PanamaJIT Demo

PanamaJIT.java

import jdk.incubator.foreign.*;
import java.lang.invoke.*;

// Original code (C++): https://stackoverflow.com/a/40937610/7659948
public class PanamaJIT {

    public static final int MEM_COMMIT = 0x00001000;

    public static final int PAGE_READWRITE = 0x04;
    public static final int PAGE_EXECUTE_READ = 0x20;

    public static final int PAGE_SIZE = 4096;

    private static final MethodHandle virtualAllocHandle;
    private static final MethodHandle virtualProtectHandle;

    static {
        System.loadLibrary("Kernel32");

        virtualAllocHandle = CLinker.getInstance().downcallHandle(
                SymbolLookup.loaderLookup().lookup("VirtualAlloc").orElseThrow(UnsatisfiedLinkError::new),
                MethodType.methodType(MemoryAddress.class, MemoryAddress.class, long.class, int.class, int.class),
                FunctionDescriptor.of(CLinker.C_POINTER, CLinker.C_POINTER, CLinker.C_LONG_LONG, CLinker.C_LONG, CLinker.C_LONG)
        );

        virtualProtectHandle = CLinker.getInstance().downcallHandle(
                SymbolLookup.loaderLookup().lookup("VirtualProtect").orElseThrow(UnsatisfiedLinkError::new),
                MethodType.methodType(int.class, MemoryAddress.class, long.class, int.class, MemoryAddress.class),
                FunctionDescriptor.of(CLinker.C_INT, CLinker.C_POINTER, CLinker.C_LONG_LONG, CLinker.C_LONG, CLinker.C_POINTER)
        );

    }

    public static MemoryAddress VirtualAlloc(MemoryAddress lpAddress, long dwSize, int flAllocationType, int flProtect) {
        try {
            return (MemoryAddress) virtualAllocHandle.invokeExact(lpAddress, dwSize, flAllocationType, flProtect);
        } catch (Throwable e) {
            throw new AssertionError(e);
        }
    }

    public static boolean VirtualProtect(MemoryAddress lpAddress, long dwSize, int flNewProtect, MemoryAddress lpflOldProtect) {
        try {
            return (int) virtualProtectHandle.invokeExact(lpAddress, dwSize, flNewProtect, lpflOldProtect) != 0;
        } catch (Throwable e) {
            throw new AssertionError(e);
        }
    }

    /*
     * Return: () -> value
     */
    private static byte[] compile(int value) {
        /*
         *  movl    $value, %eax
         *  retq
         */
        return new byte[]{
                (byte) 0xb8,
                (byte) (value), (byte) (value >>> 8), (byte) (value >>> 16), (byte) (value >>> 24),
                (byte) 0xc3
        };
    }

    public static void main(String[] args) throws Throwable {
        byte[] code = compile(Integer.parseInt(args[0]));

        MemoryAddress buffer = VirtualAlloc(MemoryAddress.NULL, PAGE_SIZE, MEM_COMMIT, PAGE_READWRITE);
        buffer.asSegment(code.length, ResourceScope.globalScope()).copyFrom(MemorySegment.ofArray(code));

        MemoryAddress dummy = CLinker.allocateMemory(4);
        VirtualProtect(buffer, code.length, PAGE_EXECUTE_READ, dummy);

        MethodHandle functionPtr = CLinker.getInstance().downcallHandle(
                buffer,
                MethodType.methodType(int.class),
                FunctionDescriptor.of(CLinker.C_INT)
        );

        int result = (int) functionPtr.invokeExact();
        assert result == Integer.parseInt(args[0]);
        System.out.println("Result: " + result);
    }
}

Platform: Windows AMD64, Java 17
Use java --add-modules jdk.incubator.foreign --enable-native-access=ALL-UNNAMED PanamaJIT.java <any integer> to run it.