[Bug tree-optimization/113105] Missing optimzation: fold `div(v, a) * b + rem(v, a)` to `div(v, a) * (b - a) + v`

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113105

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So, e.g. on x86_64,
unsigned int
f1 (unsigned val)
{
  return val / 10 * 16 + val % 10;
}

unsigned int
f2 (unsigned val)
{
  return val / 10 * 6 + val;
}

unsigned int
f3 (unsigned val, unsigned a, unsigned b)
{
  return val / a * b + val % a;
}

unsigned int
f4 (unsigned val, unsigned a, unsigned b)
{
  return val / a * (b - a) + val % a;
}

unsigned int
f5 (unsigned val)
{
  return val / 93 * 127 + val % 93;
}

unsigned int
f6 (unsigned val)
{
  return val / 93 * (127 - 93) + val;
}

f2, f3 and f5 are shorter compared to f1, f4 and f6 at -O2.
With -Os, f3 is shorter than f4, while f1/f2 and f5/f6 are the same size (and
also same number of insns there, perhaps f1 better than f2 as it uses shift
rather than imul).
So, this is really something that needs to take into account the machine
specific expansion etc., isn't a clear winner all the time.