RFR: 8359965: Enable paired pushp and popp instruction usage for APX enabled CPUs

[David Holmes]

On Thu, 19 Jun 2025 05:15:40 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

> The goal of this PR is to enhance the existing x86 assembly stubs using PUSH and POP instructions with paired PUSHP/POPP instructions which are part of Intel APX technology.
> 
> In Intel APX, the PUSHP and POPP instructions are modern, compact replacements for the legacy PUSH and POP, designed to work seamlessly with the expanded set of 32 general-purpose registers (R0–R31). Unlike their predecessors, they use the new APX (REX2-based) encoding, enabling more uniform and efficient instruction formats. These instructions improve code density, simplify register access, and are optimized for performance on APX-enabled CPUs.
> 
> Pairing PUSHP and POPP in Intel APX provides CPU-level benefits such as more efficient instruction decoding, better stack pointer tracking, and improved register dependency management. Their uniform encoding allows for streamlined execution, reduced pipeline stalls, and potential micro-op fusion, all of which enhance performance and power efficiency. This pairing helps the processor optimize speculative execution and register lifetimes, making code faster and more scalable on modern architectures.

Just a drive-by comment as this isn't code I normally have much to do with but to me it would look a lot cleaner to define `push_paired`/`pop_paired` (maybe abbreviating directly to `pushp`/`popp`?) rather than passing the boolean.

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PR Review: https://git.openjdk.org/jdk/pull/25889#pullrequestreview-2941765167