From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: JellySquid Date: Fri, 5 Feb 2021 00:16:30 -0600 Subject: [PATCH] lithium: CompactSineLUT Original code by CaffeineMC, licensed under GNU Lesser General Public License v3.0 You can find the original code on https://github.com/CaffeineMC/lithium-fabric (Yarn mappings) diff --git a/src/main/java/me/jellysquid/mods/lithium/common/util/math/CompactSineLUT.java b/src/main/java/me/jellysquid/mods/lithium/common/util/math/CompactSineLUT.java new file mode 100644 index 0000000000000000000000000000000000000000..b8c9cb28876c2c1781cd72870076d528b9647916 --- /dev/null +++ b/src/main/java/me/jellysquid/mods/lithium/common/util/math/CompactSineLUT.java @@ -0,0 +1,90 @@ +package me.jellysquid.mods.lithium.common.util.math; + +import net.minecraft.util.Mth; + +/** + * A replacement for the sine angle lookup table used in {@link Mth}, both reducing the size of LUT and improving + * the access patterns for common paired sin/cos operations. + * + * sin(-x) = -sin(x) + * ... to eliminate negative angles from the LUT. + * + * sin(x) = sin(pi/2 - x) + * ... to eliminate supplementary angles from the LUT. + * + * Using these identities allows us to reduce the LUT from 64K entries (256 KB) to just 16K entries (64 KB), enabling + * it to better fit into the CPU's caches at the expense of some cycles on the fast path. The implementation has been + * tightly optimized to avoid branching where possible and to use very quick integer operations. + * + * Generally speaking, reducing the size of a lookup table is always a good optimization, but since we need to spend + * extra CPU cycles trying to maintain parity with vanilla, there is the potential risk that this implementation ends + * up being slower than vanilla when the lookup table is able to be kept in cache memory. + * + * Unlike other "fast math" implementations, the values returned by this class are *bit-for-bit identical* with those + * from {@link Mth}. Validation is performed during runtime to ensure that the table is correct. + * + * @author coderbot16 Author of the original (and very clever) implementation in Rust: + * https://gitlab.com/coderbot16/i73/-/tree/master/i73-trig/src + * @author jellysquid3 Additional optimizations, port to Java + */ +public class CompactSineLUT { + private static final int[] SINE_TABLE_INT = new int[16384 + 1]; + private static final float SINE_TABLE_MIDPOINT; + + static { + final float[] SINE_TABLE = Mth.getSinTable(); + // Copy the sine table, covering to raw int bits + for (int i = 0; i < SINE_TABLE_INT.length; i++) { + SINE_TABLE_INT[i] = Float.floatToRawIntBits(SINE_TABLE[i]); + } + + SINE_TABLE_MIDPOINT = SINE_TABLE[SINE_TABLE.length / 2]; + + // Test that the lookup table is correct during runtime + for (int i = 0; i < SINE_TABLE.length; i++) { + float expected = SINE_TABLE[i]; + float value = lookup(i); + + if (expected != value) { + throw new IllegalArgumentException(String.format("LUT error at index %d (expected: %s, found: %s)", i, expected, value)); + } + } + } + + // [VanillaCopy] MathHelper#sin(float) + public static float sin(float f) { + return lookup((int) (f * 10430.378f) & 0xFFFF); + } + + // [VanillaCopy] MathHelper#cos(float) + public static float cos(float f) { + return lookup((int) (f * 10430.378f + 16384.0f) & 0xFFFF); + } + + private static float lookup(int index) { + // A special case... Is there some way to eliminate this? + if (index == 32768) { + return SINE_TABLE_MIDPOINT; + } + + // Trigonometric identity: sin(-x) = -sin(x) + // Given a domain of 0 <= x <= 2*pi, just negate the value if x > pi. + // This allows the sin table size to be halved. + int neg = (index & 0x8000) << 16; + + // All bits set if (pi/2 <= x), none set otherwise + // Extracts the 15th bit from 'half' + int mask = (index << 17) >> 31; + + // Trigonometric identity: sin(x) = sin(pi/2 - x) + int pos = (0x8001 & mask) + (index ^ mask); + + // Wrap the position in the table. Moving this down to immediately before the array access + // seems to help the Hotspot compiler optimize the bit math better. + pos &= 0x7fff; + + // Fetch the corresponding value from the LUT and invert the sign bit as needed + // This directly manipulate the sign bit on the float bits to simplify logic + return Float.intBitsToFloat(SINE_TABLE_INT[pos] ^ neg); + } +} \ No newline at end of file diff --git a/src/main/java/net/minecraft/util/Mth.java b/src/main/java/net/minecraft/util/Mth.java index 9b22034aa655ceb0da151d9d8ca3147f6487889a..ec587cf6592a1dc0d90d6f54af1bdfab97aec7c6 100644 --- a/src/main/java/net/minecraft/util/Mth.java +++ b/src/main/java/net/minecraft/util/Mth.java @@ -32,6 +32,7 @@ public class Mth { }); private static final RandomSource RANDOM = RandomSource.createThreadSafe(); + public static float[] getSinTable() { return SIN; } // DivineMC - lithium: CompactSineLUT private static final int[] MULTIPLY_DE_BRUIJN_BIT_POSITION = new int[]{0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9}; private static final double ONE_SIXTH = 0.16666666666666666D; private static final int FRAC_EXP = 8; @@ -41,11 +42,11 @@ public class Mth { private static final double[] COS_TAB = new double[257]; public static float sin(float value) { - return SIN[(int)(value * 10430.378F) & '\uffff']; + return me.jellysquid.mods.lithium.common.util.math.CompactSineLUT.sin(value); // DivineMC - lithium: CompactSineLUT } public static float cos(float value) { - return SIN[(int)(value * 10430.378F + 16384.0F) & '\uffff']; + return me.jellysquid.mods.lithium.common.util.math.CompactSineLUT.cos(value); // DivineMC - lithium: CompactSineLUT } public static float sqrt(float value) {