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

[Robinson, Paul via cfe-dev]

> 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).

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.

> 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?

> 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