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Re: Miscompilation of remainder expressions
Andrew Haley wrote:
Roberto Bagnara writes:
> Andrew Haley wrote:
> > Roberto Bagnara writes:
> > >
> > > Reading the thread "Autoconf manual's coverage of signed integer
> > > overflow & portability" I was horrified to discover about GCC's
> > > miscompilation of the remainder expression that causes INT_MIN % -1
> > > to cause a SIGFPE on CPUs of the i386 family. Are there plans to
> > > fix this bug (which, to me, looks quite serious)?
> >
> > No, there aren't. It would make more sense for you to wrap % in some
> > code that checks for this, rather than for us to slow down every division
> > for this one special case.
>
> With all due respect, I must say I am shocked. I always thought
> (and taught) that we, Free Software people, value standard conformance
> and getting things right.
This is a disgreement about interpretation of the langauge in the
standard, which is:
"The result of the / operator is the quotient from the division of the
first operand by the second; the result of the % operator is the
remainder. In both operations, if the value of the second operand is
zero, the behavior is undefined. When integers are divided, the result
of the / operator is the algebraic quotient with any fractional part
discarded.87) If the quotient a/b is representable, the expression
(a/b)*b + a%b shall equal a."
If the quotient a/b is *not* representable, is the behaviour of %
well-defined or not? It doesn't say.
To the point that, when a/b is not representable, raising SIGFPE
for a%b is standard conformant behavior?
Note however that I am not saying that the standard is not defective.
Who knows how to write and submit C/C++ standard defect reports?
Let us do that, assuming that such a report has not been submitted
already.
--
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara@cs.unipr.it