Re: Loop unrolling Problem !!

[Daniel Jacobowitz <drow at mvista dot com>]

On Thu, Jul 25, 2002 at 11:18:04AM +0800, Aladdin Chang wrote:
> On Wed, 24 Jul 2002, Peter Barada wrote:
> 
> > 
> > >I compiler the source main.c with m68k-elf-gcc(gcc-3.0.4 and binutils-2.12)
> > >and  dissemble the binary file, we can find that the code inside the
> > >infinte
> > >loop while(1) j = 10 and i = 10 have two copy in binary file.
> > >Is there any option to avoid this kind of duplication and
> > >where to find the source related with.
> > >
> > >
> > >thanks,
> > >
> > >By the way, I compiled the source with following command
> > >m68k-elf-gcc -m68000 -nostartfiles -gstabs+ -nostdlib -fno-unroll-loops  -c
> > >main.c
> > >
> > >SOURCE: main.c
> > >================================
> > >main ()
> > >{
> > >  int j = 10;
> > >  int i = 10;
> > >  while(1)
> > >  {
> > >    j = 10;
> > >    i = 10;
> > >  }
> > >
> > >}
> > >
> > 
> > If you turn the optimizer on(by adding -O2):
> > m68k-elf-gcc -O2 -m68000 -nostartfiles -gstabs+ -nostdlib -fno-unroll-loops  -c
> > 
> > You should see something like:
> > 
> > 00000000 <main>:
> >    0:	4e56 0000      	linkw %fp,#0
> >    4:	60fe           	bras 4 <main+0x4>
> >    6:	4e71           	nop
> > 
> > The optimizer determined that the assignments had no effect outside
> > the scope of the function, so it wiped them out.  Do you have a better
> > testcase that shows a problem with the optimizer tuirned *on*?
> > 
> > 
> 
> i, 
> thanks your help,
> 
> I know that the optimazation will solve this problem,
> but I am going to development a source level debugging tool(link gdb).
> if the code within an loop will be duplicate when specify -O0 ,
> I have no idea how to implement the break point, because the same source
> line will be map to more than one address in binary code in stabs debug
> information.
> 
> if possible, will you give me a hand to tell me where the source 
> related with this loop unrolling.
> 
> thanks in advance.

You'll have to be able to handle that.  It happens in all sorts of
places; any optimized code, inlining, C++ constructors, and others.  If
you use DWARF-2 instead you'll at least have more accurate information
about it; just break on all locations.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer