Code optimization by compiler in Java- Time taken for execution of "if" condition is Zero

Question

I have the following code..

 public static void main(String[] args) {
            int i = 1234;
            int j = 1234;
            int k = 4321;
            long l1 = System.nanoTime();
            if (i == j) {
                System.out.println("equal");
            }
            System.out.println(System.nanoTime() - l1);
            l1 = System.nanoTime();

            if (i != k) {
                System.out.println("equal");
            }

            System.out.println(System.nanoTime() - l1);
            l1 = System.nanoTime();

            if (i == k) {
                System.out.println("equal");
            }

            System.out.println(System.nanoTime() - l1);


            l1 = System.nanoTime();

            if (i != j) {
                System.out.println("equal");
            }
            System.out.println(System.nanoTime() - l1);

        }

Now when I run this program, the OPs are:

1. 
equal
190295
equal
22237
427
0

2. 
equal
156512
equal
18388
0
428

3. 
equal
155656
equal
20099
0
0

My question : how is it that the time taken for execution is "0" ??.. If the compiler does some optimization, then why doesn't it do it always?. I think the "0" is because that statement wasnt executed by the compiler.

Answer 1

The actual reason

The output varies and is sometimes 0, sometimes not as you already noticed. The likeliest issue is that System.nanoTime:

provides nanosecond precision, but not necessarily nanosecond resolution (that is, how frequently the value changes)

So 0 does not necessarily mean 0 - it just means: less than the smallest "tick" that your clock can represent.

---

Can we be sure that the code hasn't been optimised by the compiler?

If you use the -XX:+PrintCompilation flag when running the program, you will get an output similar to below. If you read carefully, you will notice that main has not been compiled by the JIT yet (which makes sense since it is not really hot) - so your code can't have been optimised away.

 62    1       3       java.lang.String::hashCode (55 bytes)
 62    3       3       java.lang.String::indexOf (70 bytes)
 63    2       3       java.lang.String::charAt (29 bytes)
 65    4       3       java.lang.AbstractStringBuilder::ensureCapacityInternal (16 bytes)
 65    5       3       java.lang.Object::<init> (1 bytes)
 65    8     n 0       java.lang.System::arraycopy (native)   (static)
 65    6       3       java.lang.CharacterData::of (120 bytes)
 66    7       3       java.lang.CharacterDataLatin1::getProperties (11 bytes)
 66    9       3       java.lang.Character::toLowerCase (9 bytes)
 66   10       3       java.lang.CharacterDataLatin1::toLowerCase (39 bytes)
 66   11       3       java.lang.String::length (6 bytes)
 66   12       3       java.lang.AbstractStringBuilder::append (29 bytes)
 66   13       3       java.lang.StringBuilder::append (8 bytes)
 67   14       3       java.io.WinNTFileSystem::isSlash (18 bytes)
 67   15  s    3       java.lang.StringBuffer::append (13 bytes)
 67   16       3       java.lang.String::<init> (62 bytes)
 68   17       3       java.lang.String::equals (81 bytes)
 68   18       1       java.lang.Object::<init> (1 bytes)
 68    5       3       java.lang.Object::<init> (1 bytes)   made not entrant
 68   19       3       java.lang.String::getChars (62 bytes)
 74   20       3       java.lang.Math::min (11 bytes)
 74   21       3       java.io.BufferedInputStream::getBufIfOpen (21 bytes)
 74   22       3       java.util.Arrays::copyOfRange (63 bytes)
 75   24       3       java.io.DataInputStream::readUTF (501 bytes)
 76   29       3       java.io.DataInputStream::readFully (63 bytes)
 77   26  s    3       java.io.BufferedInputStream::read (113 bytes)
 77   23  s    3       java.io.BufferedInputStream::read (49 bytes)
 77   30       3       java.io.DataInputStream::readShort (40 bytes)
 78   25       3       java.io.DataInputStream::readUTF (5 bytes)
 78   27       3       java.io.BufferedInputStream::read1 (108 bytes)
 78   28       3       java.io.DataInputStream::readUnsignedShort (39 bytes)
 78   31       3       java.util.HashMap::hash (20 bytes)
 79   32  s    3       java.io.ByteArrayInputStream::read (36 bytes)
 79   33       3       java.io.DataInputStream::readByte (23 bytes)
 79   34       3       sun.util.calendar.ZoneInfoFile::indexOf (32 bytes)
 80   35       3       java.util.zip.CRC32::update (16 bytes)
 81   36     n 0       java.util.zip.CRC32::updateBytes (native)   (static)
 81   37       3       sun.util.calendar.ZoneInfoFile$Checksum::update (39 bytes)
 83   39       3       java.util.HashMap::putVal (300 bytes)
 84   41       4       java.lang.String::equals (81 bytes)
 84   38       3       java.util.HashMap::put (13 bytes)
 84   42   !   3       java.io.BufferedReader::readLine (304 bytes)
 86   43       3       java.util.LinkedList::indexOf (73 bytes)
 86   44       3       java.util.HashMap::getNode (148 bytes)
 86   46       3       java.util.HashMap::get (23 bytes)
 87   47       3       sun.misc.JarIndex::addToList (59 bytes)
 87   17       3       java.lang.String::equals (81 bytes)   made not entrant
 87   45       3       java.util.HashMap$Node::<init> (26 bytes)
 87   40       3       java.util.HashMap::newNode (13 bytes)
 88   48       4       java.lang.String::charAt (29 bytes)
 88    2       3       java.lang.String::charAt (29 bytes)   made not entrant
 92   49       3       java.lang.AbstractStringBuilder::append (50 bytes)
 92   50       3       java.lang.System::getSecurityManager (4 bytes)
 93   51       3       java.lang.Math::max (11 bytes)
 93   52       3       java.lang.String::endsWith (17 bytes)
 96   53       3       java.lang.StringBuilder::append (8 bytes)
equal
189546
equal
17232
367
0

 97   54       1       java.nio.Buffer::position (5 bytes)

Answer 2

Run > run configuration > arguments > VM arguments
input as follows: -server -XX:+PrintCompilation -XX:+CITime