[XRay] Reserve memory space ahead-of-time when reading native format …

…log (#76853)

XRay used to struggle reading large log files. It turned out the
bottleneck was primarily caused by the reallocation happens when
appending log entries into a std::vector.
This patch reserves the memory space ahead-of-time since the number of
entries is known for most cases. Making llvm-xray runs 1.8 times faster
and uses 1.4 times less physical memory when reading large (~2.6GB) log
files.

llvm/lib/XRay/Trace.cpp

@@ -51,6 +51,9 @@ Error loadNaiveFormatLog(StringRef Data, bool IsLittleEndian,
     return FileHeaderOrError.takeError();
   FileHeader = std::move(FileHeaderOrError.get());
 
+  size_t NumReservations = llvm::divideCeil(Reader.size() - OffsetPtr, 32U);
+  Records.reserve(NumReservations);
+
   // Each record after the header will be 32 bytes, in the following format:
   //
   //   (2)   uint16 : record type