Question

Which function is faster (p is atomic public int property of MyObject):

public static boolean check(MyObject o1, List<MyObject> list) {

    int p = o1.p;
    for (int i = 0; i < 1000000; i++) {
        MyObject o = list.get(i);
        if (o.p < p) return false;
    }

    return true;
}

or

public static boolean check(MyObject o1, List<MyObject> list) {

    for (int i = 0; i < 1000000; i++) {
        MyObject o = list.get(i);
        if (o.p < o1.p) return false;
    }

    return true;
}

So by using the local variable p we cache the object property call or it is done inline by the compiler optimization?

Answer 1

Short Answer: It depends

Slightly longer Answer: It depends on the compiler, the VM and the settings of your VM.

Background: Using the HotSpot VM (most common flavor) in server mode will make both variants equal since the VM does loop invariant hoisting in server mode. In client mode this may be done, may not be done or even may be done later if the VM considers it worthy of optimisations.

Loop invariant hoisting is one of the loop optimisation, that is implemented into most modern compilers (or in the case of Java, VMs). As for the code generated by javac: Without further optimization done by the VM your first code sniplet will perfom faster.

public static boolean check(Test$MyObject, java.util.List);
  Code:
   0:   aload_0
   1:   getfield        #7; //Field Test$MyObject.p:I
   4:   istore_2
   5:   iconst_0
   6:   istore_3
   7:   iload_3
   8:   ldc     #4; //int 1000000
   10:  if_icmpge       42
   13:  aload_1
   14:  iload_3
   15:  invokeinterface #11,  2; //InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
   20:  checkcast       #5; //class Test$MyObject
   23:  astore  4
   25:  aload   4
   27:  getfield        #7; //Field Test$MyObject.p:I
   30:  iload_2
   31:  if_icmpge       36
   34:  iconst_0
   35:  ireturn
   36:  iinc    3, 1
   39:  goto    7
   42:  iconst_1
   43:  ireturn

—

public static boolean check(Test$MyObject, java.util.List);
  Code:
   0:   iconst_0
   1:   istore_2
   2:   iload_2
   3:   ldc     #4; //int 1000000
   5:   if_icmpge       38
   8:   aload_1
   9:   iload_2
   10:  invokeinterface #11,  2; //InterfaceMethod java/util/List.get:(I)Ljava/lang/Object;
   15:  checkcast       #5; //class Test$MyObject
   18:  astore_3
   19:  aload_3
   20:  getfield        #7; //Field Test$MyObject.p:I
   23:  aload_0
   24:  getfield        #7; //Field Test$MyObject.p:I
   27:  if_icmpge       32
   30:  iconst_0
   31:  ireturn
   32:  iinc    2, 1
   35:  goto    2
   38:  iconst_1
   39:  ireturn

As you can see, the getfield operation from line 20 in the second example lies in line 1 in the first and is outside of the loop (lines 7 to 39 in variant 1 and lines 2 to 35 in variant 2) and is therefore only executed ondced instead of 1000000 times.