Why jvm expands byte & short to int before pushing on stack?

Question

java code : ,

byte a_b = 12;
short c_d = 14

replaces in bytecodes with

bipush  12 // expands byte1 (a byte type) to an int and pushes it onto the stack
sipush   14 // expands byte1, byte2 (a short type) to an int and pushes it onto the stack

Why jvm does that expansion, and not use byte & short ?

Also when i open bytecode of my file

EDIT : short var = 14 is replaced by bipush 14 rather than sipush 14

Is my understanding is not clear or is there a bug ?

enter image description here

I am using following version

java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) Client VM (build 20.1-b02, mixed mode, sharing)

Mainly because that's the way they defined it, and there's no compelling need to have it defined differently. (The stack is conceptually only incremented/decremented in 4-byte increments, though a little tap-dance obviously must occur in 8-byte JVMs.) — Hot Licks
– Hot Licks, Commented Aug 27, 2013 at 15:38
And a bipush is 2 bytes long, while a sipush is 3 bytes long. Why use the longer instruction when the shorter works just as well? — Hot Licks
– Hot Licks, Commented Aug 27, 2013 at 15:44
@HotLicks see the snap added from following link docs.oracle.com/javase/specs/jvms/se7/html/… — Prateek
– Prateek, Commented Aug 27, 2013 at 15:50
You mean "At any point in time, an operand stack has an associated depth, where a value of type long or double contributes two units to the depth and a value of any other type contributes one unit. "? — Hot Licks
– Hot Licks, Commented Aug 27, 2013 at 16:14

Lars Brinkhoff · Accepted Answer · 2013-08-27 15:31:30Z

8

Because (conceptually) the smallest unit of data on the JVM stack is 32 bits. So there is no way to increase the size of the stack with just 8 bits.

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6.2

At any point in time, an operand stack has an associated depth, where a value of type long or double contributes two units to the depth and a value of any other type contributes one unit.

answered Aug 27, 2013 at 15:31

Lars Brinkhoff

14.5k2 gold badges30 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

André Stannek Over a year ago

I think you mean decrease ;-) Also you could only write 8 bit of data into a 32 bit stackframe. I just would be a waste of space. I think the real reason is found in the answer @TheTerribleSwiftTomato has linked in the comments.

Hot Licks Over a year ago

@stonedsquirrel - They do write the 8 bits of data into the 32-bit stack frame. And then they sign-extend into the rest of the frame.

André Stannek Over a year ago

@HotLicks Of course. That you not only could do it but have to with a fixed stack frame size skipped my mind ;-) Thanks!

Collectives™ on Stack Overflow

Why jvm expands byte & short to int before pushing on stack?

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related