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c++ccompiler-constructionllvm

Is there a backend optimizer in LLVM?

I can get the optimization level from the command llc -help

-O=<char>               - Optimization level. [-O0, -O1, -O2, or -O3] (default = '-O2')

I want to know what the optimization does exactly.

So, I'm searching the source code of the backend optimizer.

I google it by "llvm backend optimizer", but there is no information about it, only some target-independent pass source code.

I want to know what does the optimization do for div-rem-pars.

It can combine two instructions of llvm IR to one instruction for assembly code.

Solution

  • Apparently there are backend optimizers options in llvm. They are however not well documented [1,2]. The TargetMachine [3] class has functions getOptLevel and setOptLevel to set an optimization level from 0-3 for a specific target machine, so starting from there you can try to track where it is used.

    [1] https://llvm.org/docs/CodeGenerator.html#ssa-based-machine-code-optimizations
    [2] https://llvm.org/docs/CodeGenerator.html#late-machine-code-optimizations
    [3] https://llvm.org/doxygen/classllvm_1_1TargetMachine.html