[ProfData] Improve efficiency of reader

llvm/include/llvm/ProfileData/SampleProf.h

@@ -1658,6 +1658,7 @@ class ProfileSymbolList {
   }
 
   unsigned size() { return Syms.size(); }
+  void reserve(size_t Size) { Syms.reserve(Size); }
 
   void setToCompress(bool TC) { ToCompress = TC; }
   bool toCompress() { return ToCompress; }

llvm/lib/ProfileData/SampleProf.cpp

@@ -22,6 +22,8 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <cstdint>
 #include <string>
 #include <system_error>
 
@@ -398,6 +400,10 @@ LLVM_DUMP_METHOD void FunctionSamples::dump() const { print(dbgs(), 0); }
 
 std::error_code ProfileSymbolList::read(const uint8_t *Data,
                                         uint64_t ListSize) {
+  // Scan forward to see how many elements we expect.
+  reserve(std::min<uint64_t>(ProfileSymbolListCutOff,
+                             std::count(Data, Data + ListSize, 0)));
+
   const char *ListStart = reinterpret_cast<const char *>(Data);
   uint64_t Size = 0;
   uint64_t StrNum = 0;