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

[Claes Redestad]

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

> 
> - 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).

/Claes