[Bug tree-optimization/36905] New: IV-opts needs a little help with a[i+1]

["pinskia at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

Take:
void fred(unsigned short in, unsigned short *out1)
{
    __SIZE_TYPE__ i;
    for (i=0;i<100;i++)
        out1[i+1] = in;
}

For PPC we currently generate:
.L2:
        addi 9,9,1
        slwi 0,9,1
        sthx 3,4,0
        bdnz .L2

But if change the code just so slightly to:
void fred(unsigned short in, unsigned short *out1)
{
    __SIZE_TYPE__ i;
    out1 ++;
    for (i=0;i<100;i++)
      out1[i] = in;
}
--- CUT ---
we get great code:
.L2:
        sthu 3,2(4)
        bdnz .L2
Even without update we still get:
.L2:
        sth 3,2(4)
        addi 4,4,2
        bdnz .L2

Even if we use the variable out1 afterwards (by a return), we still get the
better code in the second case.


-- 
           Summary: IV-opts needs a little help with a[i+1]
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org
GCC target triplet: powerpc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36905