Dump Block Liveness of source code using Clang - c++

Question

I need to dump the block liveness of source code using clang's API. I have tried printing the block liveness but got no success. Below is the code that I have tried
bool MyASTVisitor::VisitFunctionDecl(FunctionDecl *f) {
std::cout<<"Dump Liveness\n";
clang::AnalysisDeclContextManager adcm;
clang::AnalysisDeclContext *adc = adcm.getContext(llvm::cast<clang::Decl>(f));
//clang::LiveVariables *lv = clang::LiveVariables::create(*adc);
//clang::LiveVariables *lv = clang::LiveVariables::computeLiveness(*adc,false);
clang::LiveVariables *lv = adc->getAnalysis<LiveVariables>();
clang::LiveVariables::Observer *obs = new clang::LiveVariables::Observer();
lv->runOnAllBlocks(*obs);
lv->dumpBlockLiveness((f->getASTContext()).getSourceManager());
return true;
}
I have override Visitor Functions and have tried printing the liveness of a function. I have tried using create, computeLiveness and getAnalysis methods to get the LiveVariables object, but all the approaches failed. However no Liveness Information is displayed except the block numbers.
When I use command line arguments of clang to print the liveness it displays the output correctly.
I am using the following source code as test case taken from Live Variable Analysis Wikipedia
.
int main(int argc, char *argv[])
{
int a,b,c,d,x;
a = 3;
b = 5;
d = 4;
x = 100;
if(a>b){
c = a+b;
d = 2;
}
c = 4;
return b * d + c;
}
Could someone please point out where could I be wrong?
Thanks in advance.

Answer 1

I had the same issue, after some debugging of clang -cc1 -analyze -analyzer-checker=debug.DumpLiveVars I finally found the answer !
The issue is that the LiveVariables analysis does not explores sub-expressions (such as DeclRefExpr) by itself. It only relies on the CFG enumeration. By default the CFG only enumerates top-level statements.
You must call adc->getCFGBuildOptions().setAllAlwaysAdd() before getting any analysis from your AnalysisDeclContext. This will create elements for all sub-expressions in the CFGBlocks of the control-flow-graph.