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Date: Wed, 27 Feb 2008 16:15:21 +0200
From: "Kirill A. Shutemov" <k.shutemov@gmail.com>
To: ledest@gmail.com,
	ALT Linux Sisyphus discussions <sisyphus@lists.altlinux.org>
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Subject: Re: [sisyphus] IQ: kernel-image-std-smp-2.6.24-alt0.3
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On [Wed, 27.02.2008 15:55], Led wrote:
> =D0=9D=D0=B5 =D0=B7=D0=BD=D0=B0=D1=8E =D0=B8=D0=B7-=D0=B7=D0=B0 =D1=8D=D1=
=82=D0=BE=D0=B3=D0=BE =D0=B8=D0=BB=D0=B8 =D0=BD=D0=B5=D1=82, =D0=BD=D0=BE q=
emu-kvm =D0=B2 =D1=81=D0=B8=D0=B7=D0=B8=D1=84=D0=B5 =D0=BF=D1=80=D0=B8=D0=
=BD=D1=83=D0=B4=D0=B8=D1=82=D0=B5=D0=BB=D1=8C=D0=BD=D0=BE =D1=81=D0=BE=D0=
=B1=D1=80=D0=B0=D0=BD gcc4.1,=20
> =D0=B0 =D0=BD=D0=B5 gcc3.4

=D0=AD=D1=82=D0=BE=D0=B3=D0=BE =D0=B4=D0=B5=D0=BB=D0=B0=D1=82=D1=8C =D0=BD=
=D0=B5=D0=BB=D1=8C=D0=B7=D1=8F:

The issue is that gcc4 optimizes better, but this breaks assumptions of=20
QEmu.

Example: The basic idea (simplified!) of QEmu is writing C functions which=
=20
implement the instructions of the target CPU. Then, code to be emulated is=
=20
translated by chaining the _compiled_ functions (corresponding to the=20
target code) together, but _leaving_ out the return instruction at the end=
=20
of the function (otherwise, the resulting code would return already after=
=20
the first emulated instruction).

Now, gcc4 can produce code with several return instructions (with no=20
option to turn that of, as far as I understand). You cannot cut them out,=
=20
and therefore you cannot chain the simple functions.

--=20
Regards,  Kirill A. Shutemov
 + Belarus, Minsk
 + Velesys Ltd, http://www.velesys.com/
 + ALT Linux Team, http://www.altlinux.com/

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