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package org.graalvm.compiler.replacements.aarch64;

import org.graalvm.compiler.api.replacements.ClassSubstitution;
import org.graalvm.compiler.api.replacements.MethodSubstitution;

AArch64 ISA offers a count leading zeros instruction which can be used to implement numberOfLeadingZeros more efficiently than using BitScanReverse.
/** * AArch64 ISA offers a count leading zeros instruction which can be used to implement * numberOfLeadingZeros more efficiently than using BitScanReverse. */
@ClassSubstitution(Integer.class) public class AArch64IntegerSubstitutions { @MethodSubstitution public static int bitCount(int value) { // Based on Warren, Hacker's Delight, slightly adapted to profit from Aarch64 add + shift // instruction. // Assuming the peephole optimizer optimizes all x - y >>> z into a single instruction // this takes 10 instructions. int x = value; x = x - ((x & 0xaaaaaaaa) >>> 1); x = (x & 0x33333333) + ((x & 0xcccccccc) >>> 2); x = (x + (x >>> 4)) & 0x0f0f0f0f; x = x + (x >>> 8); x = x + (x >>> 16); return x & 0x3f; } }