[ProfileData] Refactor VTableNamePtr and CompressedVTableNamesLen (NFC) (#94859)

VTableNamePtr and CompressedVTableNamesLen are always used together to
create a StringRef in getSymtab.

We can create the StringRef ahead of time in readHeader.  This way,
IndexedInstrProfReader becomes a tiny bit simpler with fewer member
variables.  Also, StringRef default-constructs itself with its Data
and Length set to nullptr and 0, respectively, which is exactly what
we need.

Author: kazutakahirata

llvm/include/llvm/ProfileData/InstrProfReader.h

@@ -693,15 +693,10 @@ class IndexedInstrProfReader : public InstrProfReader {
   /// Context sensitive profile summary data.
   std::unique_ptr<ProfileSummary> CS_Summary;
   IndexedMemProfReader MemProfReader;
-  /// VTableNamePtr points to the beginning of compressed vtable names.
-  /// When a symtab is constructed from profiles by llvm-profdata, the list of
-  /// names could be decompressed based on `VTableNamePtr` and
-  /// `CompressedVTableNamesLen`.
+  /// The compressed vtable names, to be used for symtab construction.
   /// A compiler that reads indexed profiles could construct symtab from module
   /// IR so it doesn't need the decompressed names.
-  const char *VTableNamePtr = nullptr;
-  /// The length of compressed vtable names.
-  uint64_t CompressedVTableNamesLen = 0;
+  StringRef VTableName;
   /// Total size of binary ids.
   uint64_t BinaryIdsSize{0};
   /// Start address of binary id length and data pairs.

llvm/lib/ProfileData/InstrProfReader.cpp

@@ -1397,14 +1397,16 @@ Error IndexedInstrProfReader::readHeader() {
   if (Header->getIndexedProfileVersion() >= 12) {
     const unsigned char *Ptr = Start + Header->VTableNamesOffset;
 
-    CompressedVTableNamesLen =
+    uint64_t CompressedVTableNamesLen =
         support::endian::readNext<uint64_t, llvm::endianness::little>(Ptr);
 
     // Writer first writes the length of compressed string, and then the actual
     // content.
-    VTableNamePtr = (const char *)Ptr;
+    const char *VTableNamePtr = (const char *)Ptr;
     if (VTableNamePtr > (const char *)DataBuffer->getBufferEnd())
       return make_error<InstrProfError>(instrprof_error::truncated);
+
+    VTableName = StringRef(VTableNamePtr, CompressedVTableNamesLen);
   }
 
   if (Header->getIndexedProfileVersion() >= 10 &&
@@ -1460,8 +1462,7 @@ InstrProfSymtab &IndexedInstrProfReader::getSymtab() {
 
   auto NewSymtab = std::make_unique<InstrProfSymtab>();
 
-  if (Error E = NewSymtab->initVTableNamesFromCompressedStrings(
-          StringRef(VTableNamePtr, CompressedVTableNamesLen))) {
+  if (Error E = NewSymtab->initVTableNamesFromCompressedStrings(VTableName)) {
     auto [ErrCode, Msg] = InstrProfError::take(std::move(E));
     consumeError(error(ErrCode, Msg));
   }