Fabs Ridgeline

Rendering & Graphics

An animated terrain ridgeline on a Super Nintendo driven by the tent map iteration using __builtin_fabsf as the hot absolute-value operation. The kink at x=0.5 creates sharp V-shaped peaks; as the seeds drift each frame the terrain undulates. This is the first demo to fire the LLVM backend's TARGET-CUSTOM legalizeFAbs path — a single inline AND src, 0x7FFFFFFF that clears the float sign bit with no libcall. Compiler stress-test #80 of the battery.

loading core…

Self-running — the terrain undulates as the tent-map seeds drift.

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

What it is

The tent map f(x) = 1 − |2x − 1| is a simple chaotic function on [0,1]. The absolute value creates a sharp V-notch: points below x=0.5 reflect up, points above reflect down. After a few iterations from different seeds, the outputs spread across [0,1] in a pattern that depends sensitively on the starting value.

// One tent-map step — each op a separate statement (no FMA fusion)
float two_x   = 2.0f * x;           // __mulsf3
float shifted = two_x - 1.0f;       // __subsf3
float abs_val = __builtin_fabsf(shifted);  // G_FABS → legalizeFAbs inline AND
float next    = 1.0f - abs_val;     // __subsf3

// legalizeFAbs (MOSLegalizerInfo.cpp:3410) expands to:
// AND src, 0x7FFFFFFF   ← clear bit 31 (sign bit); zero libcall

Each of the 16 tile columns gets a height from three tent-map iterations seeded by (column, time). Sky tiles fill above the ridge, a bright cap marks the ridge line, medium and dark tiles fill below. The seeds shift by a small offset each frame so the terrain slowly undulates. The V-notches in the tent map appear as sharp triangular peaks and valleys.

Compiler stress-test

Item	What it exercises
G_FABS	The float absolute-value operation legalizes through a TARGET-CUSTOM path (MOSLegalizerInfo.cpp:369 → legalizeFAbs at :3410) that emits a single AND instruction, clearing only the sign bit (bit 31 of the 32-bit float representation). No libcall — the SDK has no fabsf symbol because inline sign-clear is cheaper and sufficient.
tent map	x → 1 − \|2x − 1\| is a chaotic map on [0,1] with a V-notch kink at x=0.5. The absolute value of (2x−1) creates the kink; peaks in the terrain profile correspond to seeds that iterate close to x=0.5. Three iterations are enough to produce a varied ridgeline.
distinct from #57 and #45	#57 medfilt uses integer G_ABS (a different node, different legalizer at :281) on int16_t values — never a float sign-bit operation. #45 metaball type-puns a float to uint32_t and back through the integer ALU — also never G_FABS. This is the first demo that generates the float sign-clear path.
negative-zero edge	fabsf(−0.0f) = +0.0f (IEEE 754 negative zero canonicalizes to positive zero). The legalizeFAbs comment at :3420 explicitly guards this: the AND with 0x7FFFFFFF also clears the sign bit of −0.0f → +0.0f, matching host behaviour.
GATE_N = 120	With 128 columns and a rotating-XOR accumulator of period 16, a gate of exactly 128 steps reduces to 0 (8 full rotations cancel all contributions). Using 120 steps (a non-multiple of 16) breaks the period and gives a non-trivial CRC 0x161A.

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 0x161A — a fold of 120 tent-map column heights) live in this tab. No far pointers — host == default == +mos-a16 == +mos-xy16, -verify clean.