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Date: Sat, 17 Mar 2007 14:31:06 +0000
From: Thiemo Seufer <ths@networkno.de>
Subject: Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction
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To: Stefan Weil <weil@mail.berlios.de>
Cc: QEMU Developers <qemu-devel@nongnu.org>

Stefan Weil wrote:
> So an emulation has several options:
> 
> 1. Show undefined behaviour (this is what it does today).
> 2. Emulate the behaviour of existing CPUs as far as possible.
>    As different CPUs behave different, this must depend on the
>    current CPU.
> 3. Display an error message.

(3) is bad, as it amounts to a DoS.

> The current solution (1) is not good, because users get crashes
> and don't know the reason, and experienced users spend a lot of
> time with debugging (at least I did).
> 
> Solution (2) is needed to run existing binary code.
> 
> Solution (3) is the minimum I expect of an emulation like QEMU.
> 
> I prefer a mix of solutions (2) and (3): display a message and
> try to emulate the original behaviour.
> 
> Do you agree, and would you accept patches which implement this?

If the AR7 CPU spec defines the semantics of branch delay slots more
precisely than the architecture spec then I'll consider a patch.

If this isn't the case then I ask you to use a non-broken compiler/
assembly code.


Thiemo