[Bug tree-optimization/79489] New: Strange static branch prediction for n != 0

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79489

            Bug ID: 79489
           Summary: Strange static branch prediction for n != 0
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

http://stackoverflow.com/q/41880779/1918193

extern void foo();
extern void bar();

int some_func(int n)
{
    if (n) {
        foo();
    }
    else {
        bar();
    }
    return 0;
}

gcc's profile_estimate guesses (DS theory heuristics) that n is 0 with
probability 54%. This seems strange, as other parts of the compiler predict ==
as more likely false (at least for types with precision > 1).

Also, we may want to document somewhere that gcc tends to consider the "else"
branch as more likely (or at least does not particularly favor the "then"
branch), since there is quite a bit of information on the web that recommends
writing conditions the other way around. But the heuristics are complicated
enough that it is hard to come up with a statement that is true without being
uselessly vague.

(I won't be surprised if this ends up as invalid or wontfix, but it felt worth
forwarding)