[Bug c/80132] New: powerpc: irrelevant register move before operation

["christophe dot leroy at c-s dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80132

            Bug ID: 80132
           Summary: powerpc: irrelevant register move before operation
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christophe.leroy@c-s.fr
  Target Milestone: ---

In the function below, the two first 'mr' instructions are unneeded, the loop
should operate directly with r5 and r6

void memset64(long long *p, long long v, unsigned int c)
{
        int i;

        for (i = 0; i < c; i++)
                *p++ = v;
}

test2.o:     file format elf32-powerpc


Disassembly of section .text:

0000001c <memset64>:
  1c:   2c 07 00 00     cmpwi   r7,0
  20:   7c cb 33 78     mr      r11,r6
  24:   7c aa 2b 78     mr      r10,r5
  28:   38 63 ff f8     addi    r3,r3,-8
  2c:   4d 82 00 20     beqlr   
  30:   7c e9 03 a6     mtctr   r7
  34:   95 43 00 08     stwu    r10,8(r3)
  38:   91 63 00 04     stw     r11,4(r3)
  3c:   42 00 ff f8     bdnz    34 <memset64+0x18>
  40:   4e 80 00 20     blr