/// Remove any debug info for global variables/functions in the given module for
/// which said global variable/function no longer exists (i.e. is null).
///
/// Debugging information is encoded in llvm IR using metadata. This is designed
/// such a way that debug info for symbols preserved even if symbols are
/// optimized away by the optimizer. This special pass removes debug info for
/// such symbols.
bool StripDeadDebugInfo::runOnModule(Module &M) {
if (skipModule(M))
return false;
bool Changed = false;
LLVMContext &C = M.getContext();
// Find all debug info in F. This is actually overkill in terms of what we
// want to do, but we want to try and be as resilient as possible in the face
// of potential debug info changes by using the formal interfaces given to us
// as much as possible.
DebugInfoFinder F;
F.processModule(M);
// For each compile unit, find the live set of global variables/functions and
// replace the current list of potentially dead global variables/functions
// with the live list.
SmallVector<Metadata *, 64> LiveGlobalVariables;
DenseSet<DIGlobalVariableExpression *> VisitedSet;
std::set<DIGlobalVariableExpression *> LiveGVs;
for (GlobalVariable &GV : M.globals()) {
SmallVector<DIGlobalVariableExpression *, 1> GVEs;
GV.getDebugInfo(GVEs);
for (auto *GVE : GVEs)
LiveGVs.insert(GVE);
}
std::set<DICompileUnit *> LiveCUs;
// Any CU referenced from a subprogram is live.
for (DISubprogram *SP : F.subprograms()) {
if (SP->getUnit())
LiveCUs.insert(SP->getUnit());
}
bool HasDeadCUs = false;
for (DICompileUnit *DIC : F.compile_units()) {
// Create our live global variable list.
bool GlobalVariableChange = false;
for (auto *DIG : DIC->getGlobalVariables()) {
if (DIG->getExpression() && DIG->getExpression()->isConstant())
LiveGVs.insert(DIG);
// Make sure we only visit each global variable only once.
if (!VisitedSet.insert(DIG).second)
continue;
// If a global variable references DIG, the global variable is live.
if (LiveGVs.count(DIG))
LiveGlobalVariables.push_back(DIG);
else
GlobalVariableChange = true;
}
if (!LiveGlobalVariables.empty())
LiveCUs.insert(DIC);
else if (!LiveCUs.count(DIC))
HasDeadCUs = true;
// If we found dead global variables, replace the current global
// variable list with our new live global variable list.
if (GlobalVariableChange) {
DIC->replaceGlobalVariables(MDTuple::get(C, LiveGlobalVariables));
Changed = true;
}
// Reset lists for the next iteration.
LiveGlobalVariables.clear();
}
if (HasDeadCUs) {
// Delete the old node and replace it with a new one
NamedMDNode *NMD = M.getOrInsertNamedMetadata("llvm.dbg.cu");
NMD->clearOperands();
if (!LiveCUs.empty()) {
for (DICompileUnit *CU : LiveCUs)
NMD->addOperand(CU);
}
Changed = true;
}
return Changed;
}
