RFR (XS): 8191915: JCK tests produce incorrect results with C2

[Tobias Hartmann]

Hi Rickard,

I don't understand why casting from signed to unsigned is correct here because if there's an overflow of the signed
range, there is not necessarily and overflow of the unsigned range, right?

For example, this program:

  signed long long sv1 = LONG_MAX/2 + 1;
  signed long long sv2 = 2;
  signed long long sR = sv1 * sv2;
  printf("%li\n", sR);

  unsigned long long uv1 = (unsigned long long) sv1;	
  unsigned long long uv2 = (unsigned long long) sv2;
  unsigned long long uR = uv1 * uv2;
  printf("%lu\n", uR);

  if (uR / uv1 != uv2) {
    printf("Overflow detected");
  } else {
    printf("No overflow detected");
  }

Output:

  -9223372036854775808
  9223372036854775808
  No overflow detected

So there is an overflow of the signed long but the unsigned long does not overflow and thus an overflow is not detected.
Do I miss something?

Regarding the jtreg test: Please do not use bug numbers as test names but use something more meaningful like
TestLongMulOverflow or similar. I would also suggest to move the allocation of the test instance out of the loop and
check if 500.000 iterations are really necessary to trigger compilation (you can run with -Xbatch and maybe
-XX:-TieredCompilation).

Best regards,
Tobias

On 14.12.2017 08:30, Rickard Bäckman wrote:
> Hi, can I please have this change reviewed?
> 
> Summary is that we had an overflow check for mulitplyExact(long, long)
> intrinsic that relied on undefined behaviour. clang / llvm started
> optimizing that so that if a = b * c then a / b must be equal to c and
> removed half of the overflow check. 
> 
> Changing the multiplication and division to be unsigned.
> 
> Webrev: http://cr.openjdk.java.net/~rbackman/8191915/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8191915
> 
> Thanks
> /R
>