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

From: Alexander Graf
Subject: Re: [Qemu-devel] Patch for compiling with GCC 4
Date: Mon, 18 Feb 2008 13:07:18 +0100

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.




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.


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