Manipulate java thread stack in this way

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Yao Qi - 28 May 2007 18:28 GMT

We are doing byte code instrumentation, and at one moment (before call a
method, o.f(long);), the stack layout of our program thread is shown as follows,

top--> +--------+
| long |
+--------+
| Object |
+--------+
| ... |

we want to transform this stack layout to this, shown as follows,

top--> +--------+
| Object |
+--------+
| long |
+--------+
| Object |
+--------+
| Object |
+--------+
| ... |

I read the JVM spec on byte code instructions from

http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2.doc3.html

but I still could not find a byte code sequence to transform stack
layout in this way.

Could anybody here give some advice on this problem? Thanks.

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Yao Qi <qiyaoltc@gmail.com> GNU/Linux Developer
http://duewayqi.googlepages.com/

Yevtushenko has... an ego that can crack crystal at a distance of twenty feet.
-- John Cheever

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Tom Hawtin - 28 May 2007 23:25 GMT

> top--> +--------+
> | long |

long is two entries. This is the real problem, as swap wont work as
intended. (Not a popular piece of byte code design, but presumably a bit
easier for interpreters on 32-bit systems.)

> +--------+
> | Object |
[quoted text clipped - 13 lines]
> +--------+
> | ... |

So you have

..., C, B, A

where A and B are the long and C the reference, and you want

..., C, C, B, A, C

The trick is to use dup_x and then discard unwanted values.

So

..., C, B, A
dup2_x1
..., B, A, C, B, A
pop2
..., B, A, C
dup_x2
..., C, B, A, C
dup_x2
..., C, C, B, A, C

I think. (And make sure you set the max frame size appropriately.)

If you are writing code from scratch, then local variables are very
useful. Probably not so convenient for instrumentation.

Tom Hawtin

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Manipulate java thread stack in this way

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