Different behaviour of java bytecode

Question

I am a newbee in Java Bytecode. I was understanding the bytecode through some examples but I got stuck in an example.
These are my java and bytecode file

class SimpleAdd{
    public static void main(char args[]){
        int a,b,c,d;
        a = 9;
        b = 4;
        c = 3;
        d = a + b + c;
        System.out.println(d);
    }
}  
Compiled from "SimpleAdd.java"
class SimpleAdd extends java.lang.Object{
SimpleAdd();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(char[]);
  Code:
   0:   bipush  9
   2:   istore_1
   3:   iconst_4
   4:   istore_2
   5:   iconst_3
   6:   istore_3
   7:   iload_1
   8:   iload_2
   9:   iadd
   10:  iload_3
   11:  iadd
   12:  istore  4
   14:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   17:  iload   4
   19:  invokevirtual   #3; //Method java/io/PrintStream.println:(I)V
   22:  return

}

I just want to know why there is bipush 9 when we have instruction a = 9
And in all other case there is iconst.

`bipush 9` pushes the integer literal 9 onto the stack. `istore_1` stores that value into local variable #1. `iconst_4` pushes a literal 4 onto the stack, and does it in one byte vs two for the `bipush`, but there are only the `iconst_1` through `iconst_5` bytecodes -- no `iconst_9`. — Hot Licks, Aug 04 '12 at 20:31

Mark Byers · Accepted Answer · 2015-08-19T08:24:24.440

25

iconst can push constant values -1 to 5. It is a single-byte instruction.

bipush can push constant values between -128 and 127. It is a two-byte instruction.

To push 9 you cannot use iconst. There is no iconst_9 instruction.

edited Aug 19 '15 at 08:24

answered Aug 04 '12 at 20:32

Mark Byers

811,555
193
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1,452

score 6 · Answer 2 · edited Dec 10 '15 at 03:17

6

iconst_n is defined for n from 0 to 5

There's no iconst_9, so you have to use the equivalent (but less efficent) bipush

edited Dec 10 '15 at 03:17

Austin Schmidt

316
1
13

answered Aug 04 '12 at 20:33

Haile

3,120
3
24
40

1

`iconst` supports values [from minus one to five](http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.5.iconst_i)… – Holger Aug 18 '15 at 15:33

score 0 · Answer 3 · answered Aug 04 '12 at 20:33

0

there is no iconst_9 instruction

answered Aug 04 '12 at 20:33

ddyer

1,792
19
26

score 0 · Answer 4 · answered Mar 13 '13 at 04:20

0

the i_const instruction only range from 0~5, so it must spit the instuction by push and store

answered Mar 13 '13 at 04:20

mannnnerd

109
1
4

score 0 · Answer 5 · 2018-08-04T13:55:15.627

0

The instructions iconst_* are optimised to work with small and specific numbers while bipush (push a byte onto the stack as an integer value) works for bigger numbers.

edited Aug 04 '18 at 13:55

answered Aug 04 '18 at 13:49

score 0 · Answer 6 · answered Aug 16 '18 at 17:35

0

There is no iconst_9 instruction. So to push 9 you cannot use iconst. You must go for bipush

answered Aug 16 '18 at 17:35

Ratnesh

298
3
14

Different behaviour of java bytecode

6 Answers6

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