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

[Pengfei Li (Arm Technology China)]

Hi Vladimir Ivanov,

Thanks for your suggestion. I've investigated the effort to extend it to long. But unfortunately, different graph nodes are created for _irem and _lrem by 2. In bytecode parsing for _irem, a conditional negation with Phi node inside is created when the divisor is power-of-2 (see http://hg.openjdk.java.net/jdk/jdk/file/8c7198cac800/src/hotspot/share/opto/parse2.cpp#l1170 ). But C2 parser does not do in the similar way for _lrem. A "ModLNode" is created whatever the divisor equals (see http://hg.openjdk.java.net/jdk/jdk/file/8c7198cac800/src/hotspot/share/opto/parse2.cpp#l2539 ). So the "x % 2L == 0L" could not get benefit even this is extended to long. Maybe the long integer pattern can be optimized in other techniques. So I just wanted to do on 32bit integer at this time.

--
Thanks again,
Pengfei


> -----Original Message-----
> From: Vladimir Ivanov <vladimir.x.ivanov at oracle.com>
> Sent: Tuesday, September 11, 2018 03:38
> To: Pengfei Li (Arm Technology China) <Pengfei.Li at arm.com>; hotspot-
> dev at openjdk.java.net
> Cc: nd <nd at arm.com>
> Subject: Re: [PING] RE: RFR(S): 8210152: Optimize integer divisible by power-
> of-2 check
> 
> 
> 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;
> >>      }
> >> }