[LLVMdev] Impact of an analysis pass on program run time

[Suresh Purini]

Hello,

I am working on finding good optimization sequences for a given program
(phase ordering problem). I have the following setup.

1) The source programs are translated into LLVM IR using -O0 + -scalarrepl.
2) Find an optimization sequence using some strategy which translates the
IR generated in the
    previous step into another IR.
3) Apply llc -O2 and map the IR in to target assembly code.

Using the above setup I found that the sequence [-functionattrs,
-loop-rotate, -licm, -basicaa] reduces the runtime of the program from 3.1
seconds (when compiled using -O2) down to 0.1 seconds. Removing any of the
optimizations in the sequence including the analysis phase -basicaa
increases the runtime back to 3.1 seconds or above. When I was explaining
this strange behaviour in the HiPEAC conference some one asked me how can
dropping an analysis phase will have an impact on the program runtime. Can
some one explain what is happening here?

The program is office_stringsearch1 from the MiBench suite.

On the same lines the way we hit upon the sequence  [-functionattrs,
-loop-rotate, -licm, -basicaa] is   by applying a sequence reduction
algorithm wherein we take a long sequence (which we hit using genetic or
random search algorithm) and keep dropping the optimizations until the
program runtime doesn't go up. So again the same question, why is that
dropping -basicaa increases the program runtime?

-Suresh Purini
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