[Bug target/29083] New: useless clrlwi instruction produced for 16-bit bitfield

[bonzini at gnu dot org]

This code produces this output:

struct s {
  unsigned x : 16;
};

int f(struct s *node)
{
  int a = 0;
  do
    a++;
  while ((--node)->x == a);
  return a;
}

        li r2,0
L2:
        lhzu r0,-4(r3)
        addi r2,r2,1
        rlwinm r0,r0,0,0xffff
        cmpw cr7,r0,r2
        beq+ cr7,L2
        mr r3,r2
        blr


If I change unsigned x : 16 to unsigned short x, instead I get the better code:

_f:
        mr r2,r3
        li r3,0
L2:
        lhzu r0,-2(r2)
        addi r3,r3,1
        cmpw cr7,r0,r3
        beq+ cr7,L2
        blr

(I don't care about the register allocation, the important point is that the
rlwinm instruction, which clears the left 16 bits as is clear from the
simplified mnemonic "clrlwi r0,r0,16", is useless).


-- 
           Summary: useless clrlwi instruction produced for 16-bit bitfield
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bonzini at gnu dot org
GCC target triplet: powerpc-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29083