[cfe-commits] r149344 - in /cfe/trunk: include/clang/Basic/DiagnosticASTKinds.td lib/AST/ExprConstant.cpp test/CXX/expr/expr.const/p2-0x.cpp

[Richard Smith]

On Tue, Jan 31, 2012 at 2:12 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Tue, Jan 31, 2012 at 2:04 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> > On Tue, Jan 31, 2012 at 12:44 AM, Abramo Bagnara <
> abramo.bagnara at gmail.com>
> > wrote:
> >>
> >> Il 31/01/2012 05:08, Richard Smith ha scritto:
> >> > Author: rsmith
> >> > Date: Mon Jan 30 22:08:20 2012
> >> > New Revision: 149344
> >> >
> >> > URL: http://llvm.org/viewvc/llvm-project?rev=149344&view=rev
> >> > Log:
> >> > constexpr: catch a collection of integral undefined behaviors:
> >> >   -INT_MIN and INT_MIN / -1
> >>
> >> Don't forget INT_MIN % -1. It is tied to INT_MIN / -1 in modern
> standards.
> >
> >
> > I didn't forget it, but my interpretation of the standard is that it does
> > not have undefined behavior.
> >
> > In particular, [expr.mul]p4 says: "if the quotient a/b is representable
> in
> > the type of the result, (a/b)*b + a%b is equal to a." This would
> presumably
> > give INT_MIN % -1 undefined behavior, except that the antecedent is not
> met,
> > so it does not apply. So we're left with INT_MIN % -1 being defined by
> "the
> > binary % operator yields the remainder from the division of the first
> > expression by the second", which is 0. I don't see where the standard
> would
> > require that the division itself fit within the relevant type.
>
> Both the C11 standard and common practice (from both clang and gcc)
> agree that it is undefined behavior.  If C++11 doesn't agree, we
> should get the standard fixed.
>

I see. This change is new in C11 (C99+TC1+TC2+TC3 does not contain it, for
instance), and is not in C++11.

- Richard
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20120131/f02f9f22/attachment.html>