[Bug c/56281] New: missed VRP optimization from undefined left shift in ISO C99

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56281



             Bug #: 56281

           Summary: missed VRP optimization from undefined left shift in

                    ISO C99

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Severity: enhancement

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: vincent-...@vinc17.net





It seems that GCC is unaware that a left shift is undefined in ISO C99 when it

yields an integer overflow. See the following testcase with -O3 -std=c99:



int foo (int i)

{

  if (i < 0)

    i = -i;

  i *= 2;

  if (i < 0)

    i++;

  return i;

}



int bar (int i)

{

  if (i < 0)

    i = -i;

  i <<= 1;

  if (i < 0)

    i++;

  return i;

}



GCC optimizes foo() at the .065t.vrp1 step, but not bar().