RFR: 8360192: C2: Make the type of count leading/trailing zero nodes more precise [v7]

[Qizheng Xing]

On Wed, 6 Aug 2025 12:38:05 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Qizheng Xing has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains nine additional commits since the last revision:
>> 
>>  - Merge branch 'master' into enhance-clz-type
>>  - Add checks for results of all test methods
>>  - Replace `isa_*` with `is_*` and add checks for `Type::BOTTOM`
>>  - Merge branch 'master' into enhance-clz-type
>>  - Move `TestCountBitsRange` to `compiler.c2.gvn`
>>  - Fix null checks
>>  - Narrow type bound
>>  - Use `BitsPerX` constant instead of `sizeof`
>>  - Make the type of count leading/trailing zero nodes more precise
>
> src/hotspot/share/opto/countbitsnode.cpp line 50:
> 
>> 48: //------------------------------Value------------------------------------------
>> 49: const Type* CountLeadingZerosINode::Value(PhaseGVN* phase) const {
>> 50:   // If the input is TOP, the result is also TOP.
> 
> IMO this comment is unnecessary, it is trivial to infer from the code below.

Removed.

> src/hotspot/share/opto/countbitsnode.cpp line 62:
> 
>> 60: 
>> 61:   const TypeInt* ti = t->is_int();
>> 62:   if (ti->is_con()) {
> 
> This is unnecessary, if `ti` is a constant then `~ti->_bits._zeros == ti->_bits._ones` and the below case will return a constant anyway.

Removed, and added tests for constant input CLZ/CTZ nodes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25928#discussion_r2258707369
PR Review Comment: https://git.openjdk.org/jdk/pull/25928#discussion_r2258709866