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

[Duncan Sands]

Hi Suresh, good alias analysis is essential for optimizing programs well.  With
-basicaa you get decent alias analysis, without it you get poor quality alias
analysis.  You may be wondering why -basicaa does something when it is listed
after the other options.  I suggest you run opt with the -debug-pass=Executions
option (opt -help-hidden will show you this) to see the order in which things
are actually being run.

Ciao, Duncan.

On 09/02/13 06:30, Suresh Purini wrote:
>
> 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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