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

[David Holmes]

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 ?

- can next_power_of_two build on the existing log2_* functions (or vice 
versa)?

- 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);
}


Cheers,
David

On 26/11/2019 7:44 pm, Claes Redestad wrote:
> Hi,
> 
> in various places in the hotspot we have custom code to calculate the
> next power of two, some of which have potential to go into an infinite 
> loop in case of an overflow.
> 
> This patch proposes adding next_power_of_two utility methods which
> avoid infinite loops on overflow, while providing slightly more
> efficient code in most cases.
> 
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8234331
> Webrev: http://cr.openjdk.java.net/~redestad/8234331/open.01/
> 
> Testing: tier1-3
> 
> Thanks!
> 
> /Claes