On Tue, Jan 27, 2015 at 9:29 AM, Hans Wennborg <[email protected]> wrote:
> On Thu, Jan 22, 2015 at 4:55 PM, Hans Wennborg <[email protected]> wrote: > > On Thu, Jan 22, 2015 at 2:43 PM, David Blaikie <[email protected]> > wrote: > >> > >> > >> On Thu, Jan 22, 2015 at 2:11 PM, Hans Wennborg <[email protected]> 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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