[Bug target/65010] New: ppc backend generates unnecessary signed extension

["carrot at google dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65010

            Bug ID: 65010
           Summary: ppc backend generates unnecessary signed extension
           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

I use ppc gcc to compile following code with option -O2

unsigned long c2l(unsigned char* p)
{
  unsigned long res = *p + *(p+1);
  return res;
}

long c2sl(signed char* p)
{
  long res = *p + *(p+1);
  return res;
}


Trunk gcc generates:

c2l:
    lbz 10,0(3)
    lbz 9,1(3)
    add 3,10,9
    extsw 3,3
    blr


c2sl:
    lbz 9,1(3)
    lbz 10,0(3)
    extsb 9,9
    extsb 3,10
    add 3,3,9
    extsw 3,3
    blr


The extsw instructions in both functions are unnecessary since it can't change
the value in return register.