[riscv-port] RFR: 8277884: riscv: Fix cmpxchg_narrow_value that needs to sign-extend non-bool results

29 Nov 2021

      Hi team,

According to https://bugs.openjdk.java.net/browse/JDK-8277884 and [the original patch](https://github.com/riscv-collab/riscv-openjdk/pull/15):

A very simple program inherited from jcstress could reproduce this:

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

// run with:
// build/linux-riscv64-server-release/images/jdk/bin/java -ea -XX:-TieredCompilation -XX:CompileCommand=compileonly,jdk.internal.misc.Unsafe::compareAndSetByte TestCASByte
// assert will fail.
// if we add '-XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_compareAndExchangeByte' this test will pass.

public class TestCASByte {

    byte[] array = new byte[1];

    static final VarHandle vh;
    static {
        try {
            vh = MethodHandles.arrayElementVarHandle(byte[].class);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    public void test() {
        for (int i = 0; i < 100_000; i++) { // To level 4
            // clear
            array[0] = 0;

            byte res1 = (byte)vh.getAndSet(array, 0, (byte)-1);
            byte res2 = (byte)vh.getAndSet(array, 0, (byte)2);

            assert res1 == 0 && res2 == -1;
        }
    }

    public static void main(String[] args) {
        new TestCASByte().test();
    }

}

In this case when we are getting the -1 value saved in memory, the shift is 0, the old is 0x00000000000000ff and the mask is 0x00000000000000ff at the end of cmpxchg_narrow_value. The final result is 0x00000000000000ff but it indeed should be 0xffffffffffffffff. BTW the shift is 0. Therefore, if the result is negative, we need to make a sign extension.

Thanks,
Xiaolin

-------------

Commit messages:
 - Fix cmpxchg_narrow_value that needs to sign-extend non-bool results

Changes: https://git.openjdk.java.net/riscv-port/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=15&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277884
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/15/head:pull/15

PR: https://git.openjdk.java.net/riscv-port/pull/15

[riscv-port] RFR: 8277884: riscv: Fix cmpxchg_narrow_value that needs to sign-extend non-bool results

zhengxiaolinX