[Bug tree-optimization/80281] [5/6 Regression] Wrong constant folding

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80281

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Can be reproduced on x86_64-linux with -O2 -mfma -ffast-math.
Comparing pre and post r246674 differences, the first one I see in forwprop3:
@@ -3,6 +3,7 @@

 foo (unsigned int x, unsigned int y, unsigned int z)
 {
+  unsigned int _1;
   unsigned int _3;
   unsigned int _7;
   unsigned int _9;
@@ -12,8 +13,9 @@ foo (unsigned int x, unsigned int y, uns
   _9 = z_5(D) * z_5(D);
   _10 = y_4(D) * _9;
   _3 = -_10;
-  _7 = x_6(D) - _10;
-  return _7;
+  _1 = x_6(D) - _10;
+  _7 = _1;
+  return _1;

 }

@@ -23,6 +25,7 @@ foo (unsigned int x, unsigned int y, uns

 bar (float x, float y, float z)
 {
+  float _1;
   float _3;
   float _7;
   float reassocpow_9;
@@ -32,8 +35,9 @@ bar (float x, float y, float z)
   reassocpow_9 = __builtin_powif (z_5(D), 2);
   _10 = y_4(D) * reassocpow_9;
   _3 = -_10;
-  _7 = x_6(D) - _10;
-  return _7;
+  _1 = x_6(D) - _10;
+  _7 = _1;
+  return _1;

 }

Wonder why anything changed for the float types or unsigned types.
And in bar, where the failure appears, the difference between -mfma vs.
-mno-fma appears in widening_mul pass:
   powmult_7 = z_5(D) * z_5(D);
-  _10 = y_4(D) * powmult_7;
-  _1 = x_6(D) - _10;
+  _3 = -y_4(D);
+  _1 = _3 * powmult_7 + x_6(D);

On the other side, with -mfma the actual difference for -O2 -ffast-math -mfma
before/after this commit is:
-  powmult_1 = z_5(D) * z_5(D);
-  _3 = -powmult_1;
-  _7 = _3 * y_4(D) + x_6(D);
-  return _7;
+  powmult_7 = z_5(D) * z_5(D);
+  _3 = -y_4(D);
+  _1 = _3 * powmult_7 + x_6(D);
+  return _1;
so not an actual functional change, no idea why negating the pow2 vs. the other
operand would have any advantages/disadvantages, the difference is:
-       vfnmadd231ss    %xmm2, %xmm1, %xmm0
+       vfnmadd231ss    %xmm1, %xmm2, %xmm0