Re: [Qemu-devel] Overoptimization

["Alexander E. Patrakov" <patrakov@ums.usu.ru>]

Herbert Poetzl wrote:
> On Fri, May 28, 2004 at 07:19:35AM +0600, Alexander E. Patrakov wrote:
> 
>>The CFLAGS in Makefile.target are bad for gcc 3.3.2. This is not a my 
>>problem with my gcc, because it passes the "make check" testsuite except 
>>some known errors in libstdc++, which cannot affect qemu.
>>
>>The victim is the tcp_input function in slirp. When compiling with the 
>>decault CFLAGS, for some reason all packets are dropped as having a 
>>wrong TCP checksum. I tried debugging this by placing some debug printfs 
>>in tcp_input.c file, but the problem disappeared after I inserted them. 
>>I concluded that it is very strange and compiled this tcp_input.c file 
>>without my debug statements, but with -O1 instead of -O2. The problem 
>>went away.
> 
> 
> gcc isn't supposed to change the semantics of
> a program based on the optimization level.
> 
> nevertheless several cases are known where this
> happened, and not seldom the bug was in gcc
> 
> testing the same opts with 3.3.3 or 3.4 will
> show if gcc is to blame or not.

Done (with gcc 3.4.0). I used a truly minimal configuration of qemu in 
both cases to make sure that this is not an SDL/X/whatever else problem:

./configure --prefix=/usr --target-list=i386-softmmu --enable-slirp 
--disable-sdl

GCC 3.4.0, however, required one more file to be compiled with -O1 
(because of some register allocation error), and there were some "struct 
timezone" errors that were trivial to fix by including <time.h> where 
appropriate.

The result is the same. If I compile tcp_input.c with -O2, all tcp 
packets are rejected in user-net mode because of wrong tcp checksum. If 
tcp_input.c is compiled with -O1, qemu works.

-- 
Alexander E. Patrakov