[Bug c++/68252] regiression left operand of shift expression, while we shift > 0 integer

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68252

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-11-09
            Version|unknown                     |5.2.1
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Looks like a bogus folding to me (CST << B * CST -> CST' << B).

 return -3 << a;

from .original and

int foo (int a)
{
  return (1 << a) * -3;
}

On trunk:

/* Fold (a * (1 << b)) into (a << b)  */
(simplify
 (mult:c @0 (convert? (lshift integer_onep@1 @2)))
  (if (! FLOAT_TYPE_P (type)
       && tree_nop_conversion_p (type, TREE_TYPE (@1)))
   (lshift @0 @2)))

on the branch this is still in fold-const.c.

Note that I think this undefinedness doesn't buy us anything and that we should
just make sure to not emit bogus ubsan errors or warnings.