[llvm-dev] Effectiveness of llvm optimisation passes

[Craig Topper via llvm-dev]

Have -O0 on your clang command line causes all functions to get marked with
an 'optnone' attribute that prevents opt from being able to optimize them
later. You should also add "-Xclang -disable-O0-optnone" to your command
line.

~Craig

On Thu, Sep 21, 2017 at 10:04 PM, Yi Lin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
>
> I am trying to understand the effectiveness of various llvm optimisations
> when a language targets llvm (or C) as its backend.
>
> The following is my approach (please correct me if I did anything wrong):
>
> I am trying to explicitly control the optimisations passes in llvm. I
> disable optimisation in clang, but instead emit unoptimized llvm IR, and
> use opt to optimise that. These are what I do:
>
> * clang -O0 -S -mllvm -disable-llvm-optzns -emit-llvm
> -momit-leaf-frame-pointer a.c -o a.ll
> * opt -(PASSES) a.ll -o a.bc
> * llc a.bc -filetype=obj -o a.o
>
> To evaluate the effectiveness of optimisation passes, I started with an
> 'add-one-in' approach. The baseline is no optimisations passes, and I
> iterate through all the O1 passes and explicitly allow one pass for each
> run. I didnt try understand those passes so it is a black box test. This
> will show how effective each single optimisation is (ignore correlation of
> passes). This can be iterative, e.g. identify the most effecitve pass, and
> always enable it, and then 'add-one-in' for the rest passes. I also plan to
> take a 'leave-one-out' approach as well, in which the baseline is all
> optimisations enabled, and one pass will be disabled at a time.
>
> Here is the result for the 'add-one-in' approach on some micro benchmarks:
>
> https://drive.google.com/drive/folders/0B9EKhGby1cv9YktaS3NxUVg2Zk0
>
> The result seems a bit surprising. A few passes, such as licm, sroa,
> instcombine and mem2reg, seem to deliver a very close performance as O1
> (which includes all the passes). Figure 7 is an example. If my methodology
> is correct, then my guess is those optimisations may require some common
> internal passes, which actually deliver most of the improvements. I am
> wondering if this is true.
>
> Any suggestion or critiques are welcome.
>
> Thanks,
> Yi
>
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