[PING] RE: RFR(S): 8210152: Optimize integer divisible by power-of-2 check

[Vladimir Ivanov]

On a side note: the transformation doesn't look specific to int. How 
hard would it be to extend it to cover long?

Best regards,
Vladimir Ivanov

On 10/09/2018 07:24, Pengfei Li (Arm Technology China) wrote:
> Hi Dean / Vladimir / JDK experts,
> 
> Do you have any further questions or comments on this patch? Or should I make some modifications on it, such as adding some limitations to the matching condition?
> I appreciate your help.
> 
> --
> Thanks,
> Pengfei
> 
> 
>> -----Original Message-----
>> From: Pengfei Li (Arm Technology China)
>> Sent: Monday, September 3, 2018 13:50
>> To: 'dean.long at oracle.com' <dean.long at oracle.com>; 'Vladimir Kozlov'
>> <vladimir.kozlov at oracle.com>; hotspot-compiler-dev at openjdk.java.net;
>> hotspot-dev at openjdk.java.net
>> Cc: nd <nd at arm.com>
>> Subject: RE: RFR(S): 8210152: Optimize integer divisible by power-of-2 check
>>
>> Hi Vladimir, Dean,
>>
>> Thanks for your review.
>>
>>> I don't see where negation is coming from for 'X % 2 == 0' expression.
>>> It should be only 2 instructions: 'cmp (X and 1), 0'
>> The 'cmp (X and 1), 0' is just what we expected. But there's redundant
>> conditional negation coming from the possibly negative X handling in "X % 2".
>> For instance, X = -5, "X % 2" should be -1. So only "(X and 1)" operation is not
>> enough. We have to negate the result.
>>
>>> I will look on it next week. But it would be nice if you can provide small test
>> to show this issue.
>> I've already provided a case of "if (a%2 == 0) { ... }" in JBS description. What
>> code generated and what can be optimized are listed there.
>> You could see https://bugs.openjdk.java.net/browse/JDK-8210152 for details.
>> You could also see the test case for this optimization I attached below.
>>
>>> It looks like your matching may allow more patterns than expected. I was
>> expecting it to look for < 0 or >= 0 for the conditional negation, but I don't see
>> it.
>> Yes. I didn't limit the if condition to <0 or >= 0 so it will match more patterns.
>> But nothing is going wrong if this ideal transformation applies on more cases.
>> In pseudo code, if someone writes:
>> if ( some_condition ) { x = -x; }
>> if ( x == 0 ) { do_something(); }
>> The negation in 1st if-clause could always be eliminated whatever the
>> condition is.
>>
>> --
>> Thanks,
>> Pengfei
>>
>>
>> -- my test case attached below --
>> public class Foo {
>>
>>      public static void main(String[] args) {
>>          int[] dividends = { 0, 17, 1553, -90, -35789, 0x80000000 };
>>          for (int i = 0; i < dividends.length; i++) {
>>              int x = dividends[i];
>>              System.out.println(testDivisible(x));
>>              System.out.println(testModulo(x));
>>              testCondNeg(x);
>>          }
>>          return;
>>      }
>>
>>      public static int testDivisible(int x) {
>>          // Modulo result is only for zero check
>>          if (x % 4 == 0) {
>>              return 444;
>>          }
>>          return 555;
>>      }
>>
>>      public static int testModulo(int x) {
>>          int y = x % 4;
>>          if (y == 0) {
>>              return 222;
>>          }
>>          // Modulo result is used elsewhere
>>          System.out.println(y);
>>          return 333;
>>      }
>>
>>      public static void testCondNeg(int x) {
>>          // Pure conditional negation
>>          if (printAndIfNeg(x)) {
>>              x = -x;
>>          }
>>          if (x == 0) {
>>              System.out.println("zero!");
>>          }
>>      }
>>
>>      static boolean printAndIfNeg(int x) {
>>          System.out.println(x);
>>          return x <= 0;
>>      }
>> }