[Bug target/62233] New: unnecessary shift instructions to prepare loop counter

[carrot at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62233

            Bug ID: 62233
           Summary: unnecessary shift instructions to prepare loop counter
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carrot at google dot com
            Target: powerpc64le

Compile following source code with options  -m64 -mcpu=power8 -O2

typedef struct {
      int l;
      int b[258];
} S;

void clear (S* s )
{
   int i;
   int len = s->l;

   for (i = 0; i <= len; i++)
       s->b[i] = 0;
}


Trunk gcc generates:

clear:
    lwz 9,0(3)
    cmpwi 7,9,0
    extsw 9,9
    bltlr- 7

    sldi 9,9,2      // A
    li 10,0
    srdi 9,9,2      // B
    addi 9,9,1
    mtctr 9
    .p2align 4,,15
.L3:
    stwu 10,4(3)
    bdnz .L3
    blr

Instruction A shift loop counter left by 2 bits, instruction B shift loop
counter right by 2 bits, so they do nothing when combined together, and should
be removed.