RFR: 8374926: EnableX86ECoreOpts was not enabled on some hybrid CPU [v3]

[David Holmes]

On Sun, 11 Jan 2026 07:46:33 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> `EnableX86ECoreOpts` has been introduced in [JDK-8319429](https://bugs.openjdk.org/browse/JDK-8319429), however model numbers which should be enabled are hard-coded, so the flag would not be enabled by default on some models like Arrow Lake.
>> 
>> I ran a [benchmark](https://github.com/YaSuenag/garakuta/tree/master/randminmax) to check for effectiveness of `-XX:+EnableX86ECoreOpts` with JDK 25.0.1 on Windows 11 25H2, I saw performance improvement a bit on Intel Core 5 Ultra 225U as following:
>> 
>> Disabled (default)
>> 
>> Benchmark Mode Cnt Score Error Units
>> RandMinVal.getMax thrpt 3 9707774.405 ± 27082629.015 ops/s
>> RandMinVal.getMin thrpt 3 7510319.839 ± 10923547.382 ops/s
>> 
>> 
>> Enabled
>> 
>> Benchmark Mode Cnt Score Error Units
>> RandMinVal.getMax thrpt 3 10127809.127 ± 45404142.338 ops/s
>> RandMinVal.getMin thrpt 3 8467677.056 ± 1211998.200 ops/s
>> 
>> 
>> So I think it is better to enable this flag by default on all of hybrid CPUs.
>> 
>> 
>> To check what processor would be enabled E-core optimization, I made a [commit](https://github.com/openjdk/jdk/commit/f363ea1436b8100021fdac87b48ec43bcf6820af) to add comments for CPU models at first. I couldn't find out all of models (some models has not listed in SDM vol.4 so far), but most of models (excepts Sierra Forest) could be treated as hybrid CPU. Fortunately HotSpot identify hybrid flag from `CPUID`, so we can leverage it for this purpose.
>
> Yasumasa Suenaga 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 four additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'origin/master' into ecoreopt-hybrid
>  - Merge remote-tracking branch 'origin/master' into ecoreopt-hybrid
>  - Use supports_hybrid() to check for EnableX86ECoreOpts
>  - Add comments for CPU models

src/hotspot/cpu/x86/vm_version_x86.cpp line 924:

> 922:   // Check if processor has Intel Ecore
> 923:   if (FLAG_IS_DEFAULT(EnableX86ECoreOpts) && is_intel() && is_intel_server_family() &&
> 924:     (supports_hybrid() ||

It is not easy to determine if a listed model number will report as "hybrid". But from what I can find 0xDD  is:

#define INTEL_ATOM_DARKMONT_X		IFM(6, 0xDD) /* Clearwater Forest */

which is E-Core architecture, like Sierra Forest, not a hybrid.

Is there something else in CPUID we can read to determine the presence of E-Core and so avoid still needing hard-coded lists that need to be maintained?

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PR Review Comment: https://git.openjdk.org/jdk/pull/29149#discussion_r2688889270