RFR: 8234331: Add robust and optimized utility for rounding up to next power of two+

[David Holmes]

On 26/11/2019 8:06 pm, Claes Redestad wrote:
> 
> 
> On 2019-11-26 10:50, David Holmes wrote:
>> Hi Claes,
>>
>> Just some high-level comments
>>
>> - should next_power_of_two be defined in globalDefinitions.hpp along 
>> side the related functionality ie is_power_of_two ?
> 
> I thought we are trying to move things _out_ of globalDefinitions. I

We are? I don't recall hearing that. But wherever these go seems they 
all belong together.

> agree align.hpp might not be the best place, either, though..

I thought align.hpp as strange place too. :)

>>
>> - can next_power_of_two build on the existing log2_* functions (or 
>> vice versa)?
> 
> Yes, log2_intptr et al could probably be tamed to do a single step
> operation, although we'd need to add 64-bit implementations in
> count_leading_zeros. At least these log2_* functions already deal with
> overflows without looping forever.
> 
>>
>> - do the existing ZUtils not cover the same general area?
>>
>> ./share/gc/z/zUtils.inline.hpp
>>
>> inline size_t ZUtils::round_up_power_of_2(size_t value) {
>>    assert(value != 0, "Invalid value");
>>
>>    if (is_power_of_2(value)) {
>>      return value;
>>    }
>>
>>    return (size_t)1 << (log2_intptr(value) + 1);
>> }
>>
>> inline size_t ZUtils::round_down_power_of_2(size_t value) {
>>    assert(value != 0, "Invalid value");
>>    return (size_t)1 << log2_intptr(value);
>> }
> 
> round_up_power_of_2 is similar, but not identical (next_power_of_two 
> doesn't care if the value is already a power of 2, nor should it).

Okay but seems perhaps these should also be moved out of ZUtils and 
co-located with the other "power of two" functions.

Cheers,
David
-----

> /Claes