[WIP] [FuncSpec] Handle Available Externally Linkage Function

[ChuanqiXu]

Summary

This is the successive patch of D105966. With D105544, D105966 and this diff, they construct my prototype of ThinLTO for function specialization.

To make this work for 505.mcf_r, it requires that the function specialization handle the case that the argument is already constant, which would be handled by IPSCCP by the design of function specialization. We could discuss it later.

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

for (Argument &A : F->args()) {
       // Create a version of the function in which the argument is marked
       // constant with the given value.
       for (auto *C : Constants) {
         // Clone the function. We leave the ValueToValueMap empty to allow
         // IPSCCP to propagate the constant arguments.
         ValueToValueMapTy EmptyMap;
         Function *Clone = CloneFunction(F, EmptyMap);
         Argument *ClonedArg = Clone->arg_begin() + A.getArgNo();
+        // Functions with AvailableExternallyLinkage would be deleted
+        // in the end.
+        if (Clone->hasAvailableExternallyLinkage())
+          Clone->setLinkage(GlobalValue::InternalLinkage);
 
         // Rewrite calls to the function so that they call the clone instead.
         rewriteCallSites(F, Clone, *ClonedArg, C);
 
         // Initialize the lattice state of the arguments of the function clone,
         // marking the argument on which we specialized the function constant
         // with the given value.
         Solver.markArgInFuncSpecialization(F, ClonedArg, C);
if (F.hasFnAttribute(Attribute::NoDuplicate))
       continue;
 
     LLVM_DEBUG(dbgs() << "\nFnSpecialization: Analysing decl: " << F.getName()
                       << "\n");
     Solver.addAnalysis(F, GetAnalysis(F));
 
     // Determine if we can track the function's arguments. If so, add the
     // function to the solver's set of argument-tracked functions.
-    if (canTrackArgumentsInterprocedurally(&F)) {
+    if (canTrackArgumentsInterprocedurally(&F) ||
+        F.hasAvailableExternallyLinkage()) {
       LLVM_DEBUG(dbgs() << "FnSpecialization: Can track arguments\n");
       Solver.addArgumentTrackedFunction(&F);
       continue;
     } else {
       LLVM_DEBUG(dbgs() << "FnSpecialization: Can't track arguments!\n"
                         << "FnSpecialization: Doesn't have local linkage, or "
                         << "has its address taken\n");
     }