Re: option to control code straightening

[Tim Prince <TimothyPrince at sbcglobal dot net>]

Ian Lance Taylor wrote:
> Tim Prince <TimothyPrince@sbcglobal.net> writes:
>
>   
> > Is there an option in gcc 4.4 or 4.5 to avoid code straightening in
> > loops with conditionals or select?  Intel Core i7 Loop Stream Detector
> > seems to give up when the code path has more than minimum number of
> > branches.
> >     
>
> Code straightening is normally a purely beneficial optimization.  That
> said, gcc 4.5 is much better than 4.4 at avoiding having more than four
> jumps in a 16 byte span, which I think is what is required to avoid
> branch mispredictions on Intel/AMD processors.
>
> You may be able to avoid some code straightening using
> -fno-thread-jumps.
>
> If this does not help, then I recommend that you file a bug report with
> a standalone example showing a case where mainline gcc generates
> suboptimal code.  See http://gcc.gnu.org/bugs.html .  Thanks.
>
> Ian
>   
This does make the difference for one of the cases I was looking at, a 
switch where the same branch is taken repeatedly.  I agree, the code 
straightening is better consistently for the older Intel CPUs, where the 
Loop Stream Detector isn't as effective.
Thanks.