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;
}
}
