Burn And Tremble

LLVM passes, thou shalt preserve them!

I am currently working on some LLVM passes for my intermediate thesis aka Großer Beleg at university and I came across an annoying problem this morning, due to a somewhat misleading assertion message in LLVM’s PassManager.

I have a large ModulePass, that does some inspection across functions, keeps global state and can thus not comfortably be a FunctionPass. Within it I require two FunctionPasses

void SomeModulePass::getAnalysisUsage(AnalysisUsage &AU) const {

and later call them in ModulePass::runOnModule(Module &M):

// function is a Module::iterator
FirstFunctionPass &FFP = getAnalysis<FirstFunctionPass>(*function);
SecondFunctionPass &SFP = getAnalysis<SecondFunctionPass>(*function);

This kept failing on me—precisely after executing both required passes on the correct function just fine, mind you—with a rather cryptic

Assertion 'ResultPass && "Unable to find requested analysis info"' failed.

Which after a lot of cursing and random debugging attempts turned out to be simply due to the second function pass invalidating the first function pass, because I did not add it to the second pass’s preserved set, to inform the PassManager that these two actually don’t care about each other.

void SecondFunctionPass::getAnalysisUsage(AnalysisUsage &AU) const {

I was aware of the performance implications of not specifying preserved passes, just not the fact that it would produce weird results like this. Trivial problem, lots of time wasted because the assertion wasn’t pointing me in the right direction—I know I’m whining on a pretty high level here, in general LLVM does an awesome job at telling me where exactly I’m being stupid—and there seems to be no runtime checking for “impossible” situations like this.

(There might be just among FunctionPasses if I remember correctly, but it seems that requiring things from within a ModulePass, you’re out of luck right now. Also, when I request --debug-pass=Structure it does not prefix “results known to be dead” as mentioned here, unfortunately. Was this taken out?)

Hope this helps one or two people searching for answers to the same problem.

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