RFR: 8360192: C2: Make the type of count leading/trailing zero nodes more precise [v8]
Quan Anh Mai
qamai at openjdk.org
Thu Aug 7 04:41:16 UTC 2025
On Thu, 7 Aug 2025 02:13:47 GMT, Qizheng Xing <qxing at openjdk.org> wrote:
>> The result of count leading/trailing zeros is always non-negative, and the maximum value is integer type's size in bits. In previous versions, when C2 can not know the operand value of a CLZ/CTZ node at compile time, it will generate a full-width integer type for its result. This can significantly affect the efficiency of code in some cases.
>>
>> This patch makes the type of CLZ/CTZ nodes more precise, to make C2 generate better code. For example, the following implementation runs ~115% faster on x86-64 with this patch:
>>
>>
>> public static int numberOfNibbles(int i) {
>> int mag = Integer.SIZE - Integer.numberOfLeadingZeros(i);
>> return Math.max((mag + 3) / 4, 1);
>> }
>>
>>
>> Testing: tier1, IR test
>
> Qizheng Xing has updated the pull request incrementally with one additional commit since the last revision:
>
> Remove unnecessary code
Marked as reviewed by qamai (Committer).
-------------
PR Review: https://git.openjdk.org/jdk/pull/25928#pullrequestreview-3095237759
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