RFR(S): 8221083: [ppc64] Wrong oop compare in C1-generated code

[Doerr, Martin]

Hi Volker,

I think the 2 nested loops are not really needed.
If you use System.gc() + new String you should get it allocated at the desired location.
I believe this is sufficient with the flags you are using.

The test seems to test interpreter, C1, and C2 with the loop limit of 30_000. I like that.

The C1 fix looks good.

Thanks for finding and fixing this bug.

Best regards,
Martin


-----Original Message-----
From: hotspot-compiler-dev <hotspot-compiler-dev-bounces at openjdk.java.net> On Behalf Of Florian Weimer
Sent: Dienstag, 19. März 2019 22:48
To: Volker Simonis <volker.simonis at gmail.com>
Cc: hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
Subject: Re: RFR(S): 8221083: [ppc64] Wrong oop compare in C1-generated code

* Volker Simonis:

> The regression test reproduces the issue by allocation an object at an
> address with the 32-bit least significant bits being zero and comperes
> it with another null object.

  79         for (int i = 0; i < 10; i++) {
  80             System.gc();
  81             for (int j = 0; j < 1024; j++) {
  82                 s = new String("I'm not null!!!");
  83                 if (WB.getObjectAddress(s) == 0x700000000L) break;
  84             }
  85             if (WB.getObjectAddress(s) == 0x700000000L) {
  86                 System.out.println("Got object at address 0x700000000");
  87                 break;
  88             }
  89         }

I think this could use a labeled loop, like this:

        GC_TESTS: for (int i = 0; i < 10; i++) {
            System.gc();
            for (int j = 0; j < 1024; j++) {
                s = new String("I'm not null!!!");
                if (WB.getObjectAddress(s) == 0x700000000L) {
                    System.out.println("Got object at address 0x700000000");
                    break GC_TESTS;
                }
            }
        }

(Untested.)

On the other hand, neither this version or yours properly detects when
s is *not* actually the object with the desired address, in which case
the test objective fails to materialize, I think.