Misc ARMv8.3 features

ARMv8.3-A introduced several additional features, some of which are used by Apple’s compilers and OS for performance or compatibility.

Relaxed memory ordering: RCpc

ARMv8.3 made weak memory ordering even more flexible by mandating support for Release Consistency, processor consistent (RCpc) memory operations. New instructions (e.g. load-acquire RCpc variants) allow certain memory accesses (a store-release followed by an unrelated load-acquire) to be reordered by the processor for efficiency. This can improve performance on multi-core systems by letting CPUs make use of store buffering in more cases while still guaranteeing ordering where it matters. Apple’s chips implement these instructions, and Apple’s Xcode/LLVM can generate them when targeting ARMv8.3+ to slightly optimize synchronization primitives. The memory model remains backward compatible; standard release/acquire semantics are preserved from the software view, but the hardware can take a more performant execution path where allowed. In macOS’s kernel and concurrent frameworks, this means better scalability on multi-core Apple SoCs with no loss of memory safety.

Large system extensions

ARMv8.3 added support for larger caches and richer system registers. For example, FEAT_CCIDX introduced 64-bit cache size registers to describe caches with greater associativity or number of sets. Apple’s M1/M2 have very large unified caches, and while it’s not documented in public how Apple uses CCIDX, supporting the ARMv8.3 definitions ensures the OS can accurately query and manage caches if needed. Similarly, ARMv8.3 introduced 52-bit virtual addressing as an optional extension (LVA) to go beyond 48-bit VAs. Apple’s macOS currently still uses a 48-bit user VA (256 TB) by default, but the hardware is capable of 52-bit addresses if ever needed for very large RAM configurations.

JavaScript conversion (JSCVT)

A small but nifty addition in v8.3, JSCVT provides instructions to convert double-precision floats to 32-bit ints with JavaScript semantics (saturating to 0 on overflow, etc.). This directly accelerates JavaScript engines. Apple’s WebKit JavaScriptCore likely utilizes these instructions on Apple Silicon to speed up JS execution when converting types, improving Safari performance for web apps.

Floating-point complex operations (FCMA)

ARMv8.3 introduced instructions for complex-number multiply-add for FP/SIMD. These are leveraged in scientific and media frameworks. Apple’s Accelerate and Metal libraries can use FCMA on Apple SoCs to speed up DSP and machine learning calculations that involve complex arithmetic.

Debug and statistical profiling

Features like FEAT_DoPD (“Debug over PowerDown”) and enhancements to statistical profiling (SPE v1.1) also appeared in 8.3 (and 8.2 optional). These mostly impact low-level debugging and performance analysis on development hardware. Apple’s performance tools (like Instruments) can take advantage of the Statistical Profiling Extension to sample CPU execution with low overhead, helping developers optimize code on M1/M2.

In summary, ARMv8.3-A was a broad upgrade. Apple selectively uses these “other” features in the OS and its development toolchains to deliver incremental performance boosts and maintain compatibility with ARM standards. For instance, developers targeting Apple Silicon can rely on the ARMv8.3+ instruction set for optimized math operations and memory handling, knowing that all M-series chips meet or exceed that baseline.