 226870
> >>>
> >>> URL: http://llvm.org/viewvc/llvm-project?rev=226870&view=rev
> >>> Log:
> >>> Make the ?: precedence warning handle pointers to the left of ?
> >>>
> >>> Previously, Clang would fail to warn on:
> >>>
> >>>   int n = x + foo ? 1 : 2;
> >>>
> >>> when foo is a pointer.
> >>>
> >>> Modified:
> >>>     cfe/trunk/lib/Sema/SemaExpr.cpp
> >>>     cfe/trunk/test/Sema/parentheses.c
> >>>
> >>> Modified: cfe/trunk/lib/Sema/SemaExpr.cpp
> >>> URL:
> >>>
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaExpr.cpp?rev=226870&r1=226869&r2=226870&view=diff
> >>>
> >>>
> ==============================================================================
> >>> --- cfe/trunk/lib/Sema/SemaExpr.cpp (original)
> >>> +++ cfe/trunk/lib/Sema/SemaExpr.cpp Thu Jan 22 16:11:56 2015
> >>> @@ -6129,6 +6129,8 @@ static bool ExprLooksBoolean(Expr *E) {
> >>>      return IsLogicOp(OP->getOpcode());
> >>>    if (UnaryOperator *OP = dyn_cast<UnaryOperator>(E))
> >>>      return OP->getOpcode() == UO_LNot;
> >>> +  if (E->getType()->isPointerType())
> >>
> >>
> >> Could we generalize this a bit further, somehow? (I haven't looked at
> the
> >> code in question, but it sounds like this should use some more general
> tool
> >> of "try to apply contextual conversion to bool" so that it matches the
> >> actual semantic situation we're interested in here)
> >
> > It's tricky, because we don't really want to match the actual
> > semantics, we want to figure out if the intention was to use 'foo' as
> > a conditional expression. That's what 'ExprLooksBoolean' does, and
> > it's erring on the side of caution.
> >
> > For example, we don't want to warn if 'foo' is int, even if that could
> > be used as a conditional expression. But we do want to warn if 'foo'
> > is 'a==b', even in C where the type of that expression is int.
> >
> > Having said that, I now realised I might have made the warning a
> > little too broad.. we want to warn here:
> >
> >   int x = x + ptr ? 1 : 0;
> >
> > because 'x + ptr' seems like a pretty unlikely condition expression.
> >
> > But we don't want to warn here:
> >
> >   int y = ptr1 - ptr2 ? 1 : 0;
> >
> > I'll think about that.
>
> The last example actually showed up in code yesterday:
>
> http://git.savannah.gnu.org/cgit/emacs.git/tree/src/emacs.c?id=emacs-24.4#n2307
>
> The reason I said I didn't want to warn here is because the expression
> can only be interpreted one way. For example,
>
> int y = ptr1 - (ptr2 ? 1 : 0);
>
> would not compile because the type has changed.
>
> On the other hand, parentheses would certainly make it more readable,
> and that is what the warning suggests:
>
> int y = (ptr1 - ptr2) ? 1 : 0;
>
> So I'm now thinking maybe warning here is the right thing, just as
> -Wparentheses warns about 'a && b || c'.
>

Perhaps - though &&|| is trickier because it could easily be either way. As
you point out with ?: it can't actually be the other way sometimes. So
perhaps we could/should have the smarts to detect that case, but I'm not
sure where the effort tipping point is.


>
>  - Hans
>
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