[FnSpecialization] Refactor CallUsersT to contain Idx/Constant pairs

In the future we won't know the Function at the time of insertion, so
need to store and index so we can look up the Argument later.

Author: bababuck

llvm/include/llvm/Transforms/IPO/FunctionSpecialization.h

@@ -95,12 +95,12 @@
 namespace llvm {
 struct Spec;
 
-struct SpecSig;
-
 // Map of potential specializations for each function.
 using SpecMap = DenseMap<Function *, SmallVector<unsigned>>;
 
-using CallUserT = SmallMapVector<CallBase *, std::pair<SpecSig, Function *>, 4>;
+using CallUserT = SmallMapVector<
+    CallBase *,
+    std::pair<SmallVector<std::pair<unsigned, Constant *>, 4>, Function *>, 4>;
 
 // Just a shorter abbreviation to improve indentation.
 using Cost = InstructionCost;

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

@@ -245,7 +245,7 @@ Cost InstCostVisitor::getCodeSizeSavingsForUser(Instruction *User, Value *Use,
         LLVM_DEBUG(dbgs() << "FnSpecialization:   Function called: "
                           << F->getName() << " argument number: " << Idx
                           << "\n");
-        (*CallUsers)[CI].first.Args.push_back({F->getArg(Idx), C});
+        (*CallUsers)[CI].first.push_back({Idx, C});
         (*CallUsers)[CI].second = F;
         return true;
       } else {
@@ -1139,7 +1139,9 @@ bool FunctionSpecializer::findSpecializations(
         // Since the function might not yet be known when processing the
         // constants due to a function pointer, wait to extract the argument
         // pointer at a given index.
-        SpecSig NewS = CU.second.first;
+        SpecSig NewS;
+        for (auto &P : CU.second.first)
+          NewS.Args.push_back({NewF->getArg(P.first), P.second});
 
         Spec CallSpec(NewF, /*CallSite*/ CU.first, NewS,
                       /*Status*/ CallSiteStatusT::AWAITING_PARENT);