mirror of
https://github.com/imjasonh/nescript
synced 2026-07-08 08:55:38 +00:00
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
|
|
// `<` / `>` / `<=` / `>=` on `i8` and `i16` against negative
|
|
// values. The pre-fix behaviour (unsigned BCC/BCS branches on
|
|
// signed types) gave the wrong answer for any compare that
|
|
// crossed zero — `var v: i16 = -1; if v < 0` was always false
|
|
// because $FFFF compared greater than $0000 unsigned.
|
|
//
|
|
// What this program shows on screen at frame 180 (the harness
|
|
// snapshot frame):
|
|
//
|
|
// - Sprite 0 (Marker, X = signed_x_pos) bounces between
|
|
// X = 32 and X = 224. The bounce flips when the i16
|
|
// position passes the bounds, which only works if the
|
|
// signed compare lowers correctly. By frame 180 the
|
|
// marker has executed enough bounces to land at a
|
|
// position that an unsigned-compare regression would
|
|
// not be able to reach.
|
|
//
|
|
// - Pip 1 (X=64) lights iff `i8_neg < 0` evaluates true.
|
|
// A regression to the unsigned path would treat $FF
|
|
// as >= $00 and the pip would go dark.
|
|
//
|
|
// - Pip 2 (X=96) lights iff `(i16_minus_one) < (i16_one)`.
|
|
// Same story but on the wide path — the BVC / EOR #$80
|
|
// idiom in `gen_cmp16_signed` is what lets it fire.
|
|
//
|
|
// - Pip 3 (X=128) lights iff `i8_neg <= i8_neg2` where
|
|
// i8_neg = -10, i8_neg2 = -1, so the comparison is
|
|
// -10 <= -1 (true). Wrong-path lowering would compare
|
|
// $F6 <= $FF unsigned, which is also true by accident
|
|
// — *but* if we flip to `i8_neg2 <= i8_neg` (-1 <= -10,
|
|
// false signed, true unsigned) Pip 4 should be DARK.
|
|
// Both pips together let the harness distinguish signed
|
|
// from unsigned semantics.
|
|
//
|
|
// - Pip 4 (X=160) is intentionally driven by the
|
|
// opposite-direction compare so a regression to
|
|
// unsigned semantics would light it. With the signed
|
|
// path it stays dark.
|
|
//
|
|
// Build: cargo run --release -- build examples/signed_compare.ne
|
|
|
|
game "Signed Compare" {
|
|
mapper: NROM
|
|
}
|
|
|
|
var i8_neg: i8 = -1
|
|
var i8_neg2: i8 = -10
|
|
var i16_minus_one: i16 = -1
|
|
var i16_one: i16 = 1
|
|
var signed_x_pos: i16 = 32
|
|
var dx: i16 = 1
|
|
on frame {
|
|
// Bounce the marker between 32 and 224 using i16 signed
|
|
// comparisons. The arithmetic lives in i16 land so the
|
|
// signed-compare path is the only one that can produce
|
|
// the right turnaround.
|
|
signed_x_pos += dx
|
|
if signed_x_pos >= 224 { dx = -1 }
|
|
if signed_x_pos <= 32 { dx = 1 }
|
|
|
|
// Marker — its X coordinate is the i16 position truncated
|
|
// to u8 for the draw call.
|
|
var mx: u8 = signed_x_pos as u8
|
|
draw Marker at: (mx, 120)
|
|
|
|
// Pips at the top of the screen, each gated on a signed
|
|
// comparison.
|
|
if i8_neg < 0 { draw Pip at: (64, 16) }
|
|
if i16_minus_one < i16_one { draw Pip at: (96, 16) }
|
|
if i8_neg2 <= i8_neg { draw Pip at: (128, 16) }
|
|
if i8_neg <= i8_neg2 { draw Pip at: (160, 16) }
|
|
}
|
|
|
|
start Main
|