[Bug tree-optimization/113265] [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2024-01-09

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're somehow expanding -b as x - b.  That's done by forwprop seeing

  <bb 2> :
  x.0_1 = x;
  a_8 = b_7(D) - x.0_1;
  y = 0;
  y.1_2 = y;
  _3 = a_8 + y.1_2;
  _4 = -a_8;
  _5 = _3 / _4;
  z = _5;

this transform is done irrespectively of whether the non-negated expression
is still used (its a 1:1 replacement, though unary to binary).  We're
thn missing folding of

  a_8 = b_7(D) - x.0_1;
  _4 = x.0_1 - b_7(D);
  _5 = a_8 / _4;

failing to realize this is a_8 / -a_8.

Reduced testcase which isn't optimized by GCC 7 either:

int z;
void func(int a, int b){
    z=(a-b)/-(a-b);
}
void func1(int a, int b){
    z=(a-b)/(b-a);
}