Add custom llvm optimization command (opt) after compilation within CMake projects

Question

I created my own LLVM optimization pass in LLVM 3.7.0 . I want to use this pass within a cmake project. I need to run the pass as last, after all optimization passes of -O2 (or -O3) are executed by clang (or clang++).

Unfortunately, I did not find a mechanism to invoke the pass by passing the flag directly from the clang (if you point me out a way to do that, this would already be helpful).

Assuming there is no way to run the pass by giving a flag to clang, I need an extra optimization pass in my toolchain to be placed between the compilation and the linking phase. I need it throughout the whole cmake project.

The commands I would need to generate a binary from a two source files are:

clang -c -g -emit-llvm -O3 mySource0.c -o mySource0.bc
clang -c -g -emit-llvm -O3 mySource1.c -o mySource.bc
llvm-link mySource0.bc mySource1.bc -o main.bc
opt -load myAnalysis.so -myAnalysis main.bc -o main.analysis.bc
clang <libraryRelatedFlags> main.analysis.bc -o myExecutable

My pass is registered as:

static RegisterPass<myAnalysis> X("myAnalysis", "Implement my analysis", false, false);

as in: http://llvm.org/docs/WritingAnLLVMPass.html#basic-code-required

Answer 1

If I understand your question correctly, you are simply aiming to add your pass so that it is run under the -O3.

You'll need to edit $(llvm-dir)/tools/opt/opt.cpp to get your pass to run -O3. You'll need to find where the OptLevelO3 bool is used to add passes and make sure to add your pass there as well.

If instead you just want your pass to be run on it's own flag you'll need to initialize your pass properly on top of registering it. We can look at DependenceAnalysis.cpp as a good example of how to do this:

INITIALIZE_PASS_BEGIN(DependenceAnalysis, "da", "Dependence Analysis", true, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(DependenceAnalysis, "da", "Dependence Analysis", true, true)

You also mentioned that you want your pass to run after some other passes. Simply mark them as DA did it with:

INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)

to make sure that your pass is run AFTER the pass that you want to depend on.