[LLVMdev] Question on JIT optimizations

[Duncan Sands]

Hi Brent,

> The code in this setup will be optimized according to the optimizer pipeline one
> sets up as in the code below.  I find that if I only use the passes below the
> quality of the code is not that good

code quality will be dreadful if you don't promote memory accesses to registers
right at the start, using eg mem2reg or scalarrepl.

  (for example inlining is not aggressive
> enough) -- moreover the number of passes is overwhelming and the order in which
> they are specified seems important and I do not have the expertise to set that
> up.  What I would like, is to set up the passes in exactly the same as clang -O2
> or llvm-gcc -O2 do.  Is there some way to get that behavior using the api below?

You can use a PassManagerBuilder for this.  For some reason it is tucked away in
an obscure corner of include: include/llvm/Transforms/IPO/PassManagerBuilder.h

Ciao, Duncan.

>   How do language implementors do this in the LLVM environment?
>
> Thank you for any help.  And thank you for this great tool.
>
> Brent
>
>    FunctionPassManager OurFPM(TheModule);
>
>    // Set up the optimizer pipeline.  Start with registering info about how the
>    // target lays out data structures.
>    OurFPM.add(new TargetData(*TheExecutionEngine->getTargetData()));
>    // Provide basic AliasAnalysis support for GVN.
>    OurFPM.add(createBasicAliasAnalysisPass());
>    // Do simple"peephole"  optimizations and bit-twiddling optzns.
>    OurFPM.add(createInstructionCombiningPass());
>    // Reassociate expressions.
>    OurFPM.add(createReassociatePass());
>    // Eliminate Common SubExpressions.
>    OurFPM.add(createGVNPass());
>    // Simplify the control flow graph (deleting unreachable blocks, etc).
>    OurFPM.add(createCFGSimplificationPass());
>
>    OurFPM.doInitialization();
>
>    // Set the global so the code gen can use this.
>    TheFPM =&OurFPM;
>
>    // Run the main"interpreter loop"  now.
>    MainLoop();
>
>
>
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