[memprof] Omit the key length for the record table (#89527)

The record table has a constant key length, so we don't need to
serialize or deserialize it for every key-data pair.  Omitting the key
length saves 0.06% of the indexed MemProf file size.

Note that it's OK to change the format because Version2 is still under
development.

Author: kazutakahirata

llvm/include/llvm/ProfileData/MemProf.h

@@ -471,12 +471,16 @@ class RecordLookupTrait {
 
   hash_value_type ComputeHash(uint64_t K) { return K; }
 
-  static std::pair<offset_type, offset_type>
+  std::pair<offset_type, offset_type>
   ReadKeyDataLength(const unsigned char *&D) {
     using namespace support;
 
+    // Starting with Version2, we don't read the key length because it is a
+    // constant.
     offset_type KeyLen =
-        endian::readNext<offset_type, llvm::endianness::little>(D);
+        Version < Version2
+            ? endian::readNext<offset_type, llvm::endianness::little>(D)
+            : sizeof(uint64_t);
     offset_type DataLen =
         endian::readNext<offset_type, llvm::endianness::little>(D);
     return std::make_pair(KeyLen, DataLen);
@@ -534,7 +538,9 @@ class RecordWriterTrait {
 
     endian::Writer LE(Out, llvm::endianness::little);
     offset_type N = sizeof(K);
-    LE.write<offset_type>(N);
+    // Starting with Version2, we omit the key length because it is a constant.
+    if (Version < Version2)
+      LE.write<offset_type>(N);
     offset_type M = V.serializedSize(Version);
     LE.write<offset_type>(M);
     return std::make_pair(N, M);

llvm/include/llvm/Support/OnDiskHashTable.h

@@ -377,7 +377,7 @@ template <typename Info> class OnDiskChainedHashTable {
 
       // Determine the length of the key and the data.
       const std::pair<offset_type, offset_type> &L =
-          Info::ReadKeyDataLength(Items);
+          InfoPtr->ReadKeyDataLength(Items);
       offset_type ItemLen = L.first + L.second;
 
       // Compare the hashes.  If they are not the same, skip the entry entirely.