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

[Pengfei Li (Arm Technology China)]

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;
>     }
> }