Re: [df-scan.c] Optimise DF_REFs ordering in collection_rec, use HARD_REG_SETs instead of bitmaps

[Dimitrios Apostolou <jimis at gmx dot net>]

On Fri, 8 Jul 2011, Richard Guenther wrote:
> On Fri, Jul 8, 2011 at 5:20 AM, Dimitrios Apostolou <jimis@gmx.net> wrote:
> > Hello list,
> >
> > The attached patch does two things for df_get_call_refs():
> > * First it uses HARD_REG_SETs for defs_generated and
> > regs_invalidated_by_call, instead of bitmaps. Replacing in total more than
> > 400K calls (for my testcase) to bitmap_bit_p() with the much faster
> > TEST_HARD_REG_BIT, reduces the total instruction count from about 13M to
> > 1.5M.
> > * Second it produces the REFs in REGNO order, which is important to keep the
> > collection_rec sorted most times, and avoid expensive calls to qsort().
> > Thanks to Paolo Bonzini for idea and mentoring.
> >
> > The second part makes a big difference if accompanied with another patch in
> > df_insn_refs_collect(). I'll post a followup patch, that is unfortunately
> > unstable for some of my tests, so I'd appreciate any comments.
>
> Did you check the impact on memory usage?  I suppose on targets
> with not many hard registers it should even improve, but do we expect
> memory usage to be worse in any case?

Hi Richard, I didn't check memory usage, is that important? Since the 
struct bitmap is fairly bulky, it should take an arch with lots of hard 
regs (which one has the most?).

But still a few bytes tradeoff wouldn't be acceptable for a much faster 
type? And IMHO it makes the code better to understand, since once you see 
HARD_REG_SET you know you can't expect else. FWIW I'm now in the process 
of converting all other bitmap uses for hard regs, to HARD_REG_SETs, at 
least within DF. I'm not sure whether performance gains will be visible, 
however, not much code is as hot as df_get_call_refs().


Thanks,
Dimitris