[Bug middle-end/42303] New: Shift left as wide as mode is expanded into RTL as shift

[hutchinsonandy at gcc dot gnu dot org]

Shift LEFT by the full length of integer is carried through to RTL as shift - 
rather than being ZERO.

This hidden zero prevents further optimization. For example x = a + (B<<16)
should be reduced to x = a. Instead we get x = 0 | a;
This can also be seen in rotate expressions.



int16_t shiftzero(uint16_t x,uint16_t y)
{
        return x + (y<<16);
}




shiftzero (uint16_tD.xxxx xD.xxxx, uint16_tD.xxxx yD.xxxx)
{
  uint16_tD.xxxx D.xxxx;
  uint16_tD.xxxx D.xxxx;
  int16_tD.xxxx D.xxxx;

  # BLOCK 2 freq:10000
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  D.xxxx_2 = yD.xxxx_1(D) << 16;
  D.xxxx_4 = D.xxxx_2 + xD.xxxx_3(D);
  D.xxxx_5 = (int16_tD.xxxx) D.xxxx_4;
  return D.xxxx_5;
  # SUCC: EXIT [100.0%] 

}






/* NOTE Same on rotate - since expander is uses shift */
int16_t rotatezero(uint16_t x,uint16_t y)
{
        return ROTATE (x,16);
}

rotatezero (uint16_tD.xxxx xD.xxxx, uint16_tD.xxxx yD.xxxx)
{
  uint16_tD.xxxx D.xxxx;
  uint16_tD.xxxx D.xxxx;
  int16_tD.xxxx D.xxxx;

  # BLOCK 2 freq:10000
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  D.xxxx_2 = xD.xxxx_1(D) << 16;
  D.xxxx_3 = D.xxxx_2 | xD.xxxx_1(D);
  D.xxxx_4 = (int16_tD.xxxx) D.xxxx_3;
  return D.xxxx_4;
  # SUCC: EXIT [100.0%] 

}


-- 
           Summary: Shift left as wide as mode is expanded into RTL as shift
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hutchinsonandy at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: avr-unknown-none


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42303