[Support] Change MapVector's default template parameter to SmallVector<*, 0>

SmallVector<*, 0> is often a better replacement for std::vector :
both the object size and the code size are smaller.
(SmallMapVector uses SmallVector as well, but it is not common.)

clang size decreases by 0.0226%.
instructions:u decreases 0.037% when compiling a sqlite3 amalgram.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D156016

lld/MachO/UnwindInfoSection.cpp

@@ -146,7 +146,7 @@ private:
   Symbol *canonicalizePersonality(Symbol *);
 
   uint64_t unwindInfoSize = 0;
-  std::vector<decltype(symbols)::value_type> symbolsVec;
+  SmallVector<decltype(symbols)::value_type, 0> symbolsVec;
   CompactUnwindLayout cuLayout;
   std::vector<std::pair<compact_unwind_encoding_t, size_t>> commonEncodings;
   EncodingMap commonEncodingIndexes;

llvm/include/llvm/ADT/MapVector.h

@@ -10,7 +10,7 @@
 /// This file implements a map that provides insertion order iteration. The
 /// interface is purposefully minimal. The key is assumed to be cheap to copy
 /// and 2 copies are kept, one for indexing in a DenseMap, one for iteration in
-/// a std::vector.
+/// a SmallVector.
 ///
 //===----------------------------------------------------------------------===//
 
@@ -24,16 +24,15 @@
 #include <iterator>
 #include <type_traits>
 #include <utility>
-#include <vector>
 
 namespace llvm {
 
 /// This class implements a map that also provides access to all stored values
-/// in a deterministic order. The values are kept in a std::vector and the
+/// in a deterministic order. The values are kept in a SmallVector<*, 0> and the
 /// mapping is done with DenseMap from Keys to indexes in that vector.
-template<typename KeyT, typename ValueT,
-         typename MapType = DenseMap<KeyT, unsigned>,
-         typename VectorType = std::vector<std::pair<KeyT, ValueT>>>
+template <typename KeyT, typename ValueT,
+          typename MapType = DenseMap<KeyT, unsigned>,
+          typename VectorType = SmallVector<std::pair<KeyT, ValueT>, 0>>
 class MapVector {
   MapType Map;
   VectorType Vector;

llvm/lib/Analysis/ModuleSummaryAnalysis.cpp

@@ -266,7 +266,9 @@ static void computeFunctionSummary(
   unsigned NumInsts = 0;
   // Map from callee ValueId to profile count. Used to accumulate profile
   // counts for all static calls to a given callee.
-  MapVector<ValueInfo, CalleeInfo> CallGraphEdges;
+  MapVector<ValueInfo, CalleeInfo, DenseMap<ValueInfo, unsigned>,
+            std::vector<std::pair<ValueInfo, CalleeInfo>>>
+      CallGraphEdges;
   SetVector<ValueInfo> RefEdges, LoadRefEdges, StoreRefEdges;
   SetVector<GlobalValue::GUID> TypeTests;
   SetVector<FunctionSummary::VFuncId> TypeTestAssumeVCalls,

mlir/lib/IR/AsmPrinter.cpp

@@ -975,8 +975,8 @@ static StringRef sanitizeIdentifier(StringRef name, SmallString<16> &buffer,
 void AliasInitializer::initializeAliases(
     llvm::MapVector<const void *, InProgressAliasInfo> &visitedSymbols,
     llvm::MapVector<const void *, SymbolAlias> &symbolToAlias) {
-  std::vector<std::pair<const void *, InProgressAliasInfo>> unprocessedAliases =
-      visitedSymbols.takeVector();
+  SmallVector<std::pair<const void *, InProgressAliasInfo>, 0>
+      unprocessedAliases = visitedSymbols.takeVector();
   llvm::stable_sort(unprocessedAliases, [](const auto &lhs, const auto &rhs) {
     return lhs.second < rhs.second;
   });