[cfe-dev] Question on flags/ passes selection to improve code size

Mehdi AMINI via cfe-dev cfe-dev at lists.llvm.org
Sun Dec 15 10:48:29 PST 2019

On Fri, Dec 13, 2019 at 7:27 AM Robinson, Paul via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> > I am looking at obtaining smaller code sizes for ARM cores. So far, I am
> > trying a  genetic algorithm to find the best option for the opt passes
> and
> > clang/ llc flags.
> Nice.
> > I am looking at what flags and passes -Oz and the other optimization
> levels
> > enable and use. Having done something similar for gcc, I was looking for
> a
> > similar approach.
> > However, I was not able to find many optimization flags for clang or llc,
> > which made me think that in clang/llvm the optimization changes are done
> > mostly by selecting/removing the opt passes(flags).

Most passes exposes some flags affecting their cost modeling and other
heuristics. These are not intended to be exposed to clang users but you can
play with them in `opt`, and a genetic algorithm would likely try to select
these as well. For example here is one such option:

> I'm unfamiliar with how gcc is structured; clang does not actually
> run `opt` or `llc`.  These three tools are all independent clients
> of the optimization and code-generation libraries.  Clang builds
> its pipelines with comparatively little control from the command
> line; opt and llc, which are intended to be testing tools for LLVM
> developers, provide more control.
> As a rule of thumb, `opt` runs IR optimization passes (aka the
> "middle end"), while `llc` primarily runs lower-level "machine"
> oriented passes (aka the "back end").  Both opt and llc will run
> both target-independent and target-specific passes, so the set of
> passes you will be looking at will be at least somewhat influenced
> by which target you select.
> > In order to run what opt passes I select, I split the compilation process
> > in:
> >
> > clang  CLANG_FLAGS -emit-llvm  mysource1.c  -c -o mysource1.bc
> > opt  OPT_FLAGS  mysource1.bc -o mysource1.ll
> > llc LLC_FLAGS mysource1.ll -filetype=obj -o mysource1.o
> That looks quite reasonable.  In order to have Clang produce IR
> that is optimizable, without running any optimizations itself, you
> would want CLANG_FLAGS to include the following:
>     -Xclang -disable-llvm-passes -Xclang -disable-O0-optnone
> and then opt and llc can operate on the IR files as you would like.

Actually I would likely replace `-Xclang -disable-O0-optnone` with `-Oz`:
clang will insert an attribute on functions that will impact some pass
heuristics to optimize for size.
(it is possible that -Oz for invoking `llc` will also change some
heuristics in the backend)

> > I have also seen that for -Oz for example, the Pass Arguments appears
> > multiple time, does this mean that opt is run multiple times with
> different
> > passes options?
> I'm not clear what you are asking about here.  Note that clang does
> not run opt, are you getting some kind of dump output from clang?

It may refer to the fact that there are two pass pipelines created by the
PassManagerBuilder in Clang, so the debug output prints two invocations.
And `opt` does the same when used with the -Ox levels:


> > Now, my general direction questions are:
> >
> > Am I on the right track with this?
> > Do you have any pointers or advice on this?
> Well, what you're doing (with the tweaks mentioned above) is
> probably how I would go about doing the same thing.  Once you
> have determined what a good -Oz pipeline looks like for your
> examples and target, you could bring that back to the list as a
> proposal for how Clang should build its -Oz pipeline.  I'm sure
> you will get plenty of feedback on the exact set of passes!
> --paulr
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