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`.
- C++ exceptions via `clang++ -fsjlj-exceptions`: throw, catch,
  catch-by-value, multiple catch handlers, exception destruction.
  `W65816SjLjFinalize` IR pass inserts the call-site dispatch and
  per-function catch table; `runtime/src/libcxxabiSjlj.c` provides
  the Itanium SJLJ surface (`_Unwind_SjLj_*`, `__cxa_throw`,
  `__cxa_begin_catch`, etc.) plus a no-op personality.

**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.
- `link816 --segment-cap N` packs `.text` greedily into multiple
  bank-aligned segments, capped at N bytes per segment.  Segment 1
  stays at `--text-base` in bank 0 (alongside rodata + bss + init);
  segments 2..M start at `--segment-bank-base` (default $040000)
  in successive banks.  `--manifest path.json` writes a JSON file
  listing each segment's image, base, and entry offset.
  Cross-bank `JSL` (IMM24 reloc) just works — patched at link
  time with the full 24-bit address.  Cross-bank IMM16 is
  permitted (uses DBR for bank — caller pins DBR to data's bank);
  cross-bank PCREL is rejected with a clear diagnostic.
  `scripts/runMultiSeg.sh` is a mini in-Lua loader for MAME that
  reads the manifest, places each segment's bytes, and runs from
  segment 1's entry — used by smoke to verify cross-bank JSL
  end-to-end (helper3 chain across 3 bank-aligned segments).
- `tools/omfEmit` produces OMF v2.1 files in three modes:
  (a) single-segment — `--input flat.bin --map flat.map --base
  ADDR --entry SYM`, KIND=0x0000 (CODE, dynamic), ORG=0 (loader
  picks bank); (b) multi-segment — `--manifest path.json` reads
  link816's manifest and emits one OMF segment per entry with
  KIND=0x8800 (STATIC|ABSBANK|CODE) + ORG=segment-base, asking
  the GS/OS Loader to place each at its declared bank-aligned
  address.  All intra-segment relocations were already patched by
  the linker, so no INTERSEG/RELOC opcodes are needed for v1
  static placement.  (c) `--stack-size N` (auto-enables
  `--expressload`) appends a `~Direct` DP/Stack segment
  (KIND=0x1012) of N bytes so apps can request a custom DP+stack
  allocation from GS/OS instead of the Loader's 4KB default.
  Validated end-to-end via `runViaFinder.sh` under real GS/OS
  6.0.2 — the slow Loader path silently rejects multi-segment
  OMFs, so `--stack-size` is gated behind ExpressLoad emission.
- `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 132 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), SJLJ exception
  runtime end-to-end (libcxxabiSjlj.c throw/catch round-trip via
  setjmp/longjmp + catch-table walk), C++ -fsjlj-exceptions
  compile + link (the C++ frontend → backend path is execution-
  verified manually but skipped from MAME smoke due to a
  MAME-side flakiness — see "What's next"), 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/benchCyclesPrecise.sh` measures per-call cycle counts
  via MAME's emulated time counter.  Eight benchmarks under
  `benchmarks/`.  Current numbers: popcount 6888 cyc, bsearch
  1108, memcmp 1569, strcpy 3580, dotProduct 4774, fib(10) 14152,
  sumOfSquares 49104.  Speed is the optimization priority, not
  size.

**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).

## What's next

Work is now optimization-focused; the toolchain is feature-complete
for the common-case C / minimal-C++ workload.  Priority is speed
(cycle counts), not size.

**Speed wins queued, ranked by expected impact:**

- **u16×u16 → u32 multiply path.**  sumOfSquares is 982 cyc/iter
  bottlenecked by `__mulsi3` for what's effectively a 16×16
  multiply (both inputs are zext from u16).  Adding a `__umulhi3`
  libcall + SDAG hook to detect `MUL(zext(a), zext(b))` could
  roughly halve the iteration cost.

- **Fold `while (x != 0)` for i32 to `lda lo; ora hi; bne`.**
  The combiner currently materializes a SETCC boolean and re-tests
  it, generating ~10 redundant ops in every i32-iteration loop.
  Hot in popcount, CRC, and any BigInt-style code.

- **ptr32 pointer-increment overhead.**  `*p++` under ptr32 emits
  a full 32-bit `ADC` chain even when the high half is provably
  unchanged.  strcpy and memcmp pay 30+ cycles per byte for what
  should be 15-20.  Needs a peephole or SDAG combine for `i32 + 1`
  with provably-no-carry-into-hi.

- **Greedy regalloc retry.**  Currently blocked on an upstream
  LLVM `LiveRangeEdit::eliminateDeadDef` assertion when our
  sub-register pair partial-defs reach it.  Basic regalloc works
  but leaves measurable cycle waste in load/store shuffles.

**Open limitations:**

- **Multi-bank BSS / init_array.**  Multi-segment mode splits
  `.text` across banks but BSS + init_array still live in
  segment 1's bank (bank 0).  Programs with zero-init data
  exceeding the ~60KB bank-0 budget need crt0 to walk a
  per-segment `(start, end)` table.  Not a blocker for >64KB
  *code* programs.

- **C++ exceptions absent from CI smoke.**  The SJLJ runtime
  round-trip is in smoke; the full clang++ → backend → MAME
  execution path runs reliably interactively but is excluded
  from automated smoke due to MAME-side I/O flakiness.

- **GS/OS validation uses a stub dispatcher.**  The wrapper
  contract (PHA + PEA 0 + LDX + JSL $E100A8 + post-call SP
  fixup) is verified end-to-end in MAME against a stub
  (`scripts/runInMameWithGsosStub.sh`).  Validation against a
  real bootable GS/OS volume is left out of CI as it needs a
  smartport hard-disk image and live Tool Locator init.

- **gmtime_r requires `optnone`.**  IR-level optimizer issue:
  loop rotation + IndVar simplify mis-evaluate `days >= 365L +
  (__isLeap(...) ? 1 : 0)`, folding the comparison to
  compile-time-false.  Not a backend bug; needs IR-pass-level
  diagnosis.

- **softDouble `dpack` / `dclass` require `noinline`.**
  Inlining triggers register pressure that overflows basic
  regalloc in `__adddf3`/`__muldf3`/`__divdf3`.  Architectural
  for the same reason as qsort's earlier split.