[BOLT][DWARF] Replace MD5 with hash_combine

[ayermolo]

Summary

Slight performance improvement, based on perf.

Collected on clang-17 built with DWARF4 + split dwarf.
MD5
8:46.50 real, 713.38 user, 64.19 sys, 0 amem, 41933136 mmem
8:27.44 real, 708.55 user, 63.83 sys, 0 amem, 41906576 mmem
8:40.37 real, 724.63 user, 62.56 sys, 0 amem, 42319572 mmem

hash_combine

8:03.99 real, 681.92 user, 60.04 sys, 0 amem, 42459204 mmem
8:02.92 real, 685.20 user, 62.56 sys, 0 amem, 41879164 mmem
7:57.85 real, 690.27 user, 60.12 sys, 0 amem, 41806240 mmem

[Amir]

Is this hash strictly internal, or it gets emitted somewhere? What kind of hash is used by hash_combine?

[ayermolo]

In D155764#4517071, @Amir wrote:

Is this hash strictly internal, or it gets emitted somewhere? What kind of hash is used by hash_combine?

Yes, internally only.
No idea. Some kind of home brew in ADT/hashing.h + std::rotate.
FYI same API is used in BinarySection.cpp to has BinarySections, and in SplitFunctions.

bolt/lib/Core/DIEBuilder.cpp

for (auto &Child : Die->children()) {
     generateUnitAbbrevs(&Child);
   }
 }
 
 static uint64_t getHash(const DWARFUnit &DU) {
   // Before DWARF5 TU units are in their own section, so at least one offset,
   // first one, will be the same as CUs in .debug_info.dwo section
   if (DU.getVersion() < 5 && DU.isTypeUnit()) {
-    MD5 Hash;
-    uint64_t TypeUnitHash = cast_or_null<DWARFTypeUnit>(&DU)->getTypeHash();
-    uint64_t Offset = DU.getOffset();
-    Hash.update(llvm::ArrayRef((uint8_t *)&TypeUnitHash, sizeof(TypeUnitHash)));
-    Hash.update(llvm::ArrayRef((uint8_t *)&Offset, sizeof(Offset)));
-    return Hash.final().high();
+    const uint64_t TypeUnitHash =
+        cast_or_null<DWARFTypeUnit>(&DU)->getTypeHash();
+    const uint64_t Offset = DU.getOffset();
+    return llvm::hash_combine(llvm::hash_value(TypeUnitHash),
+                              llvm::hash_value(Offset));
   }
   return DU.getOffset();
 }
 
 void DIEBuilder::registerUnit(DWARFUnit &DU, bool NeedSort) {
   auto IterGlobal = AllProcessed.insert(getHash(DU));
   // If DU is already in a current working set or was already processed we can
   // skip it.