65816-llvm-mos/STATUS.md

# llvm816 — Current Status

LLVM/Clang backend for the WDC 65816 (Apple IIgs), forked from
llvm-mos as a separate `W65816` target.

## What works

End-to-end C-to-binary toolchain that produces 65816 machine code
which runs correctly under MAME (apple2gs).

**Language coverage at -O2 (no extra flags):**

- All scalar arithmetic: i8 / i16 / i32 / i64 add, sub, mul, div, mod
  (signed and unsigned). Carry-chained multi-word ops via ADC/SBC pseudos
  + ASLA16 / shift libcalls.
- Comparisons and signed/unsigned widening (sext, zext, trunc) for all
  the above sizes.  Signed compare near INT_MIN handled via EOR-with-
  sign-bit transform.
- Pointer arithmetic, array indexing, struct field access, struct
  return-by-value (up to 8 bytes — Pair, Vec4, double).
- Pointer dereference (`*p`) lowers via `LDAptr / STAptr / STBptr`
  to `[$E0],Y` indirect-LONG with the bank byte at `$E2` forced to 0
  — DBR-independent, so `pha;plb` bank-switched callers don't corrupt
  data through callee local-pointer writes.  Const-int pointers
  (`*(volatile uint16 *)0x5000 = v` MMIO idiom) lower to `STAabs`
  (DBR-relative) so bank-2 writes still work.
- Bitfields, switch statements (verified up to ~12 cases + default),
  function pointers, function-pointer tables, indirect calls via
  `__jsl_indir` trampoline.
- Recursion: factorial, Fibonacci, depth-3 binary-tree
  insert/sum/min/max, simple recursive quicksort.
- Loops with goto / break / continue, nested loops, state machines.
- `<stdarg.h>` varargs with int / long / unsigned long long mixed args.
- Heap: `malloc` / `free` (libc.c first-fit allocator) — linked-list
  reverse with `cons` works; free-list coalesce verified.
- Strings: hand-rolled `strlen`, `strcmp`, `strcpy`, `strchr`, atoi/itoa
  roundtrip.
- Soft-float (single): all four ops + comparisons, MAME-verified.
- Soft-double: add, sub, mul, div all return correct bit patterns
  bit-for-bit against gcc with round-to-nearest-even rounding;
  3-iter Newton sqrt converges.  Compiles at -O2 throughout.  Long-
  running iterations may hit MAME's 1-second sim-time budget (test
  config issue, not a compiler bug).
- Inline assembly with `"a"`, `"x"`, `"y"` register constraints and
  arbitrary opcode bytes (used for the `pha;plb` bank-switch idiom).
- C++ minimal: clang++ compiles a class with virtual + non-trivial
  ctor (vtable + RTTI omitted; no exceptions).
- printf with `%d %x %s %c %p` and width/precision specifiers.
- sprintf / snprintf / vsprintf / vsnprintf with the same format
  coverage as printf (`%d %u %x %ld %lu %s %c %f %p %%` + width).
  C99 truncation semantics for snprintf.  `%.Nf` produces the
  correct fractional digits with round-half-up.
- qsort + bsearch over arbitrary element size with a user `cmp`
  callback.
- Standard string/stdlib glue: strcat, strncat, strpbrk, strspn,
  strcspn, atol, llabs (kept in their own translation unit so
  vprintf's branch layout doesn't shift).
- `<math.h>`: fabs, floor, ceil, fmod, copysign, sqrt, pow,
  sin, cos, tan, exp, log, atan, atan2, asin, acos, sinh, cosh,
  tanh (and float variants).  Bit-twiddling for fabs/floor/ceil/
  copysign; Newton iteration for sqrt; range-reduction + Taylor
  for sin/cos/exp/log/atan; identities for asin/acos/atan2/sinh/
  cosh/tanh.  Accuracy is in the ~1e-6 range — good enough for
  typical numeric work, far short of glibc-quality.  These are
  slow (each call is dozens to hundreds of soft-double libcalls)
  — pre-compute or cache when possible.
- `setjmp` / `longjmp` from libgcc.s.
- Static constructors via crt0's init_array walk.
- `<stdio.h>` file I/O against an in-memory FS: `mfsRegister
  (path, buf, size, cap, writable)` stages a buffer as a named
  file; `fopen`/`fread`/`fwrite`/`fseek`/`ftell`/`fclose`/`fgetc`
  /`fgets`/`ungetc`/`fprintf` operate on it via a per-FILE
  (kind, buf, size, cap, pos, eof, err, unget) record.  stdin/
  stdout/stderr route through `putchar` as before.
- `<wchar.h>`: wcslen / wcscmp / wcsncmp / wcscpy / wcsncpy /
  wcscat / wcschr / wcsrchr; mbtowc / wctomb / mbstowcs /
  wcstombs / mblen with the trivial 1:1 byte<->wide mapping
  (Latin-1).  wchar_t is 16-bit on this target.
- `<signal.h>`: in-process signal table.  signal() registers a
  handler; raise() invokes it.  Default actions: SIGABRT calls
  abort(), SIGINT/SIGTERM call exit(128+sig), others ignored.
- `<locale.h>`: setlocale always returns "C"; localeconv returns
  a fixed C-locale lconv struct.
- C++ subset: classes, single inheritance, multiple inheritance
  (Drawable+Movable through one Sprite), virtual base diamond
  (A and B virtually derive Base; Diamond inherits from both
  with one shared Base subobject), virtual functions,
  polymorphism via base-class pointer arrays, virtual dtors,
  this-pointer adjustment for non-leftmost bases, vbase offset
  tables.  RTTI / `dynamic_cast` works (downcast, MI cross-cast,
  virtual-base sibling cast) via a minimal libcxxabi shim
  (`runtime/src/libcxxabi.c`) that provides `__dynamic_cast` +
  the three typeinfo class vtables (`__class_type_info`,
  `__si_class_type_info`, `__vmi_class_type_info`) + sized
  `operator delete` + `__cxa_pure_virtual`.  Compile with
  `clang++ -fno-exceptions` (RTTI can stay on; exceptions
  remain out of scope — see "Yet to come").

**Toolchain:**

- `clang` / `llc` produce W65816 assembly + ELF object files.
- `tools/link816` resolves cross-translation-unit refs, lays out
  text/rodata/bss, emits a flat binary the IIgs ROM can load.
  Auto-relocates bss above text+rodata when the default
  `--bss-base 0x2000` would overlap text, and skips past the
  IIgs IO window ($C000-$CFFF) if needed.  `--gc-sections`
  (default ON) drops unreachable functions: a minimal program
  with full runtime linked shrinks from ~43KB to ~1.5KB.
- `tools/omfEmit` produces OMF v2.1 single-segment files (the IIgs's
  native object format) for round-tripping with classic dev tools.
- `link816 --debug-out FILE` writes a DWARF sidecar with text/
  rodata/bss/init_array relocations applied to every `.debug_*`
  section, so `.debug_addr` / `.debug_line` PC values are final-
  image addresses.
- `runtime/build.sh` builds crt0, libc, soft-float, soft-double,
  libgcc into linkable objects.
- `scripts/smokeTest.sh` runs 122 end-to-end checks at -O2:
  scalar ops, control flow, calling conventions, MAME execution
  regressions, link816 bss-base safety + weak-symbol resolution +
  heap_end-vs-heap_start sanity, iigs/toolbox.h compile + link,
  iigs/gsos.h compile + link, standalone runtime headers,
  AsmPrinter peepholes (STZ / PEA / PEI — single-STA, shared-
  LDA-multi-STA, DPF0-forwarding), malloc/free coalesce ordering,
  plus real-world coverage: Conway's Game of Life blinker
  (2D loop + neighbour bounds), binary search tree (recursive
  struct + malloc), function-pointer dispatch table (indirect
  JSL via `__jsl_indir`), memory-backed file I/O (mfsRegister +
  fopen/fread/fwrite/fseek/fprintf), C++ polymorphism (single
  inheritance), C++ multiple inheritance (Drawable+Movable),
  C++ virtual base diamond, C++ dynamic_cast (SI + MI cross-cast +
  virtual-base sibling cast through libcxxabi shim), GS/OS wrapper
  round-trip via stub dispatcher pre-loaded at $E100A8 (validates
  PHA + PEA 0 + JSL + post-call SP-fixup contract end-to-end),
  wchar / signal core APIs, hex dumper writing through fprintf,
  JSON tokenizer state machine, hash-table command shell (parser
  + dispatch + chained collisions over fprintf-to-mfs),
  scripts/bench.sh size-vs-Calypsi harness.  100% pass.

- `scripts/bench.sh` compiles a microbenchmark suite with both
  clang (this toolchain) and Calypsi cc65816, comparing emitted
  text-section size.  Current ratio: ~1.9x (down from 2.2x once
  the W65816 target started overriding `replexitval` to "never"
  by default in `LLVMInitializeW65816Target`; SCEV's closed-form
  rewrite was promoting i16 induction expressions to i64 and
  hitting `__muldi3`, which on a 16-bit target is dramatically
  bigger than the loop it replaces).  sumOfSquares went 335B →
  128B, a 2.6x shrink with no other benchmark affected.  Eight
  benchmarks shipped under `benchmarks/`.  Remaining gap is
  structural: Calypsi uses `(sr,s),Y` for stack-relative
  pointer indirection where we route through DP $E0 indirect-
  long for bank safety.

**Backend register allocation:**

- Basic regalloc as default at -O1+; fast at -O0/optnone.  We use
  basic instead of greedy because greedy fails ("ran out of
  registers during register allocation") on functions with many
  cross-call Acc16 vregs (the `ok |= bit; helper(); ok |= bit;`
  pattern across many if-blocks).  Basic handles those cleanly
  with negligible code-size overhead vs greedy on the bench
  suite (~0.6%).
- Pre-RA passes: `WidenAcc16` (Acc16→Wide16 promotion, lets
  greedy spread i16 pressure across A and 16 IMG slots);
  `TiedDefSpill` (handles tied-def-multi-use hazard);
  `ABridgeViaX` (bridges via X/Y when free).
- Post-RA passes: `SpillToX` (STA/LDA pairs → TAX/TXA bridges
  when X dead); `StackSlotCleanup` (deletes redundant adjacent
  spills); `NegYIndY` (rewrites negative-Y indirect-Y stack-rel
  ops to avoid the 24-bit-add bank-cross).
- Pre-emit: `BranchExpand` (long Bxx → INV_Bxx skip; BRA target);
  `SepRepCleanup` (coalesces adjacent SEP/REP toggles, plus a
  cross-mode-neutral coalesce that drops REP/SEP pairs sandwiching
  X-flag-only ops, branches, transfers — saves 4B / 12cyc per
  collapse).  AsmPrinter LDAi8imm peephole walks past mode-neutral
  MIs to fuse the closing REP into a following SEP.
- Imaginary registers IMG0..IMG15 backed by DP $C0..$CE +
  $D0..$DE — gives greedy 17 effective i16 carriers (A + 16 IMG)
  before stack spills kick in.

**ABI:**

- arg0 in A; arg1 in X for i32-first-arg signatures; rest pushed RTL
  on the system stack with PHA.  Caller deallocates via `tsc;clc;adc
  #N;tcs` or `PLY*N/2`.
- Return: i8/i16 in A; i32 in A:X; i64 in A:X:Y plus DP[$F0..$F1] for
  the highest 16 bits.
- Frame is empty-descending (S points to next-free); offsets account
  for the +1 skew vs LLVM's full-descending model.

**IIgs toolbox:**

- `iigs/toolbox.h` — autogenerated wrappers for all ~1300 IIgs
  toolbox routines across 35 tool sets (Tool Locator, Memory
  Manager, Misc Tools, QuickDraw II / Aux, Event Manager,
  Sound Manager, Apple Desktop Bus, SANE, Integer Math, Text
  Tools, Window Manager, Menu Manager, Control Manager,
  LineEdit, Dialog Manager, Scrap Manager, Standard File,
  Note Synth/Sequencer, Font Manager, List Manager, ACE,
  Resource Manager, MIDI, Video Overlay, TextEdit, Media
  Control, Print Manager, Scheduler, Desk Manager, …).  Names
  match Apple's IIgs Toolbox Reference exactly (TLStartUp,
  MMStartUp, NewWindow, SysBeep, …).  417 simple wrappers
  (zero/single-arg, i16-or-void return) inline in the header;
  890 multi-arg ones live in `runtime/src/iigsToolbox.s`.
  Generated by `scripts/genToolbox.py` from ORCA-C's
  `ORCACDefs/` (re-runnable when ORCA-C updates).

## In flight

(Nothing currently — the four previous in-flight items all
landed: basic-regalloc-by-default replaced greedy and resolved
the long-arg-chain failure; `time()` reads ReadTimeHex when the
program has called `iigsToolboxInit()` and `clock()` reads the
VBL counter via 24-bit absolute load; the (sr,s),Y bank-wrap
addressing is no longer emitted by any inserter and the
`W65816NegYIndY` workaround is disabled; LC ceiling extended
from $E000 to $10000 since crt0's `lda $C083` read-twice enables
RAM through $FFFF, gaining 8KB of bank-0 space.)

## Yet to come

- **C++ exceptions** — `dynamic_cast` works (via libcxxabi shim,
  see "What works"); `throw`/`try`/`catch` does not.  Implementing
  exceptions needs the full Itanium unwind ABI: `__cxa_throw`,
  `__cxa_allocate_exception`, `_Unwind_RaiseException`, a
  personality routine, and DWARF `.eh_frame` data the unwinder
  consumes to restore registers per-frame.  The 65816's lack of
  any existing unwinder makes this a real project — defer until
  someone needs exception-based code on the IIgs.

- **GS/OS validated against a real ProDOS volume** — the wrapper
  contract (PHA + PEA 0 + LDX + JSL $E100A8 + post-call SP fixup)
  is verified end-to-end in MAME against a stub dispatcher
  (`scripts/runInMameWithGsosStub.sh`).  Validating against an
  actual GS/OS-loaded volume needs a bootable system disk image
  attached as a MAME smartport hard disk and Tool Locator init —
  out of scope for an automated CI smoke.