[llvm-dev] Effectiveness of llvm optimisation passes

[Yi Lin via llvm-dev]

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