Brian Degenhardt
fe32ef2c27
recQFSRV has a fast path for Rs == Rt+1 that reads the contiguous 256-bit
{Rt:Rs} window straight out of cpuRegs.GPR with an unaligned raw Ldr. Its
comment claimed the window was "memory-coherent after the flushes above",
but those flushes are mmiFlushReg -> _deleteEEreg, which reconciles
const-prop and the scalar/NEON slots and never touches the pins.
Under lazy-dirty the pin is authoritative for UD[0] and armStoreEERegPtrRaw
elides the canonical store entirely for a pinned lane-0 write, so a pinned
source's lower half in memory is routinely stale mid-block. Nine GPRs are
pinned, which makes four adjacent pairs both-pinned -- ($at,$v0) ($v0,$v1)
($v1,$a0) ($a0,$a1) -- plus eight more with one pinned operand: exactly the
register range a funnel-shift memcpy loop uses. Failure mode is wrong data,
not a fault.
Every other raw quad-load site fixes this by merging the pin into lane 0
after the load, which cannot work here because the read straddles two guest
registers. Flush the two pins the window covers instead -- it covers exactly
r[Rt] and r[Rt+1], since sa <= 15 over their 32 bytes -- via a new
armFlushEEGPRPin. That keeps the fast path (0-2 extra Str) rather than
falling back to the ~10-instruction temp-buffer path, and the flushed pins
stay authoritative.
This was the last raw address-of-GPR read in pcsx2/arm64/; the GE-M2e sweep
in 3bc64ac11a covered the mergeable sites and missed this one. Also fixes
the comment, which is what made the hole look deliberate.
Tests: two red-on-unfixed cases dirtying a pinned Rt and a pinned Rs, plus a
non-adjacent green control that proves the divergence belongs to the fast
path. recompiler_tests 1442/1442.
ARMSX2 — Native ARM64 JIT Fork of PCSX2
ARMSX2 is a free and open-source PlayStation 2 (PS2) emulator based on PCSX2. Its purpose is to emulate the PS2's hardware, using a combination of MIPS CPU Interpreters, Recompilers and a Virtual Machine which manages hardware states and PS2 system memory. This allows you to play PS2 games on your phone, PC, or gaming handheld, with many additional features and benefits.
Thank You
The ARMSX2 team is eternally indebted to the PCSX2 project it is based on. We are so fortunate to build on their 20 years of hardcore development.
About This Fork
The upstream PCSX2 project ships an ARM64 interpreter build for ARM, but its high-performance JIT recompilers (EE, IOP, VU0, VU1, and vtlb fast memory) are x86-64 only.
This fork exists to close that gap. The goal is to preserve the correctness features of 20 years of PCSX2 development, while generating the fastest native ARM performance possible.
Current status:
- ✅ EE (Emotion Engine) recompiler — integer, float, MMI, COP0/COP1/COP2, branches, load/store
- ✅ IOP (I/O Processor / R3000A) recompiler — full integer, load/store, branches, coprocessors
- ✅ VU (Vector Unit) recompiler — microVU skeleton + Upper FMAC vector ISA complete; Lower ISA and runtime complete
- ✅ vtlb fast memory
- ✅ Native ARM64 binary builds and boots the PS2 BIOS
- ✅ 2D games are already playable
- ✅ 3D games run
Why LLMs / AI Were Used
A word on methodology:
The x86-64 JIT code in upstream ARMSX2 is already proven correct — it has run thousands of PS2 titles for years. The challenge in this port is not emulator design or JIT theory; it is mechanical translation of a large, well-understood x86-64 assembly codebase into equivalent ARM64 assembly (via VIXL) while preserving the exact same register-allocation contracts, block lifecycle, and recompiler semantics.
Large language models (LLMs) were used as an accelerant for this translation work — pattern-matching x86 JIT boilerplate to ARM64 equivalents, scaffolding emit routines, and keeping the porting velocity high. The JIT logic (block compiler, dispatcher, analysis passes, flag pipelines, clamping rules, Tri-Ace hacks, etc.) is taken directly from the upstream x86 implementation and validated against it. Nothing was hallucinated from scratch.
In other words: the hard engineering was done by the PCSX2 team over two decades. The hard typing — translating ~50k lines of x86 emitter code into ARM64 — is what AI helped compress.
System Requirements
ARMSX2 targets ARM64 across desktop (macOS, Windows, Linux) and mobile (Android, iOS/iPadOS), all from the single shared core. Our setup documentation page contains additional details on software and hardware requirements.
Please note that a BIOS dump from a legitimately-owned PS2 console is required to use the emulator. For more information, visit this page.
Building
Check out our github actions for the latest build recipe