RFR: 8268858: Determine register pressure automatically by the number of available registers for allocation [v2]

[Joshua Zhu]

On Mon, 21 Jun 2021 16:48:42 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Joshua Zhu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update comments and place test into proper location
>
> src/hotspot/cpu/x86/x86_64.ad line 1776:
> 
>> 1774:   // The following default threshold works best for LCM's register pressure
>> 1775:   // scheduling on x64.
>> 1776:   uint default_float_pressure_threshold = _FLOAT_REG_mask.Size() - 2;
> 
> Why it works best? Did you tested different values or it is required for correctness? If for correctness - why?

Not for correctness.
The value was tested and verified in SPECjvm2008 by the author of LCM's register pressure scheduling.
Therefore I continue to use the original value.
“I did in fact experiment around with different values for both float and int pressure thresholds.  The versions in changelist work best for x64 and x86.” [1]
[1] http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2015-September/019005.html

-------------

PR: https://git.openjdk.java.net/jdk/pull/4505