[LLVMdev] Phase Interactions

[Suresh Purini]

Dear all,

  I am doing few experiments to do understand optimization phase
interactions. Here is a brief description of my experiements.

1. I picked the list of machine independent optimizations acting on
llvm IR (those that are enabled at O3).
2.  for each optimzation in the optimization-list
             a) Compiled the program using 'clang -c O0 -flto program.c'
             b) opt -optimization program.o -o optprogram.o
             c) llc optprogram.o
             d) gcc optprogram.o.s
             e) Measure the performance of the generated executable.
3. for each optimization pair [opt1, opt2] from the optimization list
             a) Compiled the program using 'clang -c O0 -flto program.c'
             b) opt -opt1 -opt2 program.o -o optprogram.o
             c) llc optprogram.o
             d) gcc optprogram.o.s
             e) Measure the performance of the generated executable.

My intention is understand or model phase interactions by observing
this data and the corresponding program's static/dynamic features.
However I couldn't glean much information from this data as almost in
all cases there is no change in the runtime when compared to O0 except
for few programs where gvn and loop-rotate improved the program
performance to some extent. But the 'scalarrepl' optimization is an
exception because it almost consistently improved the program
performance and in fact it almost matches the O3 level performance of
the program.

Can some one enlighten about what is happening? Is there any thing
wrong in my experimental setup?

Thank you
-Suresh