Integrated: 8351016: RA support for EVEX to REX/REX2 demotion to optimize NDD instructions

[Jatin Bhateja]

On Mon, 14 Jul 2025 02:36:24 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Currently, while choosing the colour (register) for a definition live range during the select phase of register allocation, we pick the first available colour that does not match with already allocated neighboring live ranges.
> 
> With Intel APX NDD ISA extension, several existing two-address arithmetic instructions can now have an explicit non-destructive destination operand; this, in general, saves additional spills for two-address instructions where the destination is also the first source operand, and where the source live range surpasses the current instruction.
> 
> All NDD instructions mandate extended EVEX encoding with a bulky 4-byte prefix, [JDK-8351994](https://github.com/openjdk/jdk/pull/24431) added logic for NDD to REX/REX2 demotion in the assembler layer, but due to the existing first color selection register allocation policy, the demotions are rare. This patch biases the allocation of NDD definition to the first source operand or the second source operand for the commutative class of operations.
> 
> Biasing is a compile-time hint to the allocator and is different from live range coalescing (aggressive/conservative), which merges the two live ranges using the union find algorithm.  Given that REX encoding needs a 1-byte prefix and REX2 encoding needs a 2-byte prefix, domotion saves considerable JIT code size.
> 
> The patch shows around 5-20% improvement in code size by facilitating NDD demotion.
> 
> For the following micro, the method JIT code size reduced from 136 to 120 bytes, which is around a 13% reduction in code size footprint.
>  
> **Micro:-**
> <img width="900" height="300" alt="image" src="https://github.com/user-attachments/assets/9cbe9da8-d6af-4b1c-bb55-3e5d86eb2cf9" />
> 
> 
> **Baseline :-**
> <img width="900" height="300" alt="image" src="https://github.com/user-attachments/assets/ff5d50c6-fdfa-40e8-b93d-5f117d5a1ac6" />
> 
> **With opt:-**
> <img width="900" height="300" alt="image" src="https://github.com/user-attachments/assets/bff425b0-f7bf-4ffd-a43d-18bdeb36b000" />
> 
> Thorough validations are underway using the latest [Intel Software Development Emulator version 9.58](https://www.intel.com/content/www/us/en/download/684897/intel-software-development-emulator.html).
> 
> Kindly review and share your feedback.
> 
> Best Regards,
> Jatin

This pull request has now been integrated.

Changeset: e0311ecb
Author:    Jatin Bhateja <jbhateja at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/e0311ecb85b78b6d97387c17102a8b6759eefc36
Stats:     283 lines in 13 files changed: 205 ins; 15 del; 63 mod

8351016: RA support for EVEX to REX/REX2 demotion to optimize NDD instructions

Reviewed-by: sviswanathan, dlunden, vlivanov, qamai

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

PR: https://git.openjdk.org/jdk/pull/26283