[Bug rtl-optimization/92712] [8/9/10 Regression] Performance regression with assumed values

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92712

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Before the first revision mentioned above *.optimized dump contained just t *
v, the second one doesn't change anything in *.optimized and is a RTL costing
matter.
  _4 = (unsigned int) t_1(D);
  _10 = _4 + 4294967295;
  _8 = (int) _10;
  _13 = v_3(D) * _8;
  x_5 = v_3(D) + _13;
and can be seen even on simpler:
int
foo (int t, int v)
{
  t = t - 1U;
  v *= t;
  return v + t;
}
which we don't optimize at GIMPLE level.  We don't optimize even:
int
bar (int t, int v)
{
  t = t - 1;
  v *= t;
  return v + t;
}
Rather than hoping it is optimized during combine (the change there was that
while combining b=a-1 into c=b*d we attempted c=a*d-d we now attempt c=(a-1)*d
and similarly for the 3 insn combination with e=c+d, where we attempted and
succeeded to combine that into e=a*d while now we attempt and fail e=(a-1)*d+d:
-Successfully matched this instruction:
+Failed to match this instruction:
 (parallel [
         (set (reg/v:SI 91 [ <retval> ])
-            (mult:SI (reg/v:SI 92 [ t ])
+            (plus:SI (mult:SI (plus:SI (reg/v:SI 92 [ t ])
+                        (const_int -1 [0xffffffffffffffff]))
+                    (reg/v:SI 93 [ v ]))
                 (reg/v:SI 93 [ v ])))
         (clobber (reg:CC 17 flags))
     ])
), I think it would be useful to optimize this in match.pd, plus maybe teach
simplify-rtx.c to handle this.