[PGO]: reserve space for string to avoid excessive memory realloc/cop…

…y (non linear)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256776 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ProfileData/InstrProf.cpp

@@ -168,6 +168,12 @@ int collectPGOFuncNameStrings(const std::vector<std::string> &NameStrs,
                               bool doCompression, std::string &Result) {
   uint8_t Header[16], *P = Header;
   std::string UncompressedNameStrings;
+  size_t UncompressedStringLen = 0;
+
+  for (auto NameStr : NameStrs)
+    UncompressedStringLen += (NameStr.length() + 1);
+
+  UncompressedNameStrings.reserve(UncompressedStringLen + 1);
 
   for (auto NameStr : NameStrs) {
     UncompressedNameStrings += NameStr;