[Bug tree-optimization/56899] Wrong constant folding

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56899



Richard Biener <rguenth at gcc dot gnu.org> changed:



           What    |Removed                     |Added

----------------------------------------------------------------------------

           Keywords|                            |wrong-code



--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> 2013-04-10 
08:01:22 UTC ---

We fold (1 - v) * 214748365 to v * -214748365 + 214748365 which is wrong

as it may introduce undefined overflow:



      /* The last case is if we are a multiply.  In that case, we can

         apply the distributive law to commute the multiply and addition

         if the multiplication of the constants doesn't overflow.  */

      if (code == MULT_EXPR)

        return fold_build2 (tcode, ctype,

                            fold_build2 (code, ctype,

                                         fold_convert (ctype, op0),

                                         fold_convert (ctype, c)),

                            op1);



misses the fact that the multiplication of the non-constant part can

also overflow.  I think the case has to be removed completely.