RFR: 8339648: ZGC: Division by zero in rule_major_allocation_rate [v2]

[Axel Boldt-Christmas]

On Mon, 16 Sep 2024 08:15:38 GMT, Lutz Schmidt <lucy at openjdk.org> wrote:

>> Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Adjust division following suggestion by xmas
>
> src/hotspot/share/gc/z/zDirector.cpp line 491:
> 
>> 489:   // Calculate the GC cost for each reclaimed byte
>> 490:   const double current_young_gc_time_per_bytes_freed = double(young_gc_time) / double(reclaimed_per_young_gc);
>> 491:   const double current_old_gc_time_per_bytes_freed = reclaimed_per_old_gc == 0 ? std::numeric_limits<double>::infinity() : double(old_gc_time) / double(reclaimed_per_old_gc);
> 
> How about using some parentheses? To my understanding, the division has a higher precedence than the ternary conditional expression. See: https://en.cppreference.com/w/cpp/language/operator_precedence

I do not mind parentheses. But ternary are the lowest precedence (if you do not count the `,` which I would almost always say is wrong to use without a surrounding `() / [] / {}`), so to me it seems superfluous.

Just to clarify the intent of this code is what we are getting with a higher precedence on division. That is:

  const double current_old_gc_time_per_bytes_freed = ((reclaimed_per_old_gc == 0) ? (std::numeric_limits<double>::infinity()) : (double(old_gc_time) / double(reclaimed_per_old_gc)));


_Side Note:_
I also think I prefer immediately invoked lambdas when the ternaries get this long.

  const double current_old_gc_time_per_bytes_freed = [&]() {
    if (reclaimed_per_old_gc == 0) {
      return std::numeric_limits<double>::infinity();
    }
    return double(old_gc_time) / double(reclaimed_per_old_gc);
  }();

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PR Review Comment: https://git.openjdk.org/jdk/pull/20888#discussion_r1761248058