[LLVMdev] Passes propose passes

[Carl-Philip Hänsch]

GVN may be expensive, but for a release build, execution time is more
important than build time for releases. This fact is noted in my proposal:
the ProposePass function can decide if it will allow a further GVN or not.
ProposePass can decide to run every other pass before running into a new
GVN. ProposePass can ensure, that GVN is not called twice in a -O2 or -O1
build.
In my profilings, I noted that llc needs about 45% of the total compilation
time while opt needs 33%. In the testcase i worked with, I was able to
shrink the code size to 30% (to, not by!). This means less data passed to
the code generator and btw a faster compiler, too when compiled with clang.

2011/12/2 Eli Friedman <eli.friedman at gmail.com>

> On Fri, Dec 2, 2011 at 11:56 AM, Carl-Philip Hänsch
> <cphaensch at googlemail.com> wrote:
> > While trying to find a solution for this bug
> > http://llvm.org/bugs/show_bug.cgi?id=11235 ,
> > I came to the conclusion that the following things can happen while
> > optimizing:
> >  - after gvn, I get new constants which are inserted into br and
> > add/sum/..., so there should be at least one more jump-threading and/or
> > instsimplify
> >  - after instsimplify, I get new constants which are inserted into br, so
> > there should be one more jump-threading pass
> >  - after jump-threading, new bigger blocks occur with redundant loads
> which
> > need an other gvn pass
> >
> > At least for -O3 we will need those optimizations.
> > Is there a kind of "I need at least one more $XYZ pass" call that can be
> > invoked from a Pass?
> >
> > So my suggestion to implement in LLVM is:
> >  - Detect where exactly constants are inserted
> >  - Detect which pass is responsible to continue folding it
> >  - Insert that pass if allowed and if it is not in the queue by a
> > ProposePass function
> >
> > ProposePass should decide wether it's worth to further constant fold the
> > block by running the proposed pass. Once accepted, the pass should be
> > inserted into the pass queue at a position where it is most effective.
> >
> > What do you think about that idea? How much would be the effort of
> > implementing it? Does it fit the layering design?
>
> One thing that this proposal doesn't really address is that running
> GVN is expensive; on common testcases, it takes much more time to run
> than any other pass.  We really don't want to be inserting extra runs
> of GVN into the pass pipeline.
>
> -Eli
>
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