Re: optimizations

["Håkan Hjort" <hakan@safelogic.se>]

Wed Jan 15 2003, Reza Roboubi wrote:
> tm_gccmail@mail.kloo.net wrote:
> [snap]
> > I mentioned this on the gcc-bugs mailing list, and Mark Mitchell
> > contributed a fairly simple load hoisting improvement to the loop
> > optmiizer which restored performance on Whetstone.
> > 
> > If you look at the gcc-bugs archives for 1998, you may be able to find
> > this message thread.
> [snap]
> 
> Thanks for this input. It would be interesting to see how the issue was fixed.
Sorry for getting into this so late.
Nobody actually posted the code generated by 3.3/3.4...

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;
}

For Sun's Forte compiler one gets the following:

main:
         save    %sp,-104,%sp
         or      %g0,16,%g1
         st      %g1,[%fp-4]
         add     %fp,-4,%o1
         or      %g0,1,%o0
         call    write   ! params =  %o0 %o1 %o2 ! Result
         or      %g0,1,%o2
         ret     ! Result =  %i0
         restore %g0,0,%o0

I.e. it just stores '16' in k before the call to write, no trace left
of mm() or any loop, as should be.

Perhaps GCC now does the same after hoisting both the load and the store?

-- 
/HÃ¥kan