For the method p in the following Java program:

class Test {

  static int p(int n) {
    int i;
    while (n > 0)
      if (n < 3)
        i = 0;
      else
       i = 1;
    return 0;
  }

  public static void main(String[] args) {
    System.out.println(p(10));
  }

}

the IR's loop recognition produces suboptimal loop enter/exit nodes:

E:\gri\work>java_g -Xcomp -XX:CompileOnly=.p -XX:+PrintIR2 Test
VM option 'CompileOnly=.p'
VM option '+PrintIR2'
IR before code generation
B6 [0, 0] -> B7
__bci__use__tid____instr____________________________________
. 0 0 24 std entry B7

B7 [0, 0] -> B8
__bci__use__tid____instr____________________________________
. 0 0 6 goto B8

B8 [0, 0] -> B0
__bci__use__tid____instr____________________________________
. 0 0 31 enter loop[0]
. 0 0 30 goto B0

B3 [3, 5] -> B9 B2
__bci__use__tid____instr____________________________________
  3 1 i12 L0
  4 1 i13 3
. 5 0 14 if i12 >= i13 then B9 else B2

B2 [8, 10] -> B0
__bci__use__tid____instr____________________________________
  8 1 i15 0
. 9 0 i16 L1 := i15
. 10 0 17 goto B0

B9 [13, 13] -> B1
__bci__use__tid____instr____________________________________
. 13 0 34 exit loop[0]
. 13 0 33 goto B1

B1 [13, 14] -> B8
__bci__use__tid____instr____________________________________
  13 1 i18 1
. 14 0 i19 L1 := i18
. 14 0 20 goto B8

B0 [15, 16] -> B3 B10
__bci__use__tid____instr____________________________________
  15 1 i7 L0
  16 1 i8 0
. 16 0 9 if i7 > i8 then B3 else B10

B10 [19, 19] -> B4
__bci__use__tid____instr____________________________________
. 19 0 37 exit loop[0]
. 19 0 36 goto B4

B4 [19, 20]
__bci__use__tid____instr____________________________________
  19 1 i10 0
. 20 0 i11 ireturn i10>


In basic block B9, the loop is exited before B1 is executed; only to enter the loop again via B8. The loop should not be left at all in this branch. This may result in suboptimal code for x86 (register usage) and thus a performance regression. Note that the generated code is still correct, though.

robert.griesemer@Eng 2000-06-16
(problem found by Peter Moessenboeck)