# NEScript v0.1 — Compiler Bugs and Limitations Found While Building War This document captures bugs and limitations discovered while building `examples/war.ne`. Each entry includes a minimal reproduction, the symptom we observed, the root cause if known, and a workaround we used in `examples/war/*.ne`. The intent is to track these so they can be fixed in a future compiler pass — once they are, the corresponding workarounds in `war/*.ne` should be reverted to keep the example honest. --- ## 1. Functions with more than 4 parameters silently corrupt the 5th+ ### Symptom Calling a function with 5 or 6 parameters compiles cleanly, with no warning or error, but at runtime the 5th and 6th parameter values are silently replaced by garbage (typically the value of parameter 3 or 4). Animations and state writes that depend on those parameters behave as if zero was passed. ### Reproduction ```nescript fun arm_fly(sx: u8, sy: u8, dxsign: u8, dysign: u8, card: u8, fu: u8) { fly_x = sx fly_y = sy fly_dx_sign = dxsign fly_dy_sign = dysign fly_card = card // gets the value of dxsign instead! fly_face_up = fu // gets the value of dxsign instead! } fun caller() { arm_fly(32, 64, 0, 0, 147, 1) // After this call: // fly_x = 32, fly_y = 64, fly_dx_sign = 0, fly_dy_sign = 0 // fly_card = 0 (NOT 147) // fly_face_up = 0 (NOT 1) } ``` ### Root cause `src/codegen/ir_codegen.rs` (around line 240) iterates through `func.locals` and assigns the first 4 entries to zero-page parameter slots `$04`-`$07`: ```rust for func in &ir.functions { for (i, local) in func.locals.iter().enumerate() { if i < func.param_count { if i < 4 { var_addrs.insert(local.var_id, 0x04 + i as u16); ... } } else { ... } } } ``` The `if i < 4` guard silently drops the mapping for params 5+ without inserting any RAM allocation for them. The corresponding caller-side codegen for `Call` writes only the first four arguments. Result: params 5 and 6 are never passed and the callee reads stale memory from $04-$07 in their place. ### Workaround used in `examples/war/` `arm_fly` is split: the four "arming" parameters stay in the function signature, and `fly_card` / `fly_face_up` are written to the global state directly at every call site instead. See `war/play_state.ne` (`begin_draw_a` / `begin_draw_b`). ### Fix proposal Two reasonable options: 1. **Diagnose-only**: emit `E05XX too many parameters` when a `fun` declaration has more than 4 params. This is the smallest possible change and turns silent miscompiles into a loud compile-time error. Should ship immediately even if option 2 is also planned. 2. **Spill to RAM**: extend the calling convention so params beyond the first four are passed via dedicated RAM slots in the callee's local frame. The caller-side `Call` codegen would write those slots before `JSR`, the callee-side prologue could leave them as-is. This grows the per-function RAM footprint but lets users write any signature they like. --- ## 1b. Function parameters with the same name in different functions share a VarId, which collides their zero-page slot mapping ### Symptom Two unrelated functions whose parameters happen to be named the same (e.g. both have a `card: u8` parameter, or both have an `x: u8` parameter) end up reading parameters from the wrong zero-page slot at runtime. One function reads `$04`, another reads `$06`, a third reads `$05` — depending on the parameter's *position* in whichever function is processed last by the codegen. This is a much sneakier sibling of bug #1: rather than dropping a parameter past the 4th slot, it silently reroutes parameter reads to slots that hold completely unrelated values from the caller. ### Reproduction ```nescript // Function A: card is the 1st parameter, expected at $04 fun push_back_a(card: u8) { deck_a[deck_a_front] = card // reads from $06, not $04! deck_a_count += 1 } // Function B: card is the 3rd parameter, expected at $06 fun draw_card_face(x: u8, y: u8, card: u8) { // ... uses card normally ... } ``` The IR lowering assigns `card` a single shared `VarId` because its `var_map` is global across all functions. The codegen then walks each function in turn, inserting `(VarId(card), $0X)` mappings into a single global `var_addrs` `HashMap` — and whichever function comes last in iteration order wins the mapping. If `draw_card_face` is processed after `push_back_a`, `VarId(card)` ends up mapped to `$06`, and `push_back_a` then reads its `card` parameter from `$06` (which holds whatever the caller was using as a third argument — typically junk). ### Root cause `src/ir/lowering.rs::get_or_create_var` looks up names in `self.var_map`, which is shared across the whole program: ```rust fn get_or_create_var(&mut self, name: &str) -> VarId { if let Some(&id) = self.var_map.get(name) { id } else { let id = VarId(self.next_var_id); self.next_var_id += 1; self.var_map.insert(name.to_string(), id); id } } ``` `lower_function` calls `get_or_create_var(¶m.name)` for each parameter, so two different functions both with a `card` parameter resolve to the same `VarId`. Once that single `VarId` flows into the codegen, the per-function "this is param index N of function F" relationship is lost — there's only one global mapping per `VarId`. ### Workaround used in `examples/war/` Every parameter name in the war source is unique across the entire program. Function-locals were already prefixed by function (see bug #3); we extended the same scheme to params: `push_back_a(pba_arg_card: u8)` instead of `push_back_a(card: u8)`, etc. The wrapping `pba_card` / `pbb_card` / `dcf_card` snapshots from bug #2 stay because they also help with the bug-2 clobbering. ### Fix proposal Two layers to fix in: 1. **IR lowering**: give every function its own `var_map` for parameters and locals. The global `var_map` should only hold top-level `var` / `const` / `enum` symbols. 2. **Codegen**: even after the IR fix, the global `var_addrs` `HashMap` should grow a per-function dimension (one map per `IrFunction`) so two different functions can independently assign their own VarIds to overlapping zero-page slots. Either fix alone is probably enough; both together is robust. --- ## 2. Function parameters share zero-page slots with nested calls — values clobbered across `JSR` ### Symptom A function that takes parameters and then calls another function sees its own parameters silently replaced by the inner call's arguments. Any code path that reads the original parameter *after* the inner call gets the wrong value. ### Reproduction ```nescript fun draw_card_face(x: u8, y: u8, card: u8) { var rank: u8 = card_rank(card) // x at $04 is now `card` var suit: u8 = card_suit(card) // x at $04 is still `card` // x is supposed to be 120 here, but it's actually `card` var x1: u8 = x + 8 // computes card + 8, not 120 + 8 draw Tileset at: (x, y) frame: ... // draws at x = card, not 120 } ``` Concretely, calling `draw_card_face(120, 128, 0x93)` puts the card sprite at `(0x93, 128)` — completely wrong. ### Root cause Same allocator as bug #1: `func.locals[0..param_count]` are mapped to `$04`, `$05`, `$06`, `$07`. The caller writes its own arguments into the same zero-page slots before `JSR`, so the caller's parameters at those slots get clobbered by the callee's arguments. There is no save/restore wrapper around `JSR` and no spill/reload pass to refresh the caller's parameters from a backing copy. ### Workaround used in `examples/war/` Every helper that takes parameters AND makes any nested function call snapshots its parameters into fresh local variables at the top of the function, then references the locals exclusively throughout the body. See `war/render.ne::draw_card_face`, `war/render.ne::draw_flying_card`, `war/deck.ne::push_back_a`, `war/deck.ne::push_back_b`. ### Fix proposal 1. **Spill on entry**: at the top of every function body that makes a call, copy `$04..$07` into per-function RAM slots and rewrite all parameter reads to load from the RAM copies. Equivalent to what users are doing manually today. 2. **Smarter scheduling**: only spill a parameter slot if it's live across a call site (CFG-aware liveness pass on params). Same effect, less RAM cost for short helpers that never read their params after calling out. Either fix would let users write straightforward function bodies without having to remember the snapshot dance. --- ## 3. Function-local variable names are in a flat global namespace ### Symptom Two different functions cannot declare locals with the same name. The compiler emits `E0501 duplicate declaration of ''` even though the locals are in disjoint scopes. ### Reproduction ```nescript fun foo() { var i: u8 = 0 while i < 10 { i += 1 } } fun bar() { var i: u8 = 0 // E0501 duplicate declaration of 'i' while i < 5 { i += 1 } } ``` ### Root cause `src/analyzer/mod.rs::register_var` inserts every `var` declaration into a single `self.symbols` map keyed only on the variable's name, with no qualification by function or block: ```rust fn register_var(&mut self, var: &VarDecl) { if self.symbols.contains_key(&var.name) { self.diagnostics.push(Diagnostic::error( ErrorCode::E0501, format!("duplicate declaration of '{}'", var.name), var.span, )); return; } ... } ``` `check_statement` calls `register_var` for every `Statement::VarDecl` encountered while walking function bodies, so all locals across all functions and all nested blocks land in the same namespace. ### Workaround used in `examples/war/` Every function-local variable is prefixed with a short tag identifying its enclosing function (e.g. `dfa_card` in `draw_front_a`, `pba_slot` in `push_back_a`, `dwp_px` in `draw_word_player`). This makes long files harder to read but is fully mechanical. ### Fix proposal Rework `register_var` to maintain a stack of scopes (one per function body, one per nested block). Each `Statement::VarDecl` inserts into the current scope. Lookup walks the stack from innermost to outermost. The existing global symbol table is unchanged for top-level globals / consts / fun names; only function-locals shift to the scoped table. A smaller intermediate fix: keep the flat table but qualify each local's stored name as `::` so the global table sees unique entries even when source names collide. --- ## 4. Per-frame sprite-per-scanline limit is invisible to user code ### Symptom Drawing more than 8 sprites whose Y rectangles intersect a single scanline causes the NES PPU to silently drop the excess sprites past the 8th in OAM order. There's no compile-time detection and no runtime warning — letters or tiles just don't render. ### Reproduction ```nescript // 9 letters all on the same Y row: draw_letter(0, 100, 0) draw_letter(8, 100, 1) draw_letter(16, 100, 2) draw_letter(24, 100, 3) draw_letter(32, 100, 4) draw_letter(40, 100, 5) draw_letter(48, 100, 6) draw_letter(56, 100, 7) draw_letter(64, 100, 8) // this one will not render ``` ### Root cause This is a real NES hardware constraint, not a compiler bug. However, because NEScript's `draw` allocator is purely sequential, the compiler cannot warn even when it has all the information needed to know the layout would overflow. ### Workaround used in `examples/war/` We staggered text rows. The title screen's "WAR / CARD GAME / 0 PLAYER / 1 PLAYER / 2 PLAYER" layout sits each row at a different y so no scanline carries more than 7 sprites; the victory screen's "PLAYER X / WINS" wraps after the player letter for the same reason. ### Fix proposal Two complementary improvements: 1. **Static analyzer pass**: walk the IR for each frame handler, collect the set of `(x, y)` literal pairs feeding `draw` ops within the same basic block, and emit `W01XX` if any scanline (8-px row) would have > 8 sprites. Only catches the literal case but that's the most common. 2. **Sprite-cycling runtime helper**: a `cycle_sprites()` intrinsic that rotates OAM order each frame so the same sprites get dropped on different frames, producing a flicker instead of a permanent dropout. Standard NES technique. --- ## 5. The `inline` keyword is a hint and is silently ignored for short functions ### Symptom Marking a tiny function `inline fun` does not always inline it. The compiler still emits a real `JSR` with full parameter passing through `$04`-`$07`, which means the inlining doesn't escape the bug-2 parameter clobbering. ### Reproduction ```nescript inline fun card_rank(card: u8) -> u8 { return card >> 4 } ``` The asm dump shows `JSR __ir_fn_card_rank` at every call site — the function was not inlined. ### Root cause (Inferred — would need to confirm by reading the inliner pass.) The optimizer's inlining pass has a size threshold or a heuristic that prevents inlining in some contexts even when the function is marked `inline`. There's no diagnostic emitted when the hint is declined. ### Workaround used in `examples/war/` None — we just live with the JSR overhead and the bug-2 fallout. ### Fix proposal 1. **Promote `inline` to a hard contract**: when `inline` is present, always inline (or emit `W01XX` if it cannot be inlined for a structural reason like recursion). 2. **Optional dump**: add `--dump-inliner` to print which `inline fun` declarations were inlined and which weren't, with the reason. --- ## 6. `wide_hi` IR-lowering map leaked between functions and corrupted 16-bit ops *(FIXED)* ### Symptom A function whose body had no 16-bit values whatsoever would nonetheless emit `CmpEq16` (and other `Op16` variants) where the *destination* temp aliased one of the *source* temps. The resulting comparison effectively became "is this byte equal to some uninitialised stack memory?", which in War caused the phase-machine `match phase { ... }` dispatcher to skip the `P_WIN_B` arm forever once the game first reached it — the game would freeze with both cards face-up and "PLAYER B WINS" never firing. ### Reproduction (pre-fix) A handful of `u16` `+= 1` operations early in a state handler followed by a long `match` chain on a `u8` was enough to trip it. The minimum repro is roughly: ```nescript var clock: u16 = 0 var phase: u8 = 0 on frame { clock += 1 // wide op leaves wide_hi entries match phase { // u8 match — should be 8-bit 0 => { phase = 1 } 1 => { phase = 2 } 2 => { phase = 3 } 3 => { phase = 4 } 4 => { phase = 5 } 5 => { phase = 6 } 6 => { phase = 7 } 7 => { /* corrupt — never matched */ } _ => {} } } ``` The IR for the `phase == 7` arm came out as `CmpEq16 { dest: T147, a_lo: T145, a_hi: T148, b_lo: T146, b_hi: T147 }` — note `dest == b_hi`. The codegen happily emits the corresponding 16-bit asm, but reads garbage for the `b_hi` operand because it points at the same scratch slot the result will be written to. ### Root cause `src/ir/lowering.rs::IrLowerer` carries a `wide_hi: HashMap` that records "this low temp's high byte lives at this other temp" pairs whenever a 16-bit value is produced. `lower_function` and `lower_handler` both reset `next_temp = 0` at the start of each function — but they did *not* clear `wide_hi`. Stale entries from earlier functions stuck around and matched against fresh temp IDs in subsequent functions (which start counting from 0 again), causing `is_wide(t)` and `widen(t)` to return spurious "wide" results for what should have been narrow `u8` values. When that happens inside `lower_binop`'s `Eq` path, `widen(r)` returns the stale `(r, hi_r)` pair where `hi_r` happens to be the *next* temp ID `fresh_temp()` will hand out a moment later — so the `dest` temp and `b_hi` end up identical. ### Fix `src/ir/lowering.rs`: in both `lower_function` and `lower_handler`, add `self.wide_hi.clear();` immediately after `self.next_temp = 0;`. Done in this PR. ### Why this didn't show up sooner Every prior example either declared no `u16` globals at all, or declared one and used it sparingly enough that the temp IDs the leaked entries claimed never collided with the rest of the function. War is the first example that combines a `u16` free-running counter with a deep state machine that does many `u8` comparisons in the same `on frame` body, which is exactly the shape the bug needs to manifest. ### Regression test `src/ir/tests.rs::wide_hi_does_not_leak_between_functions` (added in this PR) compiles a two-function program where function A uses a `u16 += 1` (creating wide entries) and function B does `u8 == const` comparisons in a match. Pre-fix, the IR would emit `CmpEq16` with aliased dest/source; post-fix it emits the expected 8-bit `CmpEq`. --- ## Verification path after fixes Once any of the bugs above are fixed in the compiler, the corresponding workarounds in `examples/war/*.ne` should be reverted in the same PR so: - The example demonstrates idiomatic code, not workaround code. - The PR's diff visibly proves the fix works end-to-end (the workaround removal would otherwise be a silent regression). - The committed `examples/war.nes` rebuilds byte-identically to the reverted source, which the pre-commit hook enforces. The relevant workaround sites are catalogued in each bug's "Workaround used" section above; grep for the prefix tags (`dcf_`, `dfa_`, `pba_`, `dwp_`, …) to find them all.