codegen: emit gate markers at end of generate() to protect peephole

Move the six gate-marker label emissions (__mul_used, __div_used,
__oam_used, __default_sprite_used, __p1_input_used, __p2_input_used)
out of the inline IR-op lowering paths and into a new
`emit_trailing_markers()` helper that runs once at the end of
`generate()`. The IR walk now just flips a bool per marker; the
label emit happens after every instruction has been lowered, so
the marker never lands in the middle of a peephole-sensitive
sequence.

Fixes a real peephole interaction that surfaced after rebasing on
main's `codegen: skip parameter-spill prologue for leaf functions`
+ `peephole: drop dead LDA #imm before mem-INC/DEC + JMP`
improvements: an inline `__oam_used:` label inside `IrOp::DrawSprite`
split the dead-load-elimination block, leaving the `STA $130 /
LDA $130` redundant store+load pair that main's peephole would
otherwise have collapsed to a plain `LDA #imm`. The stale bytes
shifted the NMI handler by a few bytes, which shifted `on frame`
execution enough that `examples/palette_and_background.ne` captured
phase 1 (WarmReds) at frame 180 instead of phase 2 (CoolBlues).

Regenerates every example ROM against the new codegen (all gate
behaviour is unchanged — the linker still sees the same markers,
just at the tail of the user stream instead of interleaved) and
updates the goldens that shifted: seven audio-hash drifts (all
audio-bearing programs, same cycle-accurate-APU-timing story as
every prior NMI layout change) and two pixel goldens — the one-
pixel sprite-position drift in `comparisons.png` that we already
tolerate, plus the phase-capture flip in
`palette_and_background.png`.

https://claude.ai/code/session_016kM6P7PukktBDqTZexrrAN

src/codegen/ir_codegen.rs

@@ -882,6 +882,15 @@ impl<'a> IrCodeGen<'a> {
             self.emit(JMP, AM::Label("__debug_halt".to_string()));
         }
 
+        // Pay-as-you-go marker labels for flags that got set during
+        // the IR walk above. Emitting them here (rather than at the
+        // call sites) keeps the markers out of peephole-sensitive
+        // instruction sequences — a label inside `IrOp::DrawSprite`
+        // or `IrOp::ReadInput`, for instance, would otherwise split
+        // the dead-load-elimination block and leave stale ZP
+        // stores in the stream.
+        self.emit_trailing_markers();
+
         // Snapshot the fixed-bank instruction stream before we
         // start emitting the banked function bodies into their own
         // streams. Programs without any banked functions skip the
@@ -1229,10 +1238,13 @@ impl<'a> IrCodeGen<'a> {
             }
             IrOp::Mul(d, a, b) => {
                 // Software multiply: multiplicand in A, multiplier in $02.
-                // Drop the `__mul_used` marker so the linker knows to
+                // Flag the `__mul_used` marker so the linker knows to
                 // link the `__multiply` subroutine in — programs that
-                // don't multiply skip ~50 bytes of runtime.
-                self.emit_mul_marker();
+                // don't multiply skip ~50 bytes of runtime. The label
+                // itself is emitted at the end of generate() so it
+                // doesn't disturb peephole-sensitive instruction
+                // sequences here.
+                self.mul_used = true;
                 self.load_temp(*a);
                 self.emit(PHA, AM::Implied); // Save for __multiply contract
                 let b_addr = self.temp_addr(*b);
@@ -1279,10 +1291,11 @@ impl<'a> IrCodeGen<'a> {
             IrOp::Div(d, a, b) => {
                 // Software divide: dividend in A, divisor in $02.
                 // `__divide` returns quotient in A and leaves
-                // remainder in ZP $03. Emit the `__div_used` marker
+                // remainder in ZP $03. Flag the `__div_used` marker
                 // so the linker links the `__divide` subroutine in;
-                // programs that don't divide skip ~70 bytes.
-                self.emit_div_marker();
+                // programs that don't divide skip ~70 bytes. The
+                // label is emitted at the end of generate().
+                self.div_used = true;
                 self.load_temp(*a);
                 self.emit(PHA, AM::Implied);
                 let b_addr = self.temp_addr(*b);
@@ -1297,7 +1310,7 @@ impl<'a> IrCodeGen<'a> {
                 // of ZP $03 afterwards. Same `__div_used` marker as
                 // `IrOp::Div` — modulo doesn't have a separate
                 // runtime routine.
-                self.emit_div_marker();
+                self.div_used = true;
                 self.load_temp(*a);
                 self.emit(PHA, AM::Implied);
                 let b_addr = self.temp_addr(*b);
@@ -1421,14 +1434,16 @@ impl<'a> IrCodeGen<'a> {
                 y,
                 frame,
             } => {
-                // Drop the `__oam_used` marker so the linker knows
-                // to emit the OAM DMA + shadow-init + default-sprite
+                // Flag the `__oam_used` marker so the linker knows
+                // to emit the OAM DMA + shadow-init + default-palette
                 // machinery. Programs that never `draw` skip all of
                 // those paths — no DMA cycles per NMI, no $FE OAM
                 // shadow fill, no built-in smiley reserved in CHR,
                 // and the default palette is suppressed too when the
-                // program has no other visual output.
-                self.emit_oam_marker();
+                // program has no other visual output. The label is
+                // emitted at the end of generate() so it doesn't
+                // split a peephole block here.
+                self.oam_used = true;
                 // Runtime OAM-cursor-based draw. Each frame handler
                 // resets `ZP_OAM_CURSOR` to 0 after the OAM clear; a
                 // `draw` loads the cursor into Y, writes the four
@@ -1476,12 +1491,12 @@ impl<'a> IrCodeGen<'a> {
                 // the marker off and tile 0 becomes user-available
                 // as a blank (all-$00) background tile.
                 if let Some(f) = frame {
-                    self.emit_default_sprite_marker();
+                    self.default_sprite_used = true;
                     self.load_temp(*f);
                 } else if let Some(&tile) = self.sprite_tiles.get(sprite_name) {
                     self.emit(LDA, AM::Immediate(tile));
                 } else {
-                    self.emit_default_sprite_marker();
+                    self.default_sprite_used = true;
                     self.emit(LDA, AM::Immediate(0));
                 }
                 self.emit(STA, AM::AbsoluteY(0x0201));
@@ -1504,15 +1519,16 @@ impl<'a> IrCodeGen<'a> {
                 self.emit(INC, AM::ZeroPage(ZP_OAM_CURSOR));
             }
             IrOp::ReadInput(dest, player) => {
-                // Drop the per-player input marker so the linker
+                // Flag the per-player input marker so the linker
                 // can decide whether to keep that port's shift
                 // block inside NMI. IR uses `player_index` 0 = P1
                 // and 1 = P2; the ZP bytes the NMI populates match
                 // the constants at the top of `runtime/mod.rs`.
+                // Labels are emitted at the end of generate().
                 if *player == 1 {
-                    self.emit_p2_input_marker();
+                    self.p2_input_used = true;
                 } else {
-                    self.emit_p1_input_marker();
+                    self.p1_input_used = true;
                 }
                 // $01 = P1 input byte, $08 = P2 input byte
                 let addr = if *player == 1 { 0x08 } else { 0x01 };
@@ -2112,78 +2128,34 @@ impl<'a> IrCodeGen<'a> {
         }
     }
 
-    /// Emit the `__mul_used` marker label at most once per program.
-    /// The linker uses this to decide whether to link in the
-    /// `gen_multiply` subroutine. Programs that never multiply (or
-    /// that only multiply by constant powers of two — those are
-    /// strength-reduced to shifts) skip the runtime entirely.
-    fn emit_mul_marker(&mut self) {
-        if !self.mul_used {
+    /// Emit the marker labels for flags that got set somewhere
+    /// during `generate()`'s IR walk. Called once, at the end of
+    /// `generate()`, so the labels land after every instruction
+    /// that could have set a flag — never splitting a
+    /// peephole-sensitive block at the flag's call site.
+    ///
+    /// Each label is a zero-byte pseudo-op. The linker looks them
+    /// up by name via `has_label` and turns on the gated runtime
+    /// feature. Programs that never set a given flag emit nothing
+    /// for it — no label, no lookup hit, no gated code.
+    fn emit_trailing_markers(&mut self) {
+        if self.mul_used {
             self.emit_label("__mul_used");
-            self.mul_used = true;
         }
-    }
-
-    /// Emit the `__div_used` marker label at most once per program.
-    /// The linker uses this to decide whether to link in the
-    /// `gen_divide` subroutine. Both `IrOp::Div` and `IrOp::Mod`
-    /// trigger this marker because modulo reuses the divide
-    /// routine. Programs whose divide/modulo operations all use
-    /// constant power-of-two divisors have already been rewritten
-    /// to shifts / masks by the optimizer and never reach here.
-    fn emit_div_marker(&mut self) {
-        if !self.div_used {
+        if self.div_used {
             self.emit_label("__div_used");
-            self.div_used = true;
         }
-    }
-
-    /// Emit the `__oam_used` marker label at most once per program.
-    /// Triggered by `IrOp::DrawSprite`. The linker drops the OAM
-    /// DMA + shadow-init + default-palette paths when this marker
-    /// is absent, shaving cycles out of every NMI and bytes out of
-    /// PRG for programs that don't draw sprites.
-    fn emit_oam_marker(&mut self) {
-        if !self.oam_used {
+        if self.oam_used {
             self.emit_label("__oam_used");
-            self.oam_used = true;
         }
-    }
-
-    /// Emit the `__default_sprite_used` marker label at most once
-    /// per program. Triggered by `IrOp::DrawSprite` when the sprite
-    /// name doesn't resolve to a user declaration (fall-back to
-    /// tile 0) or when a runtime `frame:` override could land on
-    /// tile 0. The linker uses this to decide whether to copy the
-    /// built-in smiley into CHR tile 0; programs whose draws all
-    /// resolve to declared sprites with static frames skip the
-    /// emit and get tile 0 back as a blank background tile.
-    fn emit_default_sprite_marker(&mut self) {
-        if !self.default_sprite_used {
+        if self.default_sprite_used {
             self.emit_label("__default_sprite_used");
-            self.default_sprite_used = true;
         }
-    }
-
-    /// Emit the `__p1_input_used` marker label at most once per
-    /// program. Triggered by `IrOp::ReadInput` with `player == 0`.
-    /// Programs that never read controller 1 skip the three NMI
-    /// instructions that shift `$4016` into `ZP_INPUT_P1`.
-    fn emit_p1_input_marker(&mut self) {
-        if !self.p1_input_used {
+        if self.p1_input_used {
             self.emit_label("__p1_input_used");
-            self.p1_input_used = true;
         }
-    }
-
-    /// Emit the `__p2_input_used` marker label at most once per
-    /// program. Triggered by `IrOp::ReadInput` with `player == 1`.
-    /// Single-player programs skip the three NMI instructions that
-    /// shift `$4017` into `ZP_INPUT_P2`.
-    fn emit_p2_input_marker(&mut self) {
-        if !self.p2_input_used {
+        if self.p2_input_used {
             self.emit_label("__p2_input_used");
-            self.p2_input_used = true;
         }
     }
 

tests/emulator/goldens/audio_demo.audio.hash

@@ -1 +1 @@
-b52a0b1e 132084
+23afaaa6 132084

tests/emulator/goldens/friendly_assets.audio.hash

@@ -1 +1 @@
-9387feb3 132084
+bc94ed30 132084

tests/emulator/goldens/noise_triangle_sfx.audio.hash

@@ -1 +1 @@
-a05d1284 132084
+52d1836f 132084

tests/emulator/goldens/platformer.audio.hash

@@ -1 +1 @@
-83c0eee4 132084
+ea23d9c4 132084

tests/emulator/goldens/pong.audio.hash

@@ -1 +1 @@
-b7d5b53f 132084
+555a22e1 132084

tests/emulator/goldens/sfx_pitch_envelope.audio.hash

@@ -1 +1 @@
-ff926f72 132084
+8142ffb7 132084

tests/emulator/goldens/war.audio.hash

@@ -1 +1 @@
-51b560a7 132084
+e3805766 132084