[LLVMdev] LLD: Simplify LayoutPass

[Rui Ueyama]

On Mon, Jan 26, 2015 at 4:11 AM, Michael Spencer <bigcheesegs at gmail.com>
wrote:

> On Thu, Jan 22, 2015 at 4:07 AM, Rui Ueyama <ruiu at google.com> wrote:
> > In r226336 I shove off 1.2 seconds out of 9.8 seconds for lld to link
> lld.
> > That's done by parallelizing archive member parsing. But I realized that
> was
> > not the slowest pass.
> >
> > The single slowest pass in LLD is LayoutPass. Only sort() at the last of
> > Layoutpass::perform takes about 3 seconds (one third of total execution
> > time). It is because the comparison function passed to sort,
> compareAtoms,
> > does too much stuff.
> >
> > It looks to me that the entire pass is overkill. We don't really need
> that
> > complexity there. I think nobody is actually depends on the details of
> the
> > pass's behavior.
> >
> > I'd like to simplify LayoutPass, so that the pass sorts atoms only by
> file
> > position and position in file (that means atoms are sorted in the same
> order
> > as the command line.) I believe that should be enough and we can still
> keep
> > output the same.
> >
> > Any comments?
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> >
>
> Have you tried switching to a parallel sort? On windows I got a pretty
> good perf increase by doing that.
>

Done in r227132, but I only saw a minor performance increase.


> I would also love to simplify this pass, as long as we make it clear
> what guarantees we are making.
>

Yeah. I took a look at the history of the file to understand how the
feature set that pass currently provides was chosen, and found that some
features were introduced without clear reasoning and users of the features.
Now a few pieces of code depends on subtle behavior of the pass, I need to
understand the details first.
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