Signed-Bitfield Terrain

Algorithms

An animated eroding terrain on a Super Nintendo. Each cell is a 16-bit struct packing int16_t height:5, slope:4, flow:4 and uint16_t mat:3. Reading the signed fields fires G_SEXT_INREG — the sign-extend-in-register legalizer at llvm-mos line 130 — expanding each field read to a shift-pair. Ridges erode, valleys fill. The indigo valleys (signed negative heights) prove sign-extension fires correctly. Compiler stress-test #78, the first signed-bitfield demo.

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Self-running — ridges erode and valleys fill each frame.

Click the screen to focus, then watch. Tab away and it pauses.

What it is

A 16×16 grid of SBCell structs, each packing three signed bitfields into one 16-bit word. The height field (5 bits, range −16..15) drives the colour: indigo for deep negative valleys, teal for shallow valleys, orange for low ridges, gold for high ridges. Each frame, one erosion step reads all three signed fields per cell — triggering G_SEXT_INREG three times each — and updates height and flow based on the signed slope.

// SBCell: 5+4+4+3 = 16 bits, one storage word.
typedef struct {
    int16_t  height : 5;   // [-16..15] — G_SEXT_INREG on read
    int16_t  slope  : 4;   // [-8..7]   — G_SEXT_INREG on read
    int16_t  flow   : 4;   // [-8..7]   — G_SEXT_INREG on read
    uint16_t mat    : 3;   // [0..7]    — zero-extend (no sext)
} SBCell;

// One erosion step — three G_SEXT_INREG reads per cell:
int16_t h = cell->height;  // sext_inreg(word, 5): shl(11) >> 11
int16_t s = cell->slope;   // sext_inreg(word, 4): shl(12) >> 12
int16_t f = cell->flow;    // sext_inreg(word, 4): shl(12) >> 12

if (s > 0) { h--; if (h < -16) h = -16; }  // ridge erodes
else if (s < 0) { h++; if (h > 15) h = 15; } // valley fills
f = (f + s) >> 1;  // signed running average

The erosion is a classic cellular-automaton process: positive-slope cells wear down, negative-slope cells fill. The wavefront is visible as a moving boundary between gold ridges and indigo valleys.

Compiler stress-test

Item	What it exercises
G_SEXT_INREG	Reading a signed bitfield back from a struct fires G_SEXT_INREG in GlobalISel. The MOS legalizer at line 130 lowers it with .lower(): sext_inreg(x, N) → (x << (16−N)) >> (16−N), using an arithmetic shift pair (ASL to position the sign bit, then arithmetic shift right to fill). For a 5-bit field: shl(11) + ashr(11). For a 4-bit field: shl(12) + ashr(12).
distinct from #29b and #52	#29b truchet used uint16_t bitfields (orient:1/style:1/hue:3/phase:2/mark:1/energy:4) which zero-extend — no G_SEXT_INREG. #52 disbits used uint32_t bitfields — also zero-extend. Neither had an int16_t container, confirmed by grep. This demo is the first signed-bitfield path in the battery.
signed valleys as proof	A height value of −8 maps to colour 0 (indigo), while +0 maps to colour 2 (orange). If the compiler generated zero-extension instead of sign-extension, −8 stored as 5-bit 0b11000 would read back as +24 and map to colour 3 (gold), not colour 0. The indigo valleys visible on screen prove sign-extension is firing correctly.
erosion wavefront	Cells with positive slope erode (height decrements) and cells with negative slope fill (height increments). This creates a self-organising wavefront that sweeps through the grid: ridges wear down toward the floor, valleys fill toward the mean. After many frames the terrain converges toward a flat plain, making the motion a signature of the signed arithmetic working correctly.
integer-exact gate	16 erosion steps × 16×16 cells × 3 signed field reads per cell = 12,288 G_SEXT_INREG operations. Any zero-extension (wrong read) shifts negative values positive, changing the erosion trajectory and diverging the CRC immediately. All-integer, no float. Disasm shows asl=41 (ASL for the shift-left half of each sign-extend pair).

Written in C with the llvm-mos 65816 toolchain and verified against bsnes-jg and MAME. Hit Verify fidelity to reproduce the build gate's WRAM assert (gate CRC 0x40C5 — 16 steps × 16×16 cells, fold of signed height/slope/flow × mat) live in this tab. No far pointers — host == default == +mos-a16 == +mos-xy16, -verify clean.