[Bug target/41076] New: [avr] pessimal code for logical OR of 8-bit fields

[pb at gcc dot gnu dot org]

This trivial function:

unsigned short f(unsigned char a, unsigned char b)
{
  return a | (b << 8);
}

generates the code:

f:
/* prologue: function */
/* frame size = 0 */
        mov r21,r22
        ldi r20,lo8(0)
        mov r18,r24
        ldi r19,lo8(0)
        or r18,r20
        or r19,r21
        mov r24,r18
        mov r25,r19
/* epilogue start */
        ret

In other words, GCC is first zero-extending the two 8-bit quantities to 16
bits, then performing a 16-bit logical OR operation on the result.  For each
half of the OR operation, though, one side is a constant zero and hence these
operations are redundant.

In this particular trivial case, the function could be implemented as:

mov r25, r21
ret

with all the other operations omitted.


-- 
           Summary: [avr] pessimal code for logical OR of 8-bit fields
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pb at gcc dot gnu dot org
GCC target triplet: avr-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41076