On 16/01/07, Duncan Sands <[EMAIL PROTECTED]> wrote:
On Tuesday 16 January 2007 16:50, 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.
In ada/exp_ch4.adb you will find:
-- Deal with annoying case of largest negative number remainder
-- minus one. Gigi does not handle this case correctly, because
-- it generates a divide instruction which may trap in this case.
-- In fact the check is quite easy, if the right operand is -1,
-- then the mod value is always 0, and we can just ignore the
-- left operand completely in this case.
Ada semantics require INT_MIN rem -1 to be zero.
FWIW,
[EMAIL PROTECTED]:~$ gcc --version
gcc (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu5)
[EMAIL PROTECTED]:~$ cat test.c
#include <limits.h>
#include <stdio.h>
int main(void) {
int z = INT_MIN % -1;
printf("%d %% %d -> %d\n", INT_MIN, -1, z);
return 0;
}
[EMAIL PROTECTED]:~$ gcc -Wall -Wextra test.c -o test
[EMAIL PROTECTED]:~$ ./test
-2147483648 % -1 -> 0
[EMAIL PROTECTED]:~$ cat test.c
#include <limits.h>
#include <stdio.h>
int modulo(int dividend, int divisor)
{
return dividend % divisor;
}
int main(void) {
int z = modulo(INT_MIN, -1);
printf("%d %% %d -> %d\n", INT_MIN, -1, z);
return 0;
}
[EMAIL PROTECTED]:~$ gcc -Wall -Wextra test.c -o test
[EMAIL PROTECTED]:~$ ./test
Floating point exception
[EMAIL PROTECTED]:~$ gcc -O2 -Wall -Wextra test.c -o test
[EMAIL PROTECTED]:~$ ./test
Floating point exception
[EMAIL PROTECTED]:~$ gcc -O3 -Wall -Wextra test.c -o test
[EMAIL PROTECTED]:~$ ./test
-2147483648 % -1 -> 0
I don't know whether results are different in mainline.
Cheers,
Manuel.
