RFR: 8360192: C2: Make the type of count leading/trailing zero nodes more precise

[Quan Anh Mai]

On Mon, 23 Jun 2025 03:31:36 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

A stricter bound would be `TypeInt::make(~t._bits._zeros, t._bits._ones, t._widen)`

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PR Comment: https://git.openjdk.org/jdk/pull/25928#issuecomment-2995036343