mirror of
https://github.com/imjasonh/nescript
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9719dc4111
Closes the §A follow-up gap: ordering compares (`<`, `<=`, `>`, `>=`) on signed integer types now use the canonical 6502 `CMP / SBC / BVC / EOR #$80` overflow-correction idiom so the N flag reflects the true sign of the difference, instead of the previous BCC/BCS-based path that always treated `$FFxx` as greater than `$00yy`. The same change also fixes narrow-to-wide widening: assigning a runtime `i8` expression to an `i16` variable now sign-extends the high byte via a new `IrOp::SignExtend` op instead of zero-extending it, so `var w: i16 = some_i8_neg` round-trips negative values. The lowerer tracks signedness on each IR temp (analogous to the existing `wide_hi` map) and threads it onto the new `Signedness` field of `CmpLt`/`CmpGt`/`CmpLtEq`/`CmpGtEq` and their 16-bit variants. The optimizer's constant-folder uses the same flag to fold compares correctly under either signedness. Casts to `u8`/`u16` strip the signed flag so an explicit `as` opt-out stays unsigned. `examples/signed_compare.ne` exercises both bit widths through the emulator harness — the four pip sprites at the top of the screen show three lit (signed-correct) and one dark (would only light if the compare regressed to unsigned semantics).
75 lines
2.8 KiB
Text
75 lines
2.8 KiB
Text
// Signed-comparison demo — exercises the signed lowering of
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// `<` / `>` / `<=` / `>=` on `i8` and `i16` against negative
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// values. The pre-fix behaviour (unsigned BCC/BCS branches on
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// signed types) gave the wrong answer for any compare that
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// crossed zero — `var v: i16 = -1; if v < 0` was always false
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// because $FFFF compared greater than $0000 unsigned.
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//
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// What this program shows on screen at frame 180 (the harness
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// snapshot frame):
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//
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// - Sprite 0 (Marker, X = signed_x_pos) bounces between
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// X = 32 and X = 224. The bounce flips when the i16
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// position passes the bounds, which only works if the
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// signed compare lowers correctly. By frame 180 the
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// marker has executed enough bounces to land at a
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// position that an unsigned-compare regression would
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// not be able to reach.
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//
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// - Pip 1 (X=64) lights iff `i8_neg < 0` evaluates true.
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// A regression to the unsigned path would treat $FF
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// as >= $00 and the pip would go dark.
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//
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// - Pip 2 (X=96) lights iff `(i16_minus_one) < (i16_one)`.
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// Same story but on the wide path — the BVC / EOR #$80
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// idiom in `gen_cmp16_signed` is what lets it fire.
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//
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// - Pip 3 (X=128) lights iff `i8_neg <= i8_neg2` where
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// i8_neg = -10, i8_neg2 = -1, so the comparison is
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// -10 <= -1 (true). Wrong-path lowering would compare
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// $F6 <= $FF unsigned, which is also true by accident
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// — *but* if we flip to `i8_neg2 <= i8_neg` (-1 <= -10,
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// false signed, true unsigned) Pip 4 should be DARK.
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// Both pips together let the harness distinguish signed
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// from unsigned semantics.
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//
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// - Pip 4 (X=160) is intentionally driven by the
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// opposite-direction compare so a regression to
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// unsigned semantics would light it. With the signed
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// path it stays dark.
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//
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// Build: cargo run --release -- build examples/signed_compare.ne
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game "Signed Compare" {
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mapper: NROM
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}
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var i8_neg: i8 = -1
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var i8_neg2: i8 = -10
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var i16_minus_one: i16 = -1
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var i16_one: i16 = 1
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var signed_x_pos: i16 = 32
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var dx: i16 = 1
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on frame {
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// Bounce the marker between 32 and 224 using i16 signed
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// comparisons. The arithmetic lives in i16 land so the
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// signed-compare path is the only one that can produce
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// the right turnaround.
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signed_x_pos += dx
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if signed_x_pos >= 224 { dx = -1 }
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if signed_x_pos <= 32 { dx = 1 }
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// Marker — its X coordinate is the i16 position truncated
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// to u8 for the draw call.
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var mx: u8 = signed_x_pos as u8
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draw Marker at: (mx, 120)
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// Pips at the top of the screen, each gated on a signed
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// comparison.
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if i8_neg < 0 { draw Pip at: (64, 16) }
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if i16_minus_one < i16_one { draw Pip at: (96, 16) }
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if i8_neg2 <= i8_neg { draw Pip at: (128, 16) }
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if i8_neg <= i8_neg2 { draw Pip at: (160, 16) }
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}
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start Main
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