Re: [Qemu-devel] Patch for compiling with GCC 4

[Alexander Graf]

On Feb 17, 2008, at 9:22 PM, Christian Roue wrote:

> Well, I somehow felt like it was a bit brutal and probably fixing the
> symptoms which is apparently the case.
> Looking more carefully, compile fails in :
> sh4-linux-user for function op_cmp_str_T0_T1
> gcc optimization leads to a ret followed by a last assignement with  
> a jump back.
> I guess dyngen hopes to find function epilogue as the last bytes.
> It's apparently the only function where it happens.
>
> I found that adding gcc option "-fno-tree-dominator-opts" for sh4
> target avoids this (I suppose) unwanted optimization.
> It may be a bit brutal again ( disabling too many optims or wrong  
> ones).
> May be the op_cmp_str_T0_T1 function can be rewritten to something
> that avoids this optimization.
> Am I on a better track ?

This looks like the right approach to the symptoms. The "real fix"  
would be to move the sh4 target to TCG, but for the meantime I believe  
this is the way to go. You can already find a lot of these  
unoptimization flags autodetected in the configure script, so I guess  
that'd be the right place for a patch.

I am not sure if anybody with commit right listens, though.

Regards,

Alex

> Bye
> Chris.
>
>
> On Feb 16, 2008 9:01 PM, Paul Brook <address@hidden> wrote:
> > On Saturday 16 February 2008, Christian Roue wrote:
> > > Hi all,
> > > I tried to compile qemu cvs head on my x86_64 linux with gcc 4.1.2  
> > > using
> > > --disable-gcc-check, I found compile fails as stated in configure  
> > > before i
> > > disabled gcc check..
> > > Error message, points to a problem of dyngen not correctly detecting
> > > function ends on i386 when last instruction is a jump. I applied  
> > > following
> > > change and successfully compiled/run qemu i386.  This extra test  
> > > check for
> > > a relative backward jump  to function exit ret,
> > > gcc 4 apparently generates a few of these.
> >
> > You patch is wrong. The dyngen error is correct.
> >
> > Paul