[Bug target/43892] PowerPC suboptimal "add with carry" optimization

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43892

--- Comment #32 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #5)
> No.
> 
> Actually compilable testcase:
> 
> typedef unsigned int u32;
> 
> u32
> add32carry(u32 sum, u32 x)
> {
>   u32 z = sum + x;
>   if (sum + x < x)
>     z++;
>   return z;
> }
> 
> u32
> loop(u32 *buf, int len)
> {
>   u32 sum = 0;
>   for(; len; --len)
>     sum = add32carry(sum, *++buf);
>   return sum;
> }


Note on the trunk this code is recognized at least on the gimple level as add
with overflow and does:
  _7 = .ADD_OVERFLOW (sum_2(D), x_3(D));
  z_4 = REALPART_EXPR <_7>;
  _8 = IMAGPART_EXPR <_7>;
  if (_8 != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  z_5 = z_4 + 1;

  <bb 4> [local count: 1073741824]:
  # z_1 = PHI <z_4(2), z_5(3)>

---- CUT ---
So it is more about the back-end of PowerPC at this point.