[ProfData] Improve efficiency of reader

Pre-reserve space in the map before inserting. In release builds, 9.4%
of all CPU time is spent in llvm::sampleprof::ProfileSymbolList::add. Of
that 9.4%, roughly half is in llvm::DenseMapBase::grow.

Author: mxms

llvm/lib/ProfileData/SampleProf.cpp

@@ -399,30 +399,29 @@ LLVM_DUMP_METHOD void FunctionSamples::dump() const { print(dbgs(), 0); }
 std::error_code ProfileSymbolList::read(const uint8_t *Data,
                                         uint64_t ListSize) {
   const char *ListStart = reinterpret_cast<const char *>(Data);
-  uint64_t Offset = 0;
+  uint64_t Size = 0;
   uint64_t StrNum = 0;
   uint64_t ExpectedCount = 0;
 
   // Scan forward to see how many elements we expect.
   while (Offset < ListSize) {
     if (ListStart[Offset] == '\0') ExpectedCount++;
     Offset++;
-  }
 
   reserve(ExpectedCount);
 
-  Offset = 0;
+  Size = 0;
 
-  while (Offset < ListSize && StrNum < ProfileSymbolListCutOff) {
-    StringRef Str(ListStart + Offset);
+  while (Size < ListSize && StrNum < ProfileSymbolListCutOff) {
+    StringRef Str(ListStart + Size);
     add(Str);
-    Offset += Str.size() + 1;
+    Size += Str.size() + 1;
     StrNum++;
   }
 
   assert(ExpectedCount == StrNum);
 
-  if (Offset != ListSize && StrNum != ProfileSymbolListCutOff)
+  if (Size != ListSize && StrNum != ProfileSymbolListCutOff)
     return sampleprof_error::malformed;
   return sampleprof_error::success;
 }