# 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, the workaround originally used in `examples/war/*.ne`, and the compiler fix that shipped (when shipped). ## Status summary | # | Short name | Status | Fix commit | Regression test | |---|---|---|---|---| | 1 | `fun` with > 4 params silently drops the rest | **FIXED** (E0506 diagnostic) | `analyzer: reject functions with more than 4 parameters (E0506)` | `analyze_rejects_function_with_more_than_4_params`, `analyze_accepts_function_with_exactly_4_params` | | 1b | Same-named params share VarIds across functions | **FIXED** (scope-qualified keys) | `analyzer/ir: scope function locals per function body` | `analyze_allows_same_param_name_in_two_functions` | | 2 | Param transport slots $04-$07 clobbered by nested calls | **FIXED** (codegen prologue spill) | `codegen: spill parameters from $04-$07 into per-function RAM slots` | `codegen::ir_codegen::gen_function_prologue_spills_params_to_local_ram` | | 3 | Function-local `var` declarations share one flat namespace | **FIXED** (scope-qualified keys) | `analyzer/ir: scope function locals per function body` | `analyze_allows_same_local_name_in_two_functions`, `analyze_allows_same_local_name_in_two_state_handlers`, `analyze_still_rejects_duplicate_local_in_same_function` | | 4 | 8-sprites-per-scanline limit invisible to user code | **FIXED** (W0109 static analyzer warning) | `analyzer: add W0109 sprite-per-scanline budget check` | `analyze_sprite_scanline_budget_warns_over_eight`, `analyze_sprite_scanline_budget_ok_when_staggered`, `analyze_sprite_scanline_budget_skips_dynamic_coords`, `analyze_sprite_scanline_budget_expands_metasprites`, `analyze_sprite_scanline_budget_recurses_into_if` | | 5 | `inline` keyword silently declined for short functions | **FIXED** (IR lowering now inlines expression and void bodies) | `ir: real inlining for single-return and void-body `inline fun`s` | `ir::tests::inline_fun_expression_body_emits_no_call_at_use_site`, `inline_fun_void_body_statements_are_spliced`, `inline_fun_with_conditional_return_compiles_as_regular_call`, `inline_fun_nested_inlines_substitute_correctly` | | 6 | `wide_hi` IR map leaked between functions (u16→u8 aliasing) | **FIXED** (cleared per function) | `ir: clear wide_hi between functions to fix 16-bit op aliasing` | `ir::tests::wide_hi_does_not_leak_between_functions` | **Once a fix lands, revert the workaround in `examples/war/*.ne` in the same commit** so the example keeps the game honest and the PR diff visibly proves the fix works end-to-end. All seven catalogued bugs have now shipped their fixes; the example code no longer carries any workaround comments. --- ## 1. Functions with more than 4 parameters silently corrupt the 5th+ *(FIXED)* ### 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 *(FIXED)* ### 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 Both the analyzer and the IR lowerer now qualify function-body `var` / parameter declarations with the enclosing function name (or state handler name) under an internal key `"__local__{scope}__{name}"`. Each function's locals and parameters therefore get **distinct** symbol-table entries and VarIds even when the source names collide. Lookups inside a function body go through `Analyzer::resolve_symbol` / `LoweringContext::scoped_key`, which prefer the scope-qualified key over the bare one — so a function-local `var x` correctly shadows a same-named global (or another function's `var x`). State-level locals (declared at `state Foo { var x: u8 }` outside any handler) stay in the global namespace so every handler in the state can read/write them across frames. See `src/analyzer/mod.rs::resolve_symbol` / `resolve_key` / `scoped_name` and `src/ir/lowering.rs::scoped_key`. Together with fix #2 below, bugs #1b and #2 are completely gone: the workaround-prefixed locals and params in `war/*.ne` (the `dcf_`, `dwp_`, `pba_`, etc tags) are all reverted. --- ## 2. Function parameters share zero-page slots with nested calls — values clobbered across `JSR` *(FIXED)* ### 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 `codegen::ir_codegen::IrCodeGen::new` now allocates every function-local — including its parameters — into a dedicated per-function RAM slot at `$0300+`. Parameters are still passed via the zero-page transport slots `$04-$07` as the calling convention, but `gen_function` now emits a 4-instruction **prologue** at every function entry: ``` LDA $04 ; transport slot 0 STA LDA $05 ; transport slot 1 STA ... etc ... ``` By the time the body runs, every parameter lives in the function's dedicated RAM slot, so any nested call can freely clobber `$04-$07` (passing its own arguments to _its_ callee) without corrupting the caller's saved parameters. The cost is 4 LDA/STA pairs at every function entry (≈ 20 bytes of ROM, 16 cycles). Worth it to make the calling convention sound. See `codegen::ir_codegen::gen_function_prologue_spills_params_to_local_ram` for the regression test. --- ## 3. Function-local variable names are in a flat global namespace *(FIXED)* ### 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 Same as #1b: the analyzer and IR lowerer now internally qualify function-body `var` declarations with the enclosing scope's name, so `foo`'s `var i` and `bar`'s `var i` resolve to `__local__foo__i` and `__local__bar__i` respectively. The two entries coexist peacefully in the (still-flat) symbol table. What *didn't* change: two `var i` declarations inside the same function body still collide with E0501 (we scoped per function body, not per nested block). That's a deliberate trade-off — per-block scoping would require live-range analysis to reuse RAM slots across blocks, which is a much bigger change. The analyzer test `analyze_still_rejects_duplicate_local_in_same_function` pins this behaviour. --- ## 4. Per-frame sprite-per-scanline limit is invisible to user code *(FIXED)* ### 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. Letters or tiles just don't render, and prior to this fix the compiler emitted no warning even when the entire layout was a tree of literal coordinates it could have checked. ### Reproduction ```nescript // 9 letters all on the same Y row: draw Letter at: (0, 100) draw Letter at: (8, 100) draw Letter at: (16, 100) draw Letter at: (24, 100) draw Letter at: (32, 100) draw Letter at: (40, 100) draw Letter at: (48, 100) draw Letter at: (56, 100) draw Letter at: (64, 100) // past budget — silently dropped ``` Pre-fix the compiler said nothing and the 9th letter never showed up on hardware. Post-fix the analyzer emits: ``` warning[W0109]: state 'Main' draws 9 literal-coordinate sprites overlapping scanline 100; the NES renders at most 8 sprites per scanline = help: stagger draws vertically by at least 8 pixels, reduce the number of on-screen sprites, or split the draws across `on_scanline` handlers = note: the 9th and later sprites on a scanline are dropped by the PPU, causing flicker or invisible objects on real hardware ``` ### Root cause The 8-sprites-per-scanline cap is a real NES hardware constraint, not a compiler bug — but NEScript had no static check to catch the cases where user code makes the problem obvious at compile time, even though the draw allocator is sequential and the literal coords it sees are trivially checkable. ### Workaround used in `examples/war/` We staggered text rows by hand. 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. These layouts stay in place post-fix — they now pass the analyzer cleanly because they're under budget. ### Fix `src/analyzer/mod.rs::check_sprite_scanline_budget` runs at the end of `analyze_program`. For each state's `on_frame` handler it walks the block tree (including nested `if`/`while`/`for`/`loop`) collecting literal-coordinate `draw` statements into a `Vec<(y, x, span)>`. Metasprite draws expand into one tuple per tile via the metasprite's `dx`/`dy` offset arrays, so a metasprite that covers four tiles on the same y contributes four sprites to the overlap count. Non-literal coordinates are skipped entirely because the static analysis can't know where they land at runtime. With the tuples collected, the analyzer iterates every scanline 0..240 and counts sprites whose `y <= scanline < y+8`. The worst scanline is cached and, if the count exceeds 8, a `W0109` diagnostic is emitted with labels pointing at every draw site that contributed (deduplicated so metasprite expansions don't spam the message). Only `on_frame` is checked. `on_enter` / `on_exit` fire once per transition and aren't the hot sprite path; checking them would produce false positives on brief splash animations. Conditional branches are unioned (conservative over-count) — a sprite drawn inside an `if` counts for budget purposes even if its runtime branch is exclusive with a sibling's. The trade-off: the check stays local and simple, at the cost of occasionally flagging hand-sliced layouts that the user knows are actually safe. ### Regression tests Five tests in `src/analyzer/tests.rs`: - `analyze_sprite_scanline_budget_warns_over_eight` — nine literal draws on the same `y` trips W0109. - `analyze_sprite_scanline_budget_ok_when_staggered` — nine draws each on a different `y` row are silent. - `analyze_sprite_scanline_budget_skips_dynamic_coords` — draws with a `var`-backed `x` are skipped (no false positive) because the analysis can't resolve them. - `analyze_sprite_scanline_budget_expands_metasprites` — a four-tile metasprite drawn three times trips W0109 because the analyzer expands each draw into its per-tile offsets. - `analyze_sprite_scanline_budget_recurses_into_if` — nine draws inside an `if` block still trip W0109 (conservative over-count). --- ## 5. The `inline` keyword is a hint and is silently ignored for short functions *(FIXED)* ### Symptom Marking a tiny function `inline fun` did not inline it. The compiler still emitted a real `JSR` with full parameter passing through `$04`-`$07`, which meant the declared-inline helpers in War (`card_rank`, `card_suit`, `set_phase`) still paid the calling-convention overhead and still fell foul of the bug-2 clobbering until the param-spill prologue landed. ### Reproduction ```nescript inline fun card_rank(card: u8) -> u8 { return card >> 4 } ``` Pre-fix, the asm dump showed `JSR __ir_fn_card_rank` at every call site. Post-fix the body is spliced at each use and no `JSR` is emitted at all. ### Root cause The IR lowerer's old handling of `inline fun` was a no-op — `is_inline` was read off the AST but the lowering path for `Call` never branched on it. The optimizer passes also had no inlining transform. So the keyword was parsed and then dropped on the floor, producing regular out-of-line code. ### Fix `src/ir/lowering.rs` now captures inline bodies up front in `LoweringContext::capture_inline_bodies` and rewrites call sites at lowering time. Two body shapes are supported: 1. **Single-return expression** (e.g. `return card >> 4`) — captured as `InlineBody::Expression(Expr)`. At the call site, the lowerer evaluates each argument into a fresh temp, pushes a substitution frame mapping parameter names to those temps, and recursively lowers the expression in place of a `Call` op. No IR `Call`/`Return` ops are emitted; the caller ends up with the same IR it would have had if the expression were written directly. 2. **Void multi-statement body** — captured as `InlineBody::Void(Vec)`, but only when every statement passes `is_splicable_void_stmt` (plain assignments, statement-level calls, draws, palette/ background/scroll writes, `wait_frame`, inline asm, debug builtins). Any control flow (`if`/`while`/`for`/`loop`/ `return`/`break`/`continue`/`transition`) disqualifies the function from being inlined, and the call stays a regular `Call`. This mirrors War's `set_phase` (a four-statement global assign) and `reset_flight` (a similar pattern). Functions that are marked `inline` but have a body shape the simple substitution machinery can't splice — notably ones with conditional early returns like War's `wrap52` — fall back to regular out-of-line calls with no diagnostic. That's a deliberate trade-off: rather than refuse to compile the program or emit a noisy warning, we degrade gracefully. The `inline` keyword is now a best-effort hint whose "best effort" is predictable and documented here. ### Substitution stack Nested inline expansions push a fresh substitution frame so an inline body calling another inline sees the inner function's parameter substitutions, not its own. `lookup_inline_sub` walks only the top of the stack because inner bodies are lowered to completion before the stack is popped, so an unambiguous "current" frame always exists. See `LoweringContext::inline_subs_stack` and `lower_expr::Expr::Ident` (which checks the substitution stack before the global var table). ### Regression tests Four tests in `src/ir/tests.rs`: - `inline_fun_expression_body_emits_no_call_at_use_site` — a `return x * 2` inline emits no `Call`, just the multiply. - `inline_fun_void_body_statements_are_spliced` — a void three-statement inline compiles to three individual ops at the caller, not a `Call`. - `inline_fun_with_conditional_return_compiles_as_regular_call` — a body with an `if ... return` pattern falls back to a regular `Call` op. - `inline_fun_nested_inlines_substitute_correctly` — inline A calling inline B sees B's parameter substitutions, not A's. --- ## 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.