On Tue, Jan 27, 2015 at 3:15 PM, David Blaikie <[email protected]> wrote: > > > 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.
I think it would be tricky. For int y = ptr1 - ptr2 ? 1 : 0; we'd essentially have to tentatively build the 'ptr1 - (ptr2 ? 1 : 0)' expression and check if that's legal. I don't think Clang provides an easy way to do that :-/ _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
