Re: [Qemu-devel] qemu vs gcc4

[Johannes Schindelin]

Hi K. Richard,

On Fri, 20 Oct 2006, K. Richard Pixley wrote:

> Could someone please explain the issue with gcc4, please?  Or point me 
> to an existing explanation?

The issue is that gcc4 optimizes better, but this breaks assumptions of 
QEmu.

Example: The basic idea (simplified!) of QEmu is writing C functions which 
implement the instructions of the target CPU. Then, code to be emulated is 
translated by chaining the _compiled_ functions (corresponding to the 
target code) together, but _leaving_ out the return instruction at the end 
of the function (otherwise, the resulting code would return already after 
the first emulated instruction).

Now, gcc4 can produce code with several return instructions (with no 
option to turn that of, as far as I understand). You cannot cut them out, 
and therefore you cannot chain the simple functions.

There seem to be other issues, too, like not being able to correctly link 
the user emulation code, but I am not that sure about it.

> And what's being done about it or what needs to be done about it?

Paul started to implement a hand-written translator, which does not depend 
on gcc, but I guess that project is stalled.

Ciao,
Dscho