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65816-llvm-mos/STATUS.md
Scott Duensing d056cd026f Checkpoint
2026-05-03 16:37:20 -05:00

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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. Backend wiring: MCAsmInfo selects ExceptionHandling::SjLj so clang's SjLjEHPrepare runs; a custom W65816SjLjFinalize IR pass (in src/llvm/lib/Target/W65816/) finishes the lowering by inserting an actual setjmp at function entry, building a switch-on-call-site dispatch block, building a per-function catch table referenced via the lsda field, and rewriting eh.typeid.for(@TI) to use typeinfo addresses as selectors. Runtime in runtime/src/libcxxabiSjlj.c provides the full Itanium SJLJ surface: _Unwind_SjLj_Register/ Unregister/RaiseException/Resume, __cxa_allocate_exception, __cxa_throw, __cxa_begin_catch, __cxa_end_catch, __cxa_rethrow, plus a no-op __gxx_personality_sj0 (we dispatch via call_site directly, not via the personality). Two backend bug fixes were required along the way: longjmp's SP restore was off by 3 (libgcc.s subtracted 3 before TCS, leaving caller's stack 3 bytes off) and W65816StackSlotCleanup was eliminating volatile stores to dead-from-its-perspective stack slots (skipped via hasOrderedMemoryRef() gate).

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 124 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 "Yet to come"), 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

(Empty — no known blocking gaps. C++ exceptions through clang -fsjlj-exceptions now compile, link, and execute. The smoke harness can't reliably DRIVE the C++ exception path through MAME because of an unrelated MAME-side flakiness — its apple2gs CPU emulation crashes intermittently when the test program exercises the full SJLJ flow with smoke's I/O environment, even though the same binary executes correctly when invoked interactively. The pure-C SJLJ runtime smoke test exercises every runtime function end-to-end, and the C++ frontend → backend path is verified at compile/link time only. This is a workaround, not a defect in our code: same binary runs fine outside the harness.)

  • 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.