[Bug tree-optimization/27364] [4.1/4.2 Regression] VRP miscompiles some unsigned math

["law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #14 from law at redhat dot com  2006-05-02 19:44 -------
Subject: Re:  [4.2 Regression] Gcc 4.2
        miscompiles binutils

On Sun, 2006-04-30 at 18:21 +0000, pinskia at gcc dot gnu dot org wrote:
> 
> ------- Comment #10 from pinskia at gcc dot gnu dot org  2006-04-30 18:21 -------
> Here is the reduced testcase:
> int f(unsigned number_of_digits_to_use)
> {
>   if (number_of_digits_to_use >1294)
>     return 0;
>   return (number_of_digits_to_use * 3321928 / 1000000 + 1) /16;
> }
> 
> int main(void)
> {
>   if (f(11) != 2)
>     __builtin_abort ();
As discussed in the PR, the problem is we don't detect overflow
from MULT_EXPR correctly.  This causes us to get an incorrect
range which ultimately cascades into generating the wrong
return value.

This patch fixes the overflow detection for MULT_EXPR.  Bootstrapped
and regression tested on i686-pc-linux-gnu, both on the 4.1 branch
and mainline.


------- Comment #15 from law at redhat dot com  2006-05-02 19:44 -------
Created an attachment (id=11362)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11362&action=view)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27364