RFR(S): 8241091: AArch64: "bad AD file" with VM option "-XX:-UsePopCountInstruction"

[Pengfei Li]

Hi,

AArch64 C2 shows an internal error of "bad AD file" when VM option "UsePopCountInstruction" is disabled. This can be reproduced by below simple case with fastdebug build.

public class Foo {
    private static int bar(int x) {
        return Integer.bitCount(x);
    }
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 0; i < 30000; i++) {
          sum += bar(i);
        }
        System.out.println(sum);
    }
}

Cause is that the PopCountINode or PopCountLNode is created but no match rule for it without "UsePopCountInstruction" enabled. One fix [1] for this is to avoid creating the nodes. As the CNT instruction is available on all AArch64 CPUs and the bitCount() would be ~7x slower (according to my local test) if it's disabled, there is another fix [2] that force the flag to be true on AArch64. But I'm not sure if it's good to ignore a general vm option in an architecture backend.

[1] http://cr.openjdk.java.net/~pli/rfr/8241091/webrev.00/
[2] http://cr.openjdk.java.net/~pli/rfr/8241091/webrev.01/
JBS: https://bugs.openjdk.java.net/browse/JDK-8241091 

Please let me know which kind of fix do you prefer, or any other comments.

--
Thanks,
Pengfei