[llvm] Use *Map::try_emplace (NFC)

- try_emplace(Key) is shorter than insert(std::make_pair(Key, 0)).
- try_emplace performs value initialization without value parameters.
- We overwrite values on successful insertion anyway.

Author: kazutakahirata

llvm/include/llvm/ADT/MapVector.h

@@ -99,8 +99,7 @@ class MapVector {
   }
 
   ValueT &operator[](const KeyT &Key) {
-    std::pair<KeyT, typename MapType::mapped_type> Pair = std::make_pair(Key, 0);
-    std::pair<typename MapType::iterator, bool> Result = Map.insert(Pair);
+    std::pair<typename MapType::iterator, bool> Result = Map.try_emplace(Key);
     auto &I = Result.first->second;
     if (Result.second) {
       Vector.push_back(std::make_pair(Key, ValueT()));
@@ -119,7 +118,7 @@ class MapVector {
 
   template <typename... Ts>
   std::pair<iterator, bool> try_emplace(const KeyT &Key, Ts &&...Args) {
-    auto [It, Inserted] = Map.insert(std::make_pair(Key, 0));
+    auto [It, Inserted] = Map.try_emplace(Key);
     if (Inserted) {
       It->second = Vector.size();
       Vector.emplace_back(std::piecewise_construct, std::forward_as_tuple(Key),
@@ -130,7 +129,7 @@ class MapVector {
   }
   template <typename... Ts>
   std::pair<iterator, bool> try_emplace(KeyT &&Key, Ts &&...Args) {
-    auto [It, Inserted] = Map.insert(std::make_pair(Key, 0));
+    auto [It, Inserted] = Map.try_emplace(Key);
     if (Inserted) {
       It->second = Vector.size();
       Vector.emplace_back(std::piecewise_construct,

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -3881,7 +3881,7 @@ void DwarfDebug::addDwarfTypeUnitType(DwarfCompileUnit &CU,
   if (!TypeUnitsUnderConstruction.empty() && AddrPool.hasBeenUsed())
     return;
 
-  auto Ins = TypeSignatures.insert(std::make_pair(CTy, 0));
+  auto Ins = TypeSignatures.try_emplace(CTy);
   if (!Ins.second) {
     CU.addDIETypeSignature(RefDie, Ins.first->second);
     return;

llvm/lib/MC/StringTableBuilder.cpp

@@ -204,7 +204,7 @@ size_t StringTableBuilder::add(CachedHashStringRef S) {
     assert(S.size() > COFF::NameSize && "Short string in COFF string table!");
 
   assert(!isFinalized());
-  auto P = StringIndexMap.insert(std::make_pair(S, 0));
+  auto P = StringIndexMap.try_emplace(S);
   if (P.second) {
     size_t Start = alignTo(Size, Alignment);
     P.first->second = Start;

llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp

@@ -375,7 +375,7 @@ void WebAssemblyFixIrreducibleControlFlow::makeSingleEntryLoop(
   // add them as successors.
   DenseMap<MachineBasicBlock *, unsigned> Indices;
   for (auto *Entry : SortedEntries) {
-    auto Pair = Indices.insert(std::make_pair(Entry, 0));
+    auto Pair = Indices.try_emplace(Entry);
     assert(Pair.second);
 
     unsigned Index = MIB.getInstr()->getNumExplicitOperands() - 1;