optimizations

Reza Roboubi reza@linisoft.com
Wed Jan 15 00:52:00 GMT 2003


Reza Roboubi wrote:
> 
> Andrew Pinski wrote:
> >
> > What version of gcc?
> > This seems like it was fixed at one point doing the 3.x series because
> > it does not happen with 3.3 (prerelease) or 3.4 (experimental).
> 
> Ah. I am using gcc version 3.2. It's very good if this has been fixed under 3.3
> and 3.4.
> 
> I would still appreciate any comments regarding the status of these
> optimizations. Are these new features, or are they old ones that temporarily got
> broken during gcc 3.2?
> 
> What part of the gcc source tree deals with these optimizations?

Could you please also tell me if 3.3 and 3.4 remove the extra mov's in and out
of %eax. Ideally, there should be no more than 4 instructions in the critical
loop.


> 
> Thanks again.
> 
> >
> > Thanks,
> > Andrew Pinski
> >
> > On Tuesday, Jan 14, 2003, at 12:35 US/Pacific, Reza Roboubi wrote:
> >
> > > In the following code, it is clear that the return value of mm() can be
> > > eliminated.  In fact, many optimizations are possible here.  Yet gcc
> > > seems not
> > > to be able to do these optimizations.  Below, I posted the assembly
> > > code that
> > > gcc generated (for the while() loop).
> > >
> > > I compiled this code with gcc -O2 -Wall.
> > >
> > > I was wondering if I am doing something wrong.  If not, then please
> > > comment on
> > > current gcc developments in this regard, and what it takes to add some
> > > of these
> > > features.
> > >
> > > Please also comment on how other compilers would compare with gcc in
> > > this case.
> > >
> > > Are there any non-obvious remedies you have for this case?
> > >
> > > PS: Please tell me if I must report this as a gcc bug.
> > >
> > > Thanks in advance for any help you provide.
> > >
> > >
> > > inline int mm(int *i)
> > > {
> > >         if((*i)==0x10){return 0;}
> > >         (*i)++;return 1;
> > > }
> > >
> > > int
> > > main(){
> > >
> > >       int k=0;
> > >       while (mm(&k)){}
> > >       write(1,&k,1);
> > >
> > >       return 0;
> > > }
> > >
> > >
> > > Associated assembly code for the while() loop:
> > >
> > > 0x80483b0 <main+16>:  mov    0xfffffffc(%ebp),%eax
> > > 0x80483b3 <main+19>:  xor    %edx,%edx
> > > 0x80483b5 <main+21>:  cmp    $0x10,%eax
> > > 0x80483b8 <main+24>:  je     0x80483c3 <main+35>
> > > 0x80483ba <main+26>:  inc    %eax
> > > 0x80483bb <main+27>:  mov    $0x1,%edx
> > > 0x80483c0 <main+32>:  mov    %eax,0xfffffffc(%ebp)
> > > 0x80483c3 <main+35>:  test   %edx,%edx
> > > 0x80483c5 <main+37>:  jne    0x80483b0 <main+16>
> > >
>



More information about the Gcc-help mailing list