RFR: 8334431: C2 SuperWord: fix performance regression due to store-to-load-forwarding failures [v2]

[Quan Anh Mai]

On Tue, 19 Nov 2024 08:17:18 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Why does the benchmark need to have so many methods, to make sure the different values are treated as constants?  I’m not sure, but JMH might turn @Param values into constants.  If so, then your benchmark can be greatly simplified.
>
> @dean-long I tried that with `@param`, but then they are not constants... sadly. And they need to be constants. Let me know if you find some better way though ;)

@eme64 FYI you can make param a constant using this pattern:

    static final MutableCallSite MUTABLE_CONSTANT = new MutableCallSite(MethodType.methodType(int.class));
    static final MethodHandle MUTABLE_CONSTANT_HANDLE = MUTABLE_CONSTANT.dynamicInvoker();

    static {
        MethodHandle init = MethodHandles.constant(int.class, 1);
        MUTABLE_CONSTANT.setTarget(init);
    }

    @Param({"1", "2"})
    int size;

    @Setup(Level.Iteration)
    public void setup() throws Throwable {
        if (size != (int) MUTABLE_CONSTANT_HANDLE.invokeExact()) {
            MethodHandle constant = MethodHandles.constant(int.class, size);
            MUTABLE_CONSTANT.setTarget(constant);
        }
    }

    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
    private int test() throws Throwable {
        return (int) MUTABLE_CONSTANT_HANDLE.invokeExact();
    }

    @Benchmark
    public void run() throws Throwable {
        test();
    }

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

PR Comment: https://git.openjdk.org/jdk/pull/21521#issuecomment-2485621523