py: Add AArch64 native emitter and inline assembler. - #19697

1. Motivation & Problem Statement

Currently, MicroPython lacks runtime native code generation (Native/Viper) and inline assembler support for the 64-bit ARM (AArch64/ARMv8-A) architecture. This limits performance optimization opportunities on modern 64-bit ARM platforms, such as native Unix hosts or QEMU-emulated environments. This PR aims to bridge this gap by providing a complete, robust AArch64 instruction emission and inline assembly framework.

2. Summary of Changes

This PR introduces ~5,400+ lines of code, structured across the following areas:

• Core Compiler (py/): Added the AArch64 assembler, Native/Viper code emitters, and inline assembler instruction encoders.
• QEMU Port (ports/qemu/):
  • Introduced VIRT_AARCH64 and VIRT_AARCH64_FLOAT board configurations.
  • Added low-level support: startup code, exception handling, generic timer, PL011 UART, and semihosting (shared/runtime/semihosting_aarch64.c).
  • Math library optimization: Added hardware-accelerated sqrt and sqrtf implementations using the AArch64 fsqrt instruction (lib/libm/ and lib/libm_dbl/).
• Unix Port: Enabled AArch64 native and inline emitters for the Unix port, supporting both native 64-bit ARM hosts and qemu-aarch64 user-mode emulation.
• Documentation: Added docs/reference/asm_aarch64.rst, detailing register calling conventions, supported instruction subsets (including Python keyword workarounds like and_), and known limitations.
• Test Suite (tests/):
  • Added targeted regression tests: 64-bit large constants (native_const64.py, viper_const64.py), large stack frame locals (viper_many_locals.py), and large pointer offsets (viper_ptr_large_offset.py).
  • Updated existing QEMU tests (asm_test.py, native_test.py, viper_test.py) to gracefully SKIP if the target lacks a .mpy architecture ID, preventing false CI failures.
• CI/CD Pipeline: Added comprehensive AArch64 jobs in .github/workflows/ and tools/ci.sh, including automated ARM GNU toolchain download/verification, QEMU bare-metal build/test, Unix user-mode emulation test, and dedicated gcov coverage collection for the new emitter files.

3. Testing & Verification

Strictly adhering to project CI standards, the following verifications have been completed:

• Local Build & Execution: Successfully built mpy-cross and the Unix port with AArch64 emitters enabled. Feature detection correctly reports aarch64.
• Test Pass Rate: Successfully compiled all 20 AArch64 inline-assembler test files and 4 new Native/Viper regression tests. Basic regression tests resulted in 313 passed, 0 failed (feature-dependent tests unsupported by the minimal host config were correctly skipped).
• CI Coverage: New CI jobs successfully build and run the QEMU VIRT_AARCH64 test suite, inline-assembler tests, and Unix AArch64 bytecode/native test suites, generating accurate gcov reports for asmaarch64.c, emitnaarch64.c, and emitinlineaarch64.c.

4. Trade-offs, Limitations & Alternatives

(Addressing MicroPython's core concerns regarding code size and architectural compatibility)

• Code Size Impact: The new assembler, emitters, and runtime helpers increase the AArch64-specific source and CI footprint. However, because other architectures do not enable these emitters, there is zero runtime code-size increase for other ports. (Confirmed by the CI Code Size Report: all non-AArch64 ports show +0 +0.000%).
• No .mpy Architecture ID (Critical Limitation): The current .mpy file format has exhausted its available architecture IDs. Consequently, persistent native .mpy loading is deliberately disabled for AArch64. Inline assembler and Native/Viper code are compiled at runtime only. This is a pragmatic trade-off; architecture-ID allocation is deferred until the .mpy v7 format and native toolchain design are updated (see Issue mpy-cross/mpy-ld/natmod Please add target for aarch64 #19386).
• Instruction Set Limitations:
  • Floating-point (SIMD/FP) registers and instructions are not supported.
  • Load/store operations only support constant offsets; register-offset addressing and pre/post-indexed forms are not supported.
  • push and pop operations automatically pad an odd number of registers with a dummy slot (using the zero register) to maintain strict 16-byte stack pointer (sp) alignment.

5. Compliance & Generative AI Declaration

• Git History: The commit history is strictly linear with no merge commits, adhering to MicroPython's rebase standards.
• Generative AI Policy: Generative AI tools were utilized during the creation of this PR. However, all code and descriptions have been thoroughly read, manually validated, and are the sole responsibility of the human contributor, fully complying with the MicroPython Generative AI Policy.