[Bug rtl-optimization/80197] pgo dramatically pessimizes scimark2 MonteCarlo benchmark

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80197

--- Comment #7 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
No, with fixed-up inlining -ftracer sees reasonable edge probabilities. The
reason tracer makes things worse here, is that it clones the path leading to a
50%/50% conditional branch (and correctly stops at that branch), making the
tiny BB under that branch ineligible(?) for if-conversion. We go from

<BB0>
if (!cond) goto <BB2>
  <BBcond>
  var = VAL;   // this can eventually become a cmov
<BB2>

to

<BB0_1>
if (!cond) goto <BB2>
goto <BBcond>

...

<BB0_2>
if (!cond) goto <BB2>
  <BBcond>
  var = VAL;  // this doesn't become a cmov
<BB2>


I think in principle if-conversion could still do its job here by duplicating
the conditional var=VAL assignment under BB0_1.

Here's a silly compile-only sample where -O2 -ftracer is worse than -O2 due to
this effect:

void f(long n, signed char *x)
{
  for (; n; n--) {
    long a=x[n], b;
    if (!a)
      a = 42;
    b = x[a];
    if (b < 0)
      b += a;
    x[b] = 0;
  }
}