[Bug other/55181] New: [4.7/4.8 Regression] Expensive shift loop where a bit-testing instruction could be used

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55181



             Bug #: 55181

           Summary: [4.7/4.8 Regression] Expensive shift loop where a

                    bit-testing instruction could be used

    Classification: Unclassified

           Product: gcc

           Version: 4.7.0

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: other

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

        ReportedBy: g...@gcc.gnu.org

            Target: avr





The following C code:



unsigned char lfsr (unsigned long number)

{

  unsigned char b = 0;

  if (number & (1L << 29)) b++;

  if (number & (1L << 13)) b++;



  return b;

}



compiles to a right shift 29 bits of number which is very expensive because AVR

has no barrel shifter.  Instead, a bit-testing instruction could be used which

takes just a few cycles and not more than 100 like with the right shift.



4.6.2 uses a bit testing instruction.



== Command line ==