Re: SPECfp2000 worsening

[Daniel Berlin <dberlin at dberlin dot org>]

On Friday, September 20, 2002, at 02:29  AM, Roger Sayle wrote:

> Hi Dan,
> > Is this due to increased register pressure (from making more values
> > live longer)?  (IE do you see more spills when you make it more
> > aggressive)?  If so, -fnew-ra might help.
> I must admit that I haven't checked for potential spill effects in
> the register allocator.  I was looking at aggressive jump bypassing
> which is a control flow optimization (converting consecutive if-thens
> into if-then-elses), which I believe shouldn't increase register
> pressure or extend pseudo lifetimes beyond making basic blocks
> larger.
You'd be surprised.
If it can have an effect on scheduling, and if you are testing on a 
platform where pre-regalloc scheduling is run, it might happen to 
*completely* screw everything up.
Of course, if this is x86, then ignore that, as pre-regalloc scheduling 
isn't run.
But even if it just makes BB's larger, it can make the local allocator 
handle more of the register allocation than it used to, and thus, could 
affect register allocation.

I don't have spec2000, but if you could send me the before and after 
assembly, i'd love to analyze it (I realize it's probably very large).