RFR: 8343700: ceil_log2 should not loop endlessly [v3]

[Kim Barrett]

On Wed, 13 Nov 2024 19:58:32 GMT, Sonia Zaldana Calles <szaldana at openjdk.org> wrote:

>> Hi all, 
>> 
>> This PR addresses [8343700](https://bugs.openjdk.org/browse/JDK-8343700) where ceil_log2 looped endlessly if the input value has the highest bit set. 
>> 
>> I also dealt with the case where we try to find ceil_log2(1) which would've returned 1 as opposed to 0. 
>> 
>> Cheers, 
>> Sonia
>
> Sonia Zaldana Calles has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Starting ret at 0

Changes requested by kbarrett (Reviewer).

src/hotspot/share/utilities/powerOfTwo.hpp line 125:

> 123: // Find log2 value greater than this input
> 124: template <typename T, ENABLE_IF(std::is_integral<T>::value)>
> 125: inline T ceil_log2(T value) {

Pre-existing: It's a mistake that this function's return type is T.  It should be int, like all the other log2
variants in this file. Maybe it should be near those.

src/hotspot/share/utilities/powerOfTwo.hpp line 126:

> 124: template <typename T, ENABLE_IF(std::is_integral<T>::value)>
> 125: inline T ceil_log2(T value) {
> 126:   assert(value > 0, "Invalid value");

This assert doesn't help when T is an unsigned type.

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PR Review: https://git.openjdk.org/jdk/pull/22074#pullrequestreview-2435354618
PR Review Comment: https://git.openjdk.org/jdk/pull/22074#discussion_r1841765688
PR Review Comment: https://git.openjdk.org/jdk/pull/22074#discussion_r1841763247